FFmpeg源代码简单分析 libswscale的sws getContext
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FFmpeg的库函数源代码分析文章列表：
【架构图】
FFmpeg源代码结构图 - 解码
FFmpeg源代码结构图 - 编码
【通用】
FFmpeg 源代码简单分析：av_register_all()
FFmpeg 源代码简单分析：avcodec_register_all()
FFmpeg 源代码简单分析：内存的分配和释放（av_malloc()、av_free()等）
FFmpeg 源代码简单分析：常见结构体的初始化和销毁（AVFormatContext，AVFrame等）
FFmpeg 源代码简单分析：avio_open2()
FFmpeg 源代码简单分析：av_find_decoder()和av_find_encoder()
FFmpeg 源代码简单分析：avcodec_open2()
FFmpeg 源代码简单分析：avcodec_close()
【解码】
图解FFMPEG打开媒体的函数avformat_open_input
FFmpeg 源代码简单分析：avformat_open_input()
FFmpeg 源代码简单分析：avformat_find_stream_info()
FFmpeg 源代码简单分析：av_read_frame()
FFmpeg 源代码简单分析：avcodec_decode_video2()
FFmpeg 源代码简单分析：avformat_close_input()
【编码】
FFmpeg 源代码简单分析：avformat_alloc_output_context2()
FFmpeg 源代码简单分析：avformat_write_header()
FFmpeg 源代码简单分析：avcodec_encode_video()
FFmpeg 源代码简单分析：av_write_frame()
FFmpeg 源代码简单分析：av_write_trailer()
【其它】
FFmpeg源代码简单分析：日志输出系统（av_log()等）
FFmpeg源代码简单分析：结构体成员管理系统-AVClass
FFmpeg源代码简单分析：结构体成员管理系统-AVOption
FFmpeg源代码简单分析：libswscale的sws_getContext()
FFmpeg源代码简单分析：libswscale的sws_scale()
FFmpeg源代码简单分析：libavdevice的avdevice_register_all()
FFmpeg源代码简单分析：libavdevice的gdigrab
【脚本】
FFmpeg源代码简单分析：makefile
FFmpeg源代码简单分析：configure
【H.264】
FFmpeg的H.264解码器源代码简单分析：概述
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打算写两篇文章记录FFmpeg中的图像处理（缩放，YUV/RGB格式转换）类库libswsscale的源代码。libswscale是一个主要用于处理图片像素数据的类库。可以完成图片像素格式的转换，图片的拉伸等工作。有关libswscale的使用可以参考文章：
《最简单的基于FFmpeg的libswscale的示例（YUV转RGB）》
libswscale常用的函数数量很少，一般情况下就3个：
sws_getContext()：初始化一个SwsContext。
sws_scale()：处理图像数据。
sws_freeContext()：释放一个SwsContext。
其中sws_getContext()也可以用sws_getCachedContext()取代。
尽管libswscale从表面上看常用函数的个数不多，它的内部却有一个大大的“世界”。做为一个几乎“万能”的图片像素数据处理类库，它的内部包含了大量的代码。因此计划写两篇文章分析它的源代码。本文首先分析它的初始化函数sws_getContext()，而下一篇文章则分析它的数据处理函数sws_scale()。
函数调用结构图
分析得到的libswscale的函数调用关系如下图所示。
Libswscale处理数据流程
Libswscale处理像素数据的流程可以概括为下图。
从图中可以看出，libswscale处理数据有两条最主要的方式：unscaled和scaled。unscaled用于处理不需要拉伸的像素数据（属于比较特殊的情况），scaled用于处理需要拉伸的像素数据。Unscaled只需要对图像像素格式进行转换；而Scaled则除了对像素格式进行转换之外，还需要对图像进行缩放。Scaled方式可以分成以下几个步骤：
- XXX to YUV Converter：首相将数据像素数据转换为8bitYUV格式；
- Horizontal scaler：水平拉伸图像，并且转换为15bitYUV；
- Vertical scaler：垂直拉伸图像；
- Output converter：转换为输出像素格式。
SwsContext
SwsContext是使用libswscale时候一个贯穿始终的结构体。但是我们在使用FFmpeg的类库进行开发的时候，是无法看到它的内部结构的。在libswscale\swscale.h中只能看到一行定义：struct SwsContext;/* This struct should be aligned on at least a 32-byte boundary. */typedef struct SwsContext { /** * info on struct for av_log */ const AVClass *av_class; /** * Note that src, dst, srcStride, dstStride will be copied in the * sws_scale() wrapper so they can be freely modified here. */ SwsFunc swscale; int srcW; ///< Width of source luma/alpha planes. int srcH; ///< Height of source luma/alpha planes. int dstH; ///< Height of destination luma/alpha planes. int chrSrcW; ///< Width of source chroma planes. int chrSrcH; ///< Height of source chroma planes. int chrDstW; ///< Width of destination chroma planes. int chrDstH; ///< Height of destination chroma planes. int lumXInc, chrXInc; int lumYInc, chrYInc; enum AVPixelFormat dstFormat; ///< Destination pixel format. enum AVPixelFormat srcFormat; ///< Source pixel format. int dstFormatBpp; ///< Number of bits per pixel of the destination pixel format. int srcFormatBpp; ///< Number of bits per pixel of the source pixel format. int dstBpc, srcBpc; int chrSrcHSubSample; ///< Binary logarithm of horizontal subsampling factor between luma/alpha and chroma planes in source image. int chrSrcVSubSample; ///< Binary logarithm of vertical subsampling factor between luma/alpha and chroma planes in source image. int chrDstHSubSample; ///< Binary logarithm of horizontal subsampling factor between luma/alpha and chroma planes in destination image. int chrDstVSubSample; ///< Binary logarithm of vertical subsampling factor between luma/alpha and chroma planes in destination image. int vChrDrop; ///< Binary logarithm of extra vertical subsampling factor in source image chroma planes specified by user. int sliceDir; ///< Direction that slices are fed to the scaler (1 = top-to-bottom, -1 = bottom-to-top). double param[2]; ///< Input parameters for scaling algorithms that need them. /* The cascaded_* fields allow spliting a scaler task into multiple * sequential steps, this is for example used to limit the maximum * downscaling factor that needs to be supported in one scaler. */ struct SwsContext *cascaded_context[2]; int cascaded_tmpStride[4]; uint8_t *cascaded_tmp[4]; uint32_t pal_yuv[256]; uint32_t pal_rgb[256]; /** * @name Scaled horizontal lines ring buffer. * The horizontal scaler keeps just enough scaled lines in a ring buffer * so they may be passed to the vertical scaler. The pointers to the * allocated buffers for each line are duplicated in sequence in the ring * buffer to simplify indexing and avoid wrapping around between lines * inside the vertical scaler code. The wrapping is done before the * vertical scaler is called. */ //@{ int16_t **lumPixBuf; ///< Ring buffer for scaled horizontal luma plane lines to be fed to the vertical scaler. int16_t **chrUPixBuf; ///< Ring buffer for scaled horizontal chroma plane lines to be fed to the vertical scaler. int16_t **chrVPixBuf; ///< Ring buffer for scaled horizontal chroma plane lines to be fed to the vertical scaler. int16_t **alpPixBuf; ///< Ring buffer for scaled horizontal alpha plane lines to be fed to the vertical scaler. int vLumBufSize; ///< Number of vertical luma/alpha lines allocated in the ring buffer. int vChrBufSize; ///< Number of vertical chroma lines allocated in the ring buffer. int lastInLumBuf; ///< Last scaled horizontal luma/alpha line from source in the ring buffer. int lastInChrBuf; ///< Last scaled horizontal chroma line from source in the ring buffer. int lumBufIndex; ///< Index in ring buffer of the last scaled horizontal luma/alpha line from source. int chrBufIndex; ///< Index in ring buffer of the last scaled horizontal chroma line from source. //@} uint8_t *formatConvBuffer; /** * @name Horizontal and vertical filters. * To better understand the following fields, here is a pseudo-code of * their usage in filtering a horizontal line: * @code * for (i = 0; i < width; i++) { * dst[i] = 0; * for (j = 0; j < filterSize; j++) * dst[i] += src[ filterPos[i] + j ] * filter[ filterSize * i + j ]; * dst[i] >>= FRAC_BITS; // The actual implementation is fixed-point. * } * @endcode */ //@{ int16_t *hLumFilter; ///< Array of horizontal filter coefficients for luma/alpha planes. int16_t *hChrFilter; ///< Array of horizontal filter coefficients for chroma planes. int16_t *vLumFilter; ///< Array of vertical filter coefficients for luma/alpha planes. int16_t *vChrFilter; ///< Array of vertical filter coefficients for chroma planes. int32_t *hLumFilterPos; ///< Array of horizontal filter starting positions for each dst[i] for luma/alpha planes. int32_t *hChrFilterPos; ///< Array of horizontal filter starting positions for each dst[i] for chroma planes. int32_t *vLumFilterPos; ///< Array of vertical filter starting positions for each dst[i] for luma/alpha planes. int32_t *vChrFilterPos; ///< Array of vertical filter starting positions for each dst[i] for chroma planes. int hLumFilterSize; ///< Horizontal filter size for luma/alpha pixels. int hChrFilterSize; ///< Horizontal filter size for chroma pixels. int vLumFilterSize; ///< Vertical filter size for luma/alpha pixels. int vChrFilterSize; ///< Vertical filter size for chroma pixels. //@} int lumMmxextFilterCodeSize; ///< Runtime-generated MMXEXT horizontal fast bilinear scaler code size for luma/alpha planes. int chrMmxextFilterCodeSize; ///< Runtime-generated MMXEXT horizontal fast bilinear scaler code size for chroma planes. uint8_t *lumMmxextFilterCode; ///< Runtime-generated MMXEXT horizontal fast bilinear scaler code for luma/alpha planes. uint8_t *chrMmxextFilterCode; ///< Runtime-generated MMXEXT horizontal fast bilinear scaler code for chroma planes. int canMMXEXTBeUsed; int dstY; ///< Last destination vertical line output from last slice. int flags; ///< Flags passed by the user to select scaler algorithm, optimizations, subsampling, etc... void *yuvTable; // pointer to the yuv->rgb table start so it can be freed() // alignment ensures the offset can be added in a single // instruction on e.g. ARM DECLARE_ALIGNED(16, int, table_gV)[256 + 2*YUVRGB_TABLE_HEADROOM]; uint8_t *table_rV[256 + 2*YUVRGB_TABLE_HEADROOM]; uint8_t *table_gU[256 + 2*YUVRGB_TABLE_HEADROOM]; uint8_t *table_bU[256 + 2*YUVRGB_TABLE_HEADROOM]; DECLARE_ALIGNED(16, int32_t, input_rgb2yuv_table)[16+40*4]; // This table can contain both C and SIMD formatted values, the C vales are always at the XY_IDX points#define RY_IDX 0#define GY_IDX 1#define BY_IDX 2#define RU_IDX 3#define GU_IDX 4#define BU_IDX 5#define RV_IDX 6#define GV_IDX 7#define BV_IDX 8#define RGB2YUV_SHIFT 15 int *dither_error[4]; //Colorspace stuff int contrast, brightness, saturation; // for sws_getColorspaceDetails int srcColorspaceTable[4]; int dstColorspaceTable[4]; int srcRange; ///< 0 = MPG YUV range, 1 = JPG YUV range (source image). int dstRange; ///< 0 = MPG YUV range, 1 = JPG YUV range (destination image). int src0Alpha; int dst0Alpha; int srcXYZ; int dstXYZ; int src_h_chr_pos; int dst_h_chr_pos; int src_v_chr_pos; int dst_v_chr_pos; int yuv2rgb_y_offset; int yuv2rgb_y_coeff; int yuv2rgb_v2r_coeff; int yuv2rgb_v2g_coeff; int yuv2rgb_u2g_coeff; int yuv2rgb_u2b_coeff;#define RED_DITHER "0*8"#define GREEN_DITHER "1*8"#define BLUE_DITHER "2*8"#define Y_COEFF "3*8"#define VR_COEFF "4*8"#define UB_COEFF "5*8"#define VG_COEFF "6*8"#define UG_COEFF "7*8"#define Y_OFFSET "8*8"#define U_OFFSET "9*8"#define V_OFFSET "10*8"#define LUM_MMX_FILTER_OFFSET "11*8"#define CHR_MMX_FILTER_OFFSET "11*8+4*4*"AV_STRINGIFY(MAX_FILTER_SIZE)#define DSTW_OFFSET "11*8+4*4*"AV_STRINGIFY(MAX_FILTER_SIZE)"*2"#define ESP_OFFSET "11*8+4*4*"AV_STRINGIFY(MAX_FILTER_SIZE)"*2+8"#define VROUNDER_OFFSET "11*8+4*4*"AV_STRINGIFY(MAX_FILTER_SIZE)"*2+16"#define U_TEMP "11*8+4*4*"AV_STRINGIFY(MAX_FILTER_SIZE)"*2+24"#define V_TEMP "11*8+4*4*"AV_STRINGIFY(MAX_FILTER_SIZE)"*2+32"#define Y_TEMP "11*8+4*4*"AV_STRINGIFY(MAX_FILTER_SIZE)"*2+40"#define ALP_MMX_FILTER_OFFSET "11*8+4*4*"AV_STRINGIFY(MAX_FILTER_SIZE)"*2+48"#define UV_OFF_PX "11*8+4*4*"AV_STRINGIFY(MAX_FILTER_SIZE)"*3+48"#define UV_OFF_BYTE "11*8+4*4*"AV_STRINGIFY(MAX_FILTER_SIZE)"*3+56"#define DITHER16 "11*8+4*4*"AV_STRINGIFY(MAX_FILTER_SIZE)"*3+64"#define DITHER32 "11*8+4*4*"AV_STRINGIFY(MAX_FILTER_SIZE)"*3+80"#define DITHER32_INT (11*8+4*4*MAX_FILTER_SIZE*3+80) // value equal to above, used for checking that the struct hasn't been changed by mistake DECLARE_ALIGNED(8, uint64_t, redDither); DECLARE_ALIGNED(8, uint64_t, greenDither); DECLARE_ALIGNED(8, uint64_t, blueDither); DECLARE_ALIGNED(8, uint64_t, yCoeff); DECLARE_ALIGNED(8, uint64_t, vrCoeff); DECLARE_ALIGNED(8, uint64_t, ubCoeff); DECLARE_ALIGNED(8, uint64_t, vgCoeff); DECLARE_ALIGNED(8, uint64_t, ugCoeff); DECLARE_ALIGNED(8, uint64_t, yOffset); DECLARE_ALIGNED(8, uint64_t, uOffset); DECLARE_ALIGNED(8, uint64_t, vOffset); int32_t lumMmxFilter[4 * MAX_FILTER_SIZE]; int32_t chrMmxFilter[4 * MAX_FILTER_SIZE]; int dstW; ///< Width of destination luma/alpha planes. DECLARE_ALIGNED(8, uint64_t, esp); DECLARE_ALIGNED(8, uint64_t, vRounder); DECLARE_ALIGNED(8, uint64_t, u_temp); DECLARE_ALIGNED(8, uint64_t, v_temp); DECLARE_ALIGNED(8, uint64_t, y_temp); int32_t alpMmxFilter[4 * MAX_FILTER_SIZE]; // alignment of these values is not necessary, but merely here // to maintain the same offset across x8632 and x86-64. Once we // use proper offset macros in the asm, they can be removed. DECLARE_ALIGNED(8, ptrdiff_t, uv_off); ///< offset (in pixels) between u and v planes DECLARE_ALIGNED(8, ptrdiff_t, uv_offx2); ///< offset (in bytes) between u and v planes DECLARE_ALIGNED(8, uint16_t, dither16)[8]; DECLARE_ALIGNED(8, uint32_t, dither32)[8]; const uint8_t *chrDither8, *lumDither8;#if HAVE_ALTIVEC vector signed short CY; vector signed short CRV; vector signed short CBU; vector signed short CGU; vector signed short CGV; vector signed short OY; vector unsigned short CSHIFT; vector signed short *vYCoeffsBank, *vCCoeffsBank;#endif int use_mmx_vfilter;/* pre defined color-spaces gamma */#define XYZ_GAMMA (2.6f)#define RGB_GAMMA (2.2f) int16_t *xyzgamma; int16_t *rgbgamma; int16_t *xyzgammainv; int16_t *rgbgammainv; int16_t xyz2rgb_matrix[3][4]; int16_t rgb2xyz_matrix[3][4]; /* function pointers for swscale() */ yuv2planar1_fn yuv2plane1; yuv2planarX_fn yuv2planeX; yuv2interleavedX_fn yuv2nv12cX; yuv2packed1_fn yuv2packed1; yuv2packed2_fn yuv2packed2; yuv2packedX_fn yuv2packedX; yuv2anyX_fn yuv2anyX; /// Unscaled conversion of luma plane to YV12 for horizontal scaler. void (*lumToYV12)(uint8_t *dst, const uint8_t *src, const uint8_t *src2, const uint8_t *src3, int width, uint32_t *pal); /// Unscaled conversion of alpha plane to YV12 for horizontal scaler. void (*alpToYV12)(uint8_t *dst, const uint8_t *src, const uint8_t *src2, const uint8_t *src3, int width, uint32_t *pal); /// Unscaled conversion of chroma planes to YV12 for horizontal scaler. void (*chrToYV12)(uint8_t *dstU, uint8_t *dstV, const uint8_t *src1, const uint8_t *src2, const uint8_t *src3, int width, uint32_t *pal); /** * Functions to read planar input, such as planar RGB, and convert * internally to Y/UV/A. */ /** @{ */ void (*readLumPlanar)(uint8_t *dst, const uint8_t *src[4], int width, int32_t *rgb2yuv); void (*readChrPlanar)(uint8_t *dstU, uint8_t *dstV, const uint8_t *src[4], int width, int32_t *rgb2yuv); void (*readAlpPlanar)(uint8_t *dst, const uint8_t *src[4], int width, int32_t *rgb2yuv); /** @} */ /** * Scale one horizontal line of input data using a bilinear filter * to produce one line of output data. Compared to SwsContext->hScale(), * please take note of the following caveats when using these: * - Scaling is done using only 7bit instead of 14bit coefficients. * - You can use no more than 5 input pixels to produce 4 output * pixels. Therefore, this filter should not be used for downscaling * by more than ~20% in width (because that equals more than 5/4th * downscaling and thus more than 5 pixels input per 4 pixels output). * - In general, bilinear filters create artifacts during downscaling * (even when <20%), because one output pixel will span more than one * input pixel, and thus some pixels will need edges of both neighbor * pixels to interpolate the output pixel. Since you can use at most * two input pixels per output pixel in bilinear scaling, this is * impossible and thus downscaling by any size will create artifacts. * To enable this type of scaling, set SWS_FLAG_FAST_BILINEAR * in SwsContext->flags. */ /** @{ */ void (*hyscale_fast)(struct SwsContext *c, int16_t *dst, int dstWidth, const uint8_t *src, int srcW, int xInc); void (*hcscale_fast)(struct SwsContext *c, int16_t *dst1, int16_t *dst2, int dstWidth, const uint8_t *src1, const uint8_t *src2, int srcW, int xInc); /** @} */ /** * Scale one horizontal line of input data using a filter over the input * lines, to produce one (differently sized) line of output data. * * @param dst pointer to destination buffer for horizontally scaled * data. If the number of bits per component of one * destination pixel (SwsContext->dstBpc) is <= 10, data * will be 15bpc in 16bits (int16_t) width. Else (i.e. * SwsContext->dstBpc == 16), data will be 19bpc in * 32bits (int32_t) width. * @param dstW width of destination image * @param src pointer to source data to be scaled. If the number of * bits per component of a source pixel (SwsContext->srcBpc) * is 8, this is 8bpc in 8bits (uint8_t) width. Else * (i.e. SwsContext->dstBpc > 8), this is native depth * in 16bits (uint16_t) width. In other words, for 9-bit * YUV input, this is 9bpc, for 10-bit YUV input, this is * 10bpc, and for 16-bit RGB or YUV, this is 16bpc. * @param filter filter coefficients to be used per output pixel for * scaling. This contains 14bpp filtering coefficients. * Guaranteed to contain dstW * filterSize entries. * @param filterPos position of the first input pixel to be used for * each output pixel during scaling. Guaranteed to * contain dstW entries. * @param filterSize the number of input coefficients to be used (and * thus the number of input pixels to be used) for * creating a single output pixel. Is aligned to 4 * (and input coefficients thus padded with zeroes) * to simplify creating SIMD code. */ /** @{ */ void (*hyScale)(struct SwsContext *c, int16_t *dst, int dstW, const uint8_t *src, const int16_t *filter, const int32_t *filterPos, int filterSize); void (*hcScale)(struct SwsContext *c, int16_t *dst, int dstW, const uint8_t *src, const int16_t *filter, const int32_t *filterPos, int filterSize); /** @} */ /// Color range conversion function for luma plane if needed. void (*lumConvertRange)(int16_t *dst, int width); /// Color range conversion function for chroma planes if needed. void (*chrConvertRange)(int16_t *dst1, int16_t *dst2, int width); int needs_hcscale; ///< Set if there are chroma planes to be converted. SwsDither dither;} SwsContext;这个结构体的定义确实比较复杂，里面包含了libswscale所需要的全部变量。一一分析这些变量是不太现实的，在后文中会简单分析其中的几个变量。
sws_getContext()
sws_getContext()是初始化SwsContext的函数。sws_getContext()的声明位于libswscale\swscale.h，如下所示。/** * Allocate and return an SwsContext. You need it to perform * scaling/conversion operations using sws_scale(). * * @param srcW the width of the source image * @param srcH the height of the source image * @param srcFormat the source image format * @param dstW the width of the destination image * @param dstH the height of the destination image * @param dstFormat the destination image format * @param flags specify which algorithm and options to use for rescaling * @return a pointer to an allocated context, or NULL in case of error * @note this function is to be removed after a saner alternative is * written */struct SwsContext *sws_getContext(int srcW, int srcH, enum AVPixelFormat srcFormat, int dstW, int dstH, enum AVPixelFormat dstFormat, int flags, SwsFilter *srcFilter, SwsFilter *dstFilter, const double *param);该函数包含以下参数：
srcW：源图像的宽成功执行的话返回生成的SwsContext，否则返回NULL。
srcH：源图像的高
srcFormat：源图像的像素格式
dstW：目标图像的宽
dstH：目标图像的高
dstFormat：目标图像的像素格式
flags：设定图像拉伸使用的算法
sws_getContext()的定义位于libswscale\utils.c，如下所示。
SwsContext *sws_getContext(int srcW, int srcH, enum AVPixelFormat srcFormat, int dstW, int dstH, enum AVPixelFormat dstFormat, int flags, SwsFilter *srcFilter, SwsFilter *dstFilter, const double *param){ SwsContext *c; if (!(c = sws_alloc_context())) return NULL; c->flags = flags; c->srcW = srcW; c->srcH = srcH; c->dstW = dstW; c->dstH = dstH; c->srcFormat = srcFormat; c->dstFormat = dstFormat; if (param) { c->param[0] = param[0]; c->param[1] = param[1]; } if (sws_init_context(c, srcFilter, dstFilter) < 0) { sws_freeContext(c); return NULL; } return c;}从sws_getContext()的定义中可以看出，它首先调用了一个函数sws_alloc_context()用于给SwsContext分配内存。然后将传入的源图像，目标图像的宽高，像素格式，以及标志位分别赋值给该SwsContext相应的字段。最后调用一个函数sws_init_context()完成初始化工作。下面我们分别看一下sws_alloc_context()和sws_init_context()这两个函数。
sws_alloc_context()
sws_alloc_context()是FFmpeg的一个API，用于给SwsContext分配内存，它的声明如下所示。/** * Allocate an empty SwsContext. This must be filled and passed to * sws_init_context(). For filling see AVOptions, options.c and * sws_setColorspaceDetails(). */struct SwsContext *sws_alloc_context(void);sws_alloc_context()的定义位于libswscale\utils.c，如下所示。
SwsContext *sws_alloc_context(void){ SwsContext *c = av_mallocz(sizeof(SwsContext)); av_assert0(offsetof(SwsContext, redDither) + DITHER32_INT == offsetof(SwsContext, dither32)); if (c) { c->av_class = &sws_context_class; av_opt_set_defaults(c); } return c;}从代码中可以看出，sws_alloc_context()首先调用av_mallocz()为SwsContext结构体分配了一块内存；然后设置了该结构体的AVClass，并且给该结构体的字段设置了默认值。
sws_init_context()
sws_init_context()的是FFmpeg的一个API，用于初始化SwsContext。/** * Initialize the swscaler context sws_context. * * @return zero or positive value on success, a negative value on * error */int sws_init_context(struct SwsContext *sws_context, SwsFilter *srcFilter, SwsFilter *dstFilter);sws_init_context()的函数定义非常的长，位于libswscale\utils.c，如下所示。
av_cold int sws_init_context(SwsContext *c, SwsFilter *srcFilter, SwsFilter *dstFilter){ int i, j; int usesVFilter, usesHFilter; int unscaled; SwsFilter dummyFilter = { NULL, NULL, NULL, NULL }; int srcW = c->srcW; int srcH = c->srcH; int dstW = c->dstW; int dstH = c->dstH; int dst_stride = FFALIGN(dstW * sizeof(int16_t) + 66, 16); int flags, cpu_flags; enum AVPixelFormat srcFormat = c->srcFormat; enum AVPixelFormat dstFormat = c->dstFormat; const AVPixFmtDescriptor *desc_src; const AVPixFmtDescriptor *desc_dst; int ret = 0; //获取 cpu_flags = av_get_cpu_flags(); flags = c->flags; emms_c(); if (!rgb15to16) sws_rgb2rgb_init(); //如果输入的宽高和输出的宽高一样，则做特殊处理 unscaled = (srcW == dstW && srcH == dstH); //如果是JPEG标准（Y取值0-255），则需要设置这两项 c->srcRange |= handle_jpeg(&c->srcFormat); c->dstRange |= handle_jpeg(&c->dstFormat); if(srcFormat!=c->srcFormat || dstFormat!=c->dstFormat) av_log(c, AV_LOG_WARNING, "deprecated pixel format used, make sure you did set range correctly\n"); //设置Colorspace if (!c->contrast && !c->saturation && !c->dstFormatBpp) sws_setColorspaceDetails(c, ff_yuv2rgb_coeffs[SWS_CS_DEFAULT], c->srcRange, ff_yuv2rgb_coeffs[SWS_CS_DEFAULT], c->dstRange, 0, 1 << 16, 1 << 16); handle_formats(c); srcFormat = c->srcFormat; dstFormat = c->dstFormat; desc_src = av_pix_fmt_desc_get(srcFormat); desc_dst = av_pix_fmt_desc_get(dstFormat); //转换大小端？ if (!(unscaled && sws_isSupportedEndiannessConversion(srcFormat) && av_pix_fmt_swap_endianness(srcFormat) == dstFormat)) { //检查输入格式是否支持 if (!sws_isSupportedInput(srcFormat)) { av_log(c, AV_LOG_ERROR, "%s is not supported as input pixel format\n", av_get_pix_fmt_name(srcFormat)); return AVERROR(EINVAL); } //检查输出格式是否支持 if (!sws_isSupportedOutput(dstFormat)) { av_log(c, AV_LOG_ERROR, "%s is not supported as output pixel format\n", av_get_pix_fmt_name(dstFormat)); return AVERROR(EINVAL); } } //检查拉伸的方法 i = flags & (SWS_POINT | SWS_AREA | SWS_BILINEAR | SWS_FAST_BILINEAR | SWS_BICUBIC | SWS_X | SWS_GAUSS | SWS_LANCZOS | SWS_SINC | SWS_SPLINE | SWS_BICUBLIN); /* provide a default scaler if not set by caller */ //如果没有指定，就使用默认的 if (!i) { if (dstW < srcW && dstH < srcH) flags |= SWS_BICUBIC; else if (dstW > srcW && dstH > srcH) flags |= SWS_BICUBIC; else flags |= SWS_BICUBIC; c->flags = flags; } else if (i & (i - 1)) { av_log(c, AV_LOG_ERROR, "Exactly one scaler algorithm must be chosen, got %X\n", i); return AVERROR(EINVAL); } /* sanity check */ //检查宽高参数 if (srcW < 1 || srcH < 1 || dstW < 1 || dstH < 1) { /* FIXME check if these are enough and try to lower them after * fixing the relevant parts of the code */ av_log(c, AV_LOG_ERROR, "%dx%d -> %dx%d is invalid scaling dimension\n", srcW, srcH, dstW, dstH); return AVERROR(EINVAL); } if (!dstFilter) dstFilter = &dummyFilter; if (!srcFilter) srcFilter = &dummyFilter; c->lumXInc = (((int64_t)srcW << 16) + (dstW >> 1)) / dstW; c->lumYInc = (((int64_t)srcH << 16) + (dstH >> 1)) / dstH; c->dstFormatBpp = av_get_bits_per_pixel(desc_dst); c->srcFormatBpp = av_get_bits_per_pixel(desc_src); c->vRounder = 4 * 0x0001000100010001ULL; usesVFilter = (srcFilter->lumV && srcFilter->lumV->length > 1) || (srcFilter->chrV && srcFilter->chrV->length > 1) || (dstFilter->lumV && dstFilter->lumV->length > 1) || (dstFilter->chrV && dstFilter->chrV->length > 1); usesHFilter = (srcFilter->lumH && srcFilter->lumH->length > 1) || (srcFilter->chrH && srcFilter->chrH->length > 1) || (dstFilter->lumH && dstFilter->lumH->length > 1) || (dstFilter->chrH && dstFilter->chrH->length > 1); av_pix_fmt_get_chroma_sub_sample(srcFormat, &c->chrSrcHSubSample, &c->chrSrcVSubSample); av_pix_fmt_get_chroma_sub_sample(dstFormat, &c->chrDstHSubSample, &c->chrDstVSubSample); if (isAnyRGB(dstFormat) && !(flags&SWS_FULL_CHR_H_INT)) { if (dstW&1) { av_log(c, AV_LOG_DEBUG, "Forcing full internal H chroma due to odd output size\n"); flags |= SWS_FULL_CHR_H_INT; c->flags = flags; } if ( c->chrSrcHSubSample == 0 && c->chrSrcVSubSample == 0 && c->dither != SWS_DITHER_BAYER //SWS_FULL_CHR_H_INT is currently not supported with SWS_DITHER_BAYER && !(c->flags & SWS_FAST_BILINEAR) ) { av_log(c, AV_LOG_DEBUG, "Forcing full internal H chroma due to input having non subsampled chroma\n"); flags |= SWS_FULL_CHR_H_INT; c->flags = flags; } } if (c->dither == SWS_DITHER_AUTO) { if (flags & SWS_ERROR_DIFFUSION) c->dither = SWS_DITHER_ED; } if(dstFormat == AV_PIX_FMT_BGR4_BYTE || dstFormat == AV_PIX_FMT_RGB4_BYTE || dstFormat == AV_PIX_FMT_BGR8 || dstFormat == AV_PIX_FMT_RGB8) { if (c->dither == SWS_DITHER_AUTO) c->dither = (flags & SWS_FULL_CHR_H_INT) ? SWS_DITHER_ED : SWS_DITHER_BAYER; if (!(flags & SWS_FULL_CHR_H_INT)) { if (c->dither == SWS_DITHER_ED || c->dither == SWS_DITHER_A_DITHER || c->dither == SWS_DITHER_X_DITHER) { av_log(c, AV_LOG_DEBUG, "Desired dithering only supported in full chroma interpolation for destination format '%s'\n", av_get_pix_fmt_name(dstFormat)); flags |= SWS_FULL_CHR_H_INT; c->flags = flags; } } if (flags & SWS_FULL_CHR_H_INT) { if (c->dither == SWS_DITHER_BAYER) { av_log(c, AV_LOG_DEBUG, "Ordered dither is not supported in full chroma interpolation for destination format '%s'\n", av_get_pix_fmt_name(dstFormat)); c->dither = SWS_DITHER_ED; } } } if (isPlanarRGB(dstFormat)) { if (!(flags & SWS_FULL_CHR_H_INT)) { av_log(c, AV_LOG_DEBUG, "%s output is not supported with half chroma resolution, switching to full\n", av_get_pix_fmt_name(dstFormat)); flags |= SWS_FULL_CHR_H_INT; c->flags = flags; } } /* reuse chroma for 2 pixels RGB/BGR unless user wants full * chroma interpolation */ if (flags & SWS_FULL_CHR_H_INT && isAnyRGB(dstFormat) && !isPlanarRGB(dstFormat) && dstFormat != AV_PIX_FMT_RGBA && dstFormat != AV_PIX_FMT_ARGB && dstFormat != AV_PIX_FMT_BGRA && dstFormat != AV_PIX_FMT_ABGR && dstFormat != AV_PIX_FMT_RGB24 && dstFormat != AV_PIX_FMT_BGR24 && dstFormat != AV_PIX_FMT_BGR4_BYTE && dstFormat != AV_PIX_FMT_RGB4_BYTE && dstFormat != AV_PIX_FMT_BGR8 && dstFormat != AV_PIX_FMT_RGB8 ) { av_log(c, AV_LOG_WARNING, "full chroma interpolation for destination format '%s' not yet implemented\n", av_get_pix_fmt_name(dstFormat)); flags &= ~SWS_FULL_CHR_H_INT; c->flags = flags; } if (isAnyRGB(dstFormat) && !(flags & SWS_FULL_CHR_H_INT)) c->chrDstHSubSample = 1; // drop some chroma lines if the user wants it c->vChrDrop = (flags & SWS_SRC_V_CHR_DROP_MASK) >> SWS_SRC_V_CHR_DROP_SHIFT; c->chrSrcVSubSample += c->vChrDrop; /* drop every other pixel for chroma calculation unless user * wants full chroma */ if (isAnyRGB(srcFormat) && !(flags & SWS_FULL_CHR_H_INP) && srcFormat != AV_PIX_FMT_RGB8 && srcFormat != AV_PIX_FMT_BGR8 && srcFormat != AV_PIX_FMT_RGB4 && srcFormat != AV_PIX_FMT_BGR4 && srcFormat != AV_PIX_FMT_RGB4_BYTE && srcFormat != AV_PIX_FMT_BGR4_BYTE && srcFormat != AV_PIX_FMT_GBRP9BE && srcFormat != AV_PIX_FMT_GBRP9LE && srcFormat != AV_PIX_FMT_GBRP10BE && srcFormat != AV_PIX_FMT_GBRP10LE && srcFormat != AV_PIX_FMT_GBRP12BE && srcFormat != AV_PIX_FMT_GBRP12LE && srcFormat != AV_PIX_FMT_GBRP14BE && srcFormat != AV_PIX_FMT_GBRP14LE && srcFormat != AV_PIX_FMT_GBRP16BE && srcFormat != AV_PIX_FMT_GBRP16LE && ((dstW >> c->chrDstHSubSample) <= (srcW >> 1) || (flags & SWS_FAST_BILINEAR))) c->chrSrcHSubSample = 1; // Note the FF_CEIL_RSHIFT is so that we always round toward +inf. c->chrSrcW = FF_CEIL_RSHIFT(srcW, c->chrSrcHSubSample); c->chrSrcH = FF_CEIL_RSHIFT(srcH, c->chrSrcVSubSample); c->chrDstW = FF_CEIL_RSHIFT(dstW, c->chrDstHSubSample); c->chrDstH = FF_CEIL_RSHIFT(dstH, c->chrDstVSubSample); FF_ALLOC_OR_GOTO(c, c->formatConvBuffer, FFALIGN(srcW*2+78, 16) * 2, fail); c->srcBpc = 1 + desc_src->comp[0].depth_minus1; if (c->srcBpc < 8) c->srcBpc = 8; c->dstBpc = 1 + desc_dst->comp[0].depth_minus1; if (c->dstBpc < 8) c->dstBpc = 8; if (isAnyRGB(srcFormat) || srcFormat == AV_PIX_FMT_PAL8) c->srcBpc = 16; if (c->dstBpc == 16) dst_stride <<= 1; if (INLINE_MMXEXT(cpu_flags) && c->srcBpc == 8 && c->dstBpc <= 14) { c->canMMXEXTBeUsed = dstW >= srcW && (dstW & 31) == 0 && c->chrDstW >= c->chrSrcW && (srcW & 15) == 0; if (!c->canMMXEXTBeUsed && dstW >= srcW && c->chrDstW >= c->chrSrcW && (srcW & 15) == 0 && (flags & SWS_FAST_BILINEAR)) { if (flags & SWS_PRINT_INFO) av_log(c, AV_LOG_INFO, "output width is not a multiple of 32 -> no MMXEXT scaler\n"); } if (usesHFilter || isNBPS(c->srcFormat) || is16BPS(c->srcFormat) || isAnyRGB(c->srcFormat)) c->canMMXEXTBeUsed = 0; } else c->canMMXEXTBeUsed = 0; c->chrXInc = (((int64_t)c->chrSrcW << 16) + (c->chrDstW >> 1)) / c->chrDstW; c->chrYInc = (((int64_t)c->chrSrcH << 16) + (c->chrDstH >> 1)) / c->chrDstH; /* Match pixel 0 of the src to pixel 0 of dst and match pixel n-2 of src * to pixel n-2 of dst, but only for the FAST_BILINEAR mode otherwise do * correct scaling. * n-2 is the last chrominance sample available. * This is not perfect, but no one should notice the difference, the more * correct variant would be like the vertical one, but that would require * some special code for the first and last pixel */ if (flags & SWS_FAST_BILINEAR) { if (c->canMMXEXTBeUsed) { c->lumXInc += 20; c->chrXInc += 20; } // we don't use the x86 asm scaler if MMX is available else if (INLINE_MMX(cpu_flags) && c->dstBpc <= 14) { c->lumXInc = ((int64_t)(srcW - 2) << 16) / (dstW - 2) - 20; c->chrXInc = ((int64_t)(c->chrSrcW - 2) << 16) / (c->chrDstW - 2) - 20; } } if (isBayer(srcFormat)) { if (!unscaled || (dstFormat != AV_PIX_FMT_RGB24 && dstFormat != AV_PIX_FMT_YUV420P)) { enum AVPixelFormat tmpFormat = AV_PIX_FMT_RGB24; ret = av_image_alloc(c->cascaded_tmp, c->cascaded_tmpStride, srcW, srcH, tmpFormat, 64); if (ret < 0) return ret; c->cascaded_context[0] = sws_getContext(srcW, srcH, srcFormat, srcW, srcH, tmpFormat, flags, srcFilter, NULL, c->param); if (!c->cascaded_context[0]) return -1; c->cascaded_context[1] = sws_getContext(srcW, srcH, tmpFormat, dstW, dstH, dstFormat, flags, NULL, dstFilter, c->param); if (!c->cascaded_context[1]) return -1; return 0; } }#define USE_MMAP (HAVE_MMAP && HAVE_MPROTECT && defined MAP_ANONYMOUS) /* precalculate horizontal scaler filter coefficients */ {#if HAVE_MMXEXT_INLINE// can't downscale !!! if (c->canMMXEXTBeUsed && (flags & SWS_FAST_BILINEAR)) { c->lumMmxextFilterCodeSize = ff_init_hscaler_mmxext(dstW, c->lumXInc, NULL, NULL, NULL, 8); c->chrMmxextFilterCodeSize = ff_init_hscaler_mmxext(c->chrDstW, c->chrXInc, NULL, NULL, NULL, 4);#if USE_MMAP c->lumMmxextFilterCode = mmap(NULL, c->lumMmxextFilterCodeSize, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); c->chrMmxextFilterCode = mmap(NULL, c->chrMmxextFilterCodeSize, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);#elif HAVE_VIRTUALALLOC c->lumMmxextFilterCode = VirtualAlloc(NULL, c->lumMmxextFilterCodeSize, MEM_COMMIT, PAGE_EXECUTE_READWRITE); c->chrMmxextFilterCode = VirtualAlloc(NULL, c->chrMmxextFilterCodeSize, MEM_COMMIT, PAGE_EXECUTE_READWRITE);#else c->lumMmxextFilterCode = av_malloc(c->lumMmxextFilterCodeSize); c->chrMmxextFilterCode = av_malloc(c->chrMmxextFilterCodeSize);#endif#ifdef MAP_ANONYMOUS if (c->lumMmxextFilterCode == MAP_FAILED || c->chrMmxextFilterCode == MAP_FAILED)#else if (!c->lumMmxextFilterCode || !c->chrMmxextFilterCode)#endif { av_log(c, AV_LOG_ERROR, "Failed to allocate MMX2FilterCode\n"); return AVERROR(ENOMEM); } FF_ALLOCZ_OR_GOTO(c, c->hLumFilter, (dstW / 8 + 8) * sizeof(int16_t), fail); FF_ALLOCZ_OR_GOTO(c, c->hChrFilter, (c->chrDstW / 4 + 8) * sizeof(int16_t), fail); FF_ALLOCZ_OR_GOTO(c, c->hLumFilterPos, (dstW / 2 / 8 + 8) * sizeof(int32_t), fail); FF_ALLOCZ_OR_GOTO(c, c->hChrFilterPos, (c->chrDstW / 2 / 4 + 8) * sizeof(int32_t), fail); ff_init_hscaler_mmxext( dstW, c->lumXInc, c->lumMmxextFilterCode, c->hLumFilter, (uint32_t*)c->hLumFilterPos, 8); ff_init_hscaler_mmxext(c->chrDstW, c->chrXInc, c->chrMmxextFilterCode, c->hChrFilter, (uint32_t*)c->hChrFilterPos, 4);#if USE_MMAP if ( mprotect(c->lumMmxextFilterCode, c->lumMmxextFilterCodeSize, PROT_EXEC | PROT_READ) == -1 || mprotect(c->chrMmxextFilterCode, c->chrMmxextFilterCodeSize, PROT_EXEC | PROT_READ) == -1) { av_log(c, AV_LOG_ERROR, "mprotect failed, cannot use fast bilinear scaler\n"); goto fail; }#endif } else#endif /* HAVE_MMXEXT_INLINE */ { const int filterAlign = X86_MMX(cpu_flags) ? 4 : PPC_ALTIVEC(cpu_flags) ? 8 : 1; if ((ret = initFilter(&c->hLumFilter, &c->hLumFilterPos, &c->hLumFilterSize, c->lumXInc, srcW, dstW, filterAlign, 1 << 14, (flags & SWS_BICUBLIN) ? (flags | SWS_BICUBIC) : flags, cpu_flags, srcFilter->lumH, dstFilter->lumH, c->param, get_local_pos(c, 0, 0, 0), get_local_pos(c, 0, 0, 0))) < 0) goto fail; if ((ret = initFilter(&c->hChrFilter, &c->hChrFilterPos, &c->hChrFilterSize, c->chrXInc, c->chrSrcW, c->chrDstW, filterAlign, 1 << 14, (flags & SWS_BICUBLIN) ? (flags | SWS_BILINEAR) : flags, cpu_flags, srcFilter->chrH, dstFilter->chrH, c->param, get_local_pos(c, c->chrSrcHSubSample, c->src_h_chr_pos, 0), get_local_pos(c, c->chrDstHSubSample, c->dst_h_chr_pos, 0))) < 0) goto fail; } } // initialize horizontal stuff /* precalculate vertical scaler filter coefficients */ { const int filterAlign = X86_MMX(cpu_flags) ? 2 : PPC_ALTIVEC(cpu_flags) ? 8 : 1; if ((ret = initFilter(&c->vLumFilter, &c->vLumFilterPos, &c->vLumFilterSize, c->lumYInc, srcH, dstH, filterAlign, (1 << 12), (flags & SWS_BICUBLIN) ? (flags | SWS_BICUBIC) : flags, cpu_flags, srcFilter->lumV, dstFilter->lumV, c->param, get_local_pos(c, 0, 0, 1), get_local_pos(c, 0, 0, 1))) < 0) goto fail; if ((ret = initFilter(&c->vChrFilter, &c->vChrFilterPos, &c->vChrFilterSize, c->chrYInc, c->chrSrcH, c->chrDstH, filterAlign, (1 << 12), (flags & SWS_BICUBLIN) ? (flags | SWS_BILINEAR) : flags, cpu_flags, srcFilter->chrV, dstFilter->chrV, c->param, get_local_pos(c, c->chrSrcVSubSample, c->src_v_chr_pos, 1), get_local_pos(c, c->chrDstVSubSample, c->dst_v_chr_pos, 1))) < 0) goto fail;#if HAVE_ALTIVEC FF_ALLOC_OR_GOTO(c, c->vYCoeffsBank, sizeof(vector signed short) * c->vLumFilterSize * c->dstH, fail); FF_ALLOC_OR_GOTO(c, c->vCCoeffsBank, sizeof(vector signed short) * c->vChrFilterSize * c->chrDstH, fail); for (i = 0; i < c->vLumFilterSize * c->dstH; i++) { int j; short *p = (short *)&c->vYCoeffsBank[i]; for (j = 0; j < 8; j++) p[j] = c->vLumFilter[i]; } for (i = 0; i < c->vChrFilterSize * c->chrDstH; i++) { int j; short *p = (short *)&c->vCCoeffsBank[i]; for (j = 0; j < 8; j++) p[j] = c->vChrFilter[i]; }#endif } // calculate buffer sizes so that they won't run out while handling these damn slices c->vLumBufSize = c->vLumFilterSize; c->vChrBufSize = c->vChrFilterSize; for (i = 0; i < dstH; i++) { int chrI = (int64_t)i * c->chrDstH / dstH; int nextSlice = FFMAX(c->vLumFilterPos[i] + c->vLumFilterSize - 1, ((c->vChrFilterPos[chrI] + c->vChrFilterSize - 1) << c->chrSrcVSubSample)); nextSlice >>= c->chrSrcVSubSample; nextSlice <<= c->chrSrcVSubSample; if (c->vLumFilterPos[i] + c->vLumBufSize < nextSlice) c->vLumBufSize = nextSlice - c->vLumFilterPos[i]; if (c->vChrFilterPos[chrI] + c->vChrBufSize < (nextSlice >> c->chrSrcVSubSample)) c->vChrBufSize = (nextSlice >> c->chrSrcVSubSample) - c->vChrFilterPos[chrI]; } for (i = 0; i < 4; i++) FF_ALLOCZ_OR_GOTO(c, c->dither_error[i], (c->dstW+2) * sizeof(int), fail); /* Allocate pixbufs (we use dynamic allocation because otherwise we would * need to allocate several megabytes to handle all possible cases) */ FF_ALLOC_OR_GOTO(c, c->lumPixBuf, c->vLumBufSize * 3 * sizeof(int16_t *), fail); FF_ALLOC_OR_GOTO(c, c->chrUPixBuf, c->vChrBufSize * 3 * sizeof(int16_t *), fail); FF_ALLOC_OR_GOTO(c, c->chrVPixBuf, c->vChrBufSize * 3 * sizeof(int16_t *), fail); if (CONFIG_SWSCALE_ALPHA && isALPHA(c->srcFormat) && isALPHA(c->dstFormat)) FF_ALLOCZ_OR_GOTO(c, c->alpPixBuf, c->vLumBufSize * 3 * sizeof(int16_t *), fail); /* Note we need at least one pixel more at the end because of the MMX code * (just in case someone wants to replace the 4000/8000). */ /* align at 16 bytes for AltiVec */ for (i = 0; i < c->vLumBufSize; i++) { FF_ALLOCZ_OR_GOTO(c, c->lumPixBuf[i + c->vLumBufSize], dst_stride + 16, fail); c->lumPixBuf[i] = c->lumPixBuf[i + c->vLumBufSize]; } // 64 / c->scalingBpp is the same as 16 / sizeof(scaling_intermediate) c->uv_off = (dst_stride>>1) + 64 / (c->dstBpc &~ 7); c->uv_offx2 = dst_stride + 16; for (i = 0; i < c->vChrBufSize; i++) { FF_ALLOC_OR_GOTO(c, c->chrUPixBuf[i + c->vChrBufSize], dst_stride * 2 + 32, fail); c->chrUPixBuf[i] = c->chrUPixBuf[i + c->vChrBufSize]; c->chrVPixBuf[i] = c->chrVPixBuf[i + c->vChrBufSize] = c->chrUPixBuf[i] + (dst_stride >> 1) + 8; } if (CONFIG_SWSCALE_ALPHA && c->alpPixBuf) for (i = 0; i < c->vLumBufSize; i++) { FF_ALLOCZ_OR_GOTO(c, c->alpPixBuf[i + c->vLumBufSize], dst_stride + 16, fail); c->alpPixBuf[i] = c->alpPixBuf[i + c->vLumBufSize]; } // try to avoid drawing green stuff between the right end and the stride end for (i = 0; i < c->vChrBufSize; i++) if(desc_dst->comp[0].depth_minus1 == 15){ av_assert0(c->dstBpc > 14); for(j=0; j2+1; j++) ((int32_t*)(c->chrUPixBuf[i]))[j] = 1<<18; } else for(j=0; j1; j++) ((int16_t*)(c->chrUPixBuf[i]))[j] = 1<<14; av_assert0(c->chrDstH <= dstH); //是否要输出 if (flags & SWS_PRINT_INFO) { const char *scaler = NULL, *cpucaps; for (i = 0; i < FF_ARRAY_ELEMS(scale_algorithms); i++) { if (flags & scale_algorithms[i].flag) { scaler = scale_algorithms[i].description; break; } } if (!scaler) scaler = "ehh flags invalid?!"; av_log(c, AV_LOG_INFO, "%s scaler, from %s to %s%s ", scaler, av_get_pix_fmt_name(srcFormat),#ifdef DITHER1XBPP dstFormat == AV_PIX_FMT_BGR555 || dstFormat == AV_PIX_FMT_BGR565 || dstFormat == AV_PIX_FMT_RGB444BE || dstFormat == AV_PIX_FMT_RGB444LE || dstFormat == AV_PIX_FMT_BGR444BE || dstFormat == AV_PIX_FMT_BGR444LE ? "dithered " : "",#else "",#endif av_get_pix_fmt_name(dstFormat)); if (INLINE_MMXEXT(cpu_flags)) cpucaps = "MMXEXT"; else if (INLINE_AMD3DNOW(cpu_flags)) cpucaps = "3DNOW"; else if (INLINE_MMX(cpu_flags)) cpucaps = "MMX"; else if (PPC_ALTIVEC(cpu_flags)) cpucaps = "AltiVec"; else cpucaps = "C"; av_log(c, AV_LOG_INFO, "using %s\n", cpucaps); av_log(c, AV_LOG_VERBOSE, "%dx%d -> %dx%d\n", srcW, srcH, dstW, dstH); av_log(c, AV_LOG_DEBUG, "lum srcW=%d srcH=%d dstW=%d dstH=%d xInc=%d yInc=%d\n", c->srcW, c->srcH, c->dstW, c->dstH, c->lumXInc, c->lumYInc); av_log(c, AV_LOG_DEBUG, "chr srcW=%d srcH=%d dstW=%d dstH=%d xInc=%d yInc=%d\n", c->chrSrcW, c->chrSrcH, c->chrDstW, c->chrDstH, c->chrXInc, c->chrYInc); } /* unscaled special cases */ //不拉伸的情况 if (unscaled && !usesHFilter && !usesVFilter && (c->srcRange == c->dstRange || isAnyRGB(dstFormat))) { //不许拉伸的情况下，初始化相应的函数 ff_get_unscaled_swscale(c); if (c->swscale) { if (flags & SWS_PRINT_INFO) av_log(c, AV_LOG_INFO, "using unscaled %s -> %s special converter\n", av_get_pix_fmt_name(srcFormat), av_get_pix_fmt_name(dstFormat)); return 0; } } //关键：设置SwsContext中的swscale()指针 c->swscale = ff_getSwsFunc(c); return 0;fail: // FIXME replace things by appropriate error codes if (ret == RETCODE_USE_CASCADE) { int tmpW = sqrt(srcW * (int64_t)dstW); int tmpH = sqrt(srcH * (int64_t)dstH); enum AVPixelFormat tmpFormat = AV_PIX_FMT_YUV420P; if (srcW*(int64_t)srcH <= 4LL*dstW*dstH) return AVERROR(EINVAL); ret = av_image_alloc(c->cascaded_tmp, c->cascaded_tmpStride, tmpW, tmpH, tmpFormat, 64); if (ret < 0) return ret; c->cascaded_context[0] = sws_getContext(srcW, srcH, srcFormat, tmpW, tmpH, tmpFormat, flags, srcFilter, NULL, c->param); if (!c->cascaded_context[0]) return -1; c->cascaded_context[1] = sws_getContext(tmpW, tmpH, tmpFormat, dstW, dstH, dstFormat, flags, NULL, dstFilter, c->param); if (!c->cascaded_context[1]) return -1; return 0; } return -1;} sws_init_context()除了对SwsContext中的各种变量进行赋值之外，主要按照顺序完成了以下一些工作：
1. 通过sws_rgb2rgb_init()初始化RGB转RGB（或者YUV转YUV）的函数（注意不包含RGB与YUV相互转换的函数）。
2. 通过判断输入输出图像的宽高来判断图像是否需要拉伸。如果图像需要拉伸，那么unscaled变量会被标记为1。
3. 通过sws_setColorspaceDetails()初始化颜色空间。
4. 一些输入参数的检测。例如：如果没有设置图像拉伸方法的话，默认设置为SWS_BICUBIC；如果输入和输出图像的宽高小于等于0的话，也会返回错误信息。
5. 初始化Filter。这一步根据拉伸方法的不同，初始化不同的Filter。
6. 如果flags中设置了“打印信息”选项SWS_PRINT_INFO，则输出信息。
7. 如果不需要拉伸的话，调用ff_get_unscaled_swscale()将特定的像素转换函数的指针赋值给SwsContext中的swscale指针。
8. 如果需要拉伸的话，调用ff_getSwsFunc()将通用的swscale()赋值给SwsContext中的swscale指针（这个地方有点绕，但是确实是这样的）。
下面分别记录一下上述步骤的实现。
sws_rgb2rgb_init()
sws_rgb2rgb_init()的定义位于libswscale\rgb2rgb.c，如下所示。av_cold void sws_rgb2rgb_init(void){ rgb2rgb_init_c(); if (ARCH_X86) rgb2rgb_init_x86();}从sws_rgb2rgb_init()代码中可以看出，有两个初始化函数：rgb2rgb_init_c()是初始化C语言版本的RGB互转（或者YUV互转）的函数，rgb2rgb_init_x86()则是初始化X86汇编版本的RGB互转的函数。
PS：在libswscale中有一点需要注意：很多的函数名称中包含类似“_c”这样的字符串，代表了该函数是C语言写的。与之对应的还有其它标记，比如“_mmx”，“sse2”等。
rgb2rgb_
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