使用libjpeg.framework压缩UIImage

+(void)writeFile:(NSString *)filePath withQuality:(int)quality
{

    //初始化图片参数
    UIImage *image=[UIImage imageNamed:@"testimg.bmp"];
    JSAMPLE *image_buffer = (JSAMPLE *)[self RGBDataForImage:image];
    int image_width = image.size.width;
    int image_height= image.size.height;
    int image_components=3;
    //输出图片参数
    const char * filename=[filePath UTF8String];
    /* This struct contains the JPEG compression parameters and pointers to
     * working space (which is allocated as needed by the JPEG library).
     * It is possible to have several such structures, representing multiple
     * compression/decompression processes, in existence at once.  We refer
     * to any one struct (and its associated working data) as a "JPEG object".
     */
    struct jpeg_compress_struct cinfo;
    /* This struct represents a JPEG error handler.  It is declared separately
     * because applications often want to supply a specialized error handler
     * (see the second half of this file for an example).  But here we just
     * take the easy way out and use the standard error handler, which will
     * print a message on stderr and call exit() if compression fails.
     * Note that this struct must live as long as the main JPEG parameter
     * struct, to avoid dangling-pointer problems.
     */
    struct jpeg_error_mgr jerr;
    /* More stuff */
    FILE * outfile;        /* target file */
    JSAMPROW row_pointer[1];    /* pointer to JSAMPLE row[s] */
    int row_stride;        /* physical row width in image buffer */
    
    /* Step 1: allocate and initialize JPEG compression object */
    
    /* We have to set up the error handler first, in case the initialization
     * step fails.  (Unlikely, but it could happen if you are out of memory.)
     * This routine fills in the contents of struct jerr, and returns jerr's
     * address which we place into the link field in cinfo.
     */
    cinfo.err = jpeg_std_error(&jerr);
    /* Now we can initialize the JPEG compression object. */
    jpeg_create_compress(&cinfo);
    
    /* Step 2: specify data destination (eg, a file) */
    /* Note: steps 2 and 3 can be done in either order. */
    
    /* Here we use the library-supplied code to send compressed data to a
     * stdio stream.  You can also write your own code to do something else.
     * VERY IMPORTANT: use "b" option to fopen() if you are on a machine that
     * requires it in order to write binary files.
     */
    if ((outfile = fopen(filename, "wb")) == NULL) {
        fprintf(stderr, "can't open %s\n", filename);
        exit(1);
    }
    jpeg_stdio_dest(&cinfo, outfile);
    
    /* Step 3: set parameters for compression */
    
    /* First we supply a description of the input image.
     * Four fields of the cinfo struct must be filled in:
     */
    cinfo.image_width = image_width;     /* image width and height, in pixels */
    cinfo.image_height = image_height;
    cinfo.input_components =image_components;        /* # of color components per pixel */
    cinfo.in_color_space = JCS_RGB;     /* colorspace of input image */
    /* Now use the library's routine to set default compression parameters.
     * (You must set at least cinfo.in_color_space before calling this,
     * since the defaults depend on the source color space.)
     */
    jpeg_set_defaults(&cinfo);
    /* Now you can set any non-default parameters you wish to.
     * Here we just illustrate the use of quality (quantization table) scaling:
     */
    jpeg_set_quality(&cinfo, quality, TRUE /* limit to baseline-JPEG values */);
    
    /* Step 4: Start compressor */
    
    /* TRUE ensures that we will write a complete interchange-JPEG file.
     * Pass TRUE unless you are very sure of what you're doing.
     */
    jpeg_start_compress(&cinfo, TRUE);
    
    /* Step 5: while (scan lines remain to be written) */
    /*           jpeg_write_scanlines(...); */
    
    /* Here we use the library's state variable cinfo.next_scanline as the
     * loop counter, so that we don't have to keep track ourselves.
     * To keep things simple, we pass one scanline per call; you can pass
     * more if you wish, though.
     */
    row_stride = image_width * 3;    /* JSAMPLEs per row in image_buffer */
    
    while (cinfo.next_scanline < cinfo.image_height) {
        /* jpeg_write_scanlines expects an array of pointers to scanlines.
         * Here the array is only one element long, but you could pass
         * more than one scanline at a time if that's more convenient.
         */
        row_pointer[0] = & image_buffer[cinfo.next_scanline * row_stride];
        (void) jpeg_write_scanlines(&cinfo, row_pointer, 1);
    }
    
    /* Step 6: Finish compression */
    
    jpeg_finish_compress(&cinfo);
    /* After finish_compress, we can close the output file. */
    fclose(outfile);
    
    /* Step 7: release JPEG compression object */
    
    /* This is an important step since it will release a good deal of memory. */
    jpeg_destroy_compress(&cinfo);
}

+(unsigned char *)RGBDataForImage:(UIImage *)image
{
    // Create a pixel buffer in an easy to use format
    CGImageRef imageRef = [image CGImage];
    int width = (int)CGImageGetWidth(imageRef);
    int height = (int)CGImageGetHeight(imageRef);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    
    unsigned char *m_PixelBuf = malloc(sizeof(unsigned char) * height * width * 4);
    unsigned char *outPixel= malloc(sizeof(unsigned char) * height * width * 3);
    
    int bytesPerPixel = 4;
    int bytesPerRow = bytesPerPixel * width;
    int bitsPerComponent = 8;
    CGContextRef context = CGBitmapContextCreate(m_PixelBuf, width, height,
                                                 bitsPerComponent, bytesPerRow, colorSpace,
                                                 kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
    
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
    CGContextRelease(context);
    

    
    for (int y=0; y<height; y++)
    {
        for (int x=0; x<width; x++)
        {
            int byteIndex = (bytesPerRow * y) + x * bytesPerPixel;
            int outIndex=(3*width*y)+x*3;
            outPixel[outIndex+0]= m_PixelBuf[byteIndex+0];
            outPixel[outIndex+1]= m_PixelBuf[byteIndex+1];
            outPixel[outIndex+2]= m_PixelBuf[byteIndex+2];
        }
    }
    
    CGColorSpaceRelease(colorSpace);
    free(m_PixelBuf);
    free(outPixel);
    return outPixel;

}