glReadPixels读取显存数据并保存图像
glReadPixels: 如果你想将渲染的结果保存下来，你可以使用glReadPixels将图像内容从现存读取到内存中，需要注意：仅限于读取Color Buffer，无法读取Depth Buffer和Stencil Buffer，当调用glReadPixels时，可以将Color Buffer中的像素值保存到预分配的内存缓冲区中。
函数定义：
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
void glReadPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, GLvoid *pixels)
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
x, y： specify the viewport coordinates of the lower-left corner of the pixel rectangle read from the color buffer.
width，height： specify the dimensions of the pixel rectangle read from the color buffer.
format： specifies the pixel format that you would like returned. Three formats are available: GL_RGBA, GL_RGBA_INTEGER, and the value returned by querying GL_IMPLEMENTATION_COLOR_READ_FORMAT, which is an implementation-specific pixel format.
type： specifies the data type of the pixels returned. Five types are available: GL_UNSIGNED_BYTE, GL_UNSIGNED_INT, GL_INT, GL_FLOAT, and the value returned from querying GL_IMPLEMENTATION_COLOR_READ_TYPE, which is an implementation-specific pixel type. pixels: a contiguous array of bytes that contain the values read from the color buffer after glReadPixels returns.
下面分几种情况讲解glReadPixels的使用方式：
1. 从GL_TEXTURE_2D中读取：
当从GL_TEXTURE_2D读取时，因为有可能当前Context并没有FrameBuffer绑定，或者FrameBuffer绑定的并不是我们需要读取的Texture，所以最好的方式是创建一个临时FrameBuffer，在临时的FrameBuffer上绑定要读取的Texture，并在使用临时FrameBuffer之前，查询当前绑定的FrameBuffer，并在读取完成后恢复当前绑定的FrameBuffer。
GLuint Tmp_Framebuffer = 0;
glGenFramebuffers ( 1, &TMP_Framebuffer );
/******* IN Rendering LOOP Begin**************/
GLint defaultFramebuffer = 0;
glGetIntegerv ( GL_FRAMEBUFFER_BINDING, &defaultFramebuffer );
glBindFramebuffer ( GL_FRAMEBUFFER, G_Framebuffer );
// textureId: 要读取的TextureID
glFramebufferTexture2D ( GL_READ_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D,
textureId, 0 );
//默认读取GL_COLOR_ATTACHMENT0，此处可以不设置
glReadBuffer ( GL_COLOR_ATTACHMENT0 );
///////////////////////////////////////////////////////////////////
////////////调用glReadPixels读取现存////////////////////////////////
///////////////////////////////////////////////////////////////////
glBindFramebuffer ( GL_FRAMEBUFFER, defaultFramebuffer );
/******* IN Rendering LOOP End **************/2. 从GL_TEXTURE_2D_ARRAY中读取：
与1中的实现相同，唯一的区别是在临时FrameBuffer绑定GL_TEXTURE_2D_ARRAY的接口有所区别，需要使用glFramebufferTextureLayer( GL_READ_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, texture, 0, 0 );
3. 从帧缓冲区中读取，WindowSurface和PBufferSurface
不需要做特殊操作，直接读取就可以读取eglMakeCurrent时设置的readSurface中的内容。
完整代码如下：
#define FILENAME_MAX 64
#define CAMERA_DUMP_FRM_LOCATION "/sdcard/"
void dumpImage(unsigned char* imagedata,
uint32_t frameid, uint32_t width, uint32_t height, uint32_t bytesPerPixel) {
LOG("20191108: %s,%d E .", __FUNCTION__, __LINE__);
char buf[FILENAME_MAX];
memset(buf, 0, sizeof(buf));
snprintf(buf, sizeof(buf), CAMERA_DUMP_FRM_LOCATION "frame_%dX%d_%d.rgb",
width,height,frameid);
LOG("20191108: %s,%d buf : %s", __FUNCTION__,__LINE__,buf);
int file_fd = open(buf, O_RDWR | O_CREAT, 0644);
uint32_t ImageSize = width*height*bytesPerPixel;
if (file_fd >= 0)
{
ssize_t written_image = write(file_fd, imagedata, ImageSize);
LOG("20191108: %s,%d : written number of %zd bytes to file %s", __FUNCTION__, __LINE__, ImageSize, buf);
close(file_fd);
}
else
{
LOG("20191108: %s,%d : failed to write data to %s", __FUNCTION__, __LINE__, buf);
}
LOG("20191108: %s,%d X .", __FUNCTION__, __LINE__);
}
GLuint G_Framebuffer = 0;
glGenFramebuffers ( 1, &G_Framebuffer );
/********************* IN Rendering LOOP Begin **************************/
//glDrawBuffer(GL_NONE);
//glReadBuffer(GL_NONE);
//glFramebufferTextureLayer(
// GL_READ_FRAMEBUFFER,GL_COLOR_ATTACHMENT0,
// texture,0,0 // Use layer 0 for subsequent operations
//);
GLint defaultFramebuffer = 0;
glGetIntegerv ( GL_FRAMEBUFFER_BINDING, &defaultFramebuffer );
static int frameid = 0;
if((frameid++ % 300) == 0)
{
glBindFramebuffer ( GL_FRAMEBUFFER, G_Framebuffer );
glFramebufferTexture2D ( GL_READ_FRAMEBUFFER,
GL_COLOR_ATTACHMENT0,
GL_TEXTURE_2D,
textureId, 0 );
int width = my_GetIntConfig(TEXTURE_WIDTH);
int height = my_GetIntConfig(TEXTURE_HEIGHT);
glReadBuffer ( GL_COLOR_ATTACHMENT0 );
GLint readType, readFormat;
GLubyte *pixels;
glGetIntegerv ( GL_IMPLEMENTATION_COLOR_READ_TYPE, &readType );
glGetIntegerv ( GL_IMPLEMENTATION_COLOR_READ_FORMAT, &readFormat );
unsigned int bytesPerPixel = 0;
switch ( readType )
{
case GL_UNSIGNED_BYTE:
case GL_BYTE:
switch ( readFormat )
{
case GL_RGBA:
bytesPerPixel = 4;
break;
case GL_RGB:
case GL_RGB_INTEGER:
bytesPerPixel = 3;
break;
case GL_RG:
case GL_RG_INTEGER:
case GL_LUMINANCE_ALPHA:
bytesPerPixel = 2;
break;
case GL_RED:
case GL_RED_INTEGER:
case GL_ALPHA:
case GL_LUMINANCE:
bytesPerPixel = 1;
break;
default:
// Undetected format/error
break;
}
break;
case GL_FLOAT:
case GL_UNSIGNED_INT:
case GL_INT:
switch ( readFormat )
{
case GL_RGBA:
case GL_RGBA_INTEGER:
bytesPerPixel = 16;
break;
case GL_RGB:
case GL_RGB_INTEGER:
bytesPerPixel = 12;
break;
case GL_RG:
case GL_RG_INTEGER:
bytesPerPixel = 8;
break;
case GL_RED:
case GL_RED_INTEGER:
case GL_DEPTH_COMPONENT:
bytesPerPixel = 4;
break;
default:
// Undetected format/error
break;
}
break;
case GL_HALF_FLOAT:
case GL_UNSIGNED_SHORT:
case GL_SHORT:
switch ( readFormat )
{
case GL_RGBA:
case GL_RGBA_INTEGER:
bytesPerPixel = 8;
break;
case GL_RGB:
case GL_RGB_INTEGER:
bytesPerPixel = 6;
break;
case GL_RG:
case GL_RG_INTEGER:
bytesPerPixel = 4;
break;
case GL_RED:
case GL_RED_INTEGER:
bytesPerPixel = 2;
break;
default:
// Undetected format/error
break;
}
break;
case GL_FLOAT_32_UNSIGNED_INT_24_8_REV: // GL_DEPTH_STENCIL
bytesPerPixel = 8;
break;
// GL_RGBA, GL_RGBA_INTEGER format
case GL_UNSIGNED_INT_2_10_10_10_REV:
case GL_UNSIGNED_INT_10F_11F_11F_REV: // GL_RGB format
case GL_UNSIGNED_INT_5_9_9_9_REV: // GL_RGB format
case GL_UNSIGNED_INT_24_8: // GL_DEPTH_STENCIL format
bytesPerPixel = 4;
break;
case GL_UNSIGNED_SHORT_4_4_4_4: // GL_RGBA format
case GL_UNSIGNED_SHORT_5_5_5_1: // GL_RGBA format
case GL_UNSIGNED_SHORT_5_6_5: // GL_RGB format
bytesPerPixel = 2;
break;
default:
break;
// Undetected type/error
}
pixels = ( GLubyte* ) malloc( width * height * bytesPerPixel );
glReadPixels ( 0, 0, width, height, readFormat, readType, pixels );
dumpImage(pixels, frameid, width, height, bytesPerPixel);
}
glBindFramebuffer ( GL_FRAMEBUFFER, defaultFramebuffer );
/********************* IN Rendering LOOP End **************************/