OpenGL byte order for read pixels

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OpenGL represent the type and pixel format as big endian byte order,
for example GL_UNSINGED_INT_8_8_8_8, with GL_BGRA, is BGRA in BIG-endian, where
GL_UNSIGNED_INT_8_8_8_8_REV, with GL_BGRA is ARGB in BIG-endian, so, if you want to read the data and send it to other device with little-endian byte order you should:

  1. create a bitmap context with little-endian byte order
  2. read opengl buffer with type and packet format as GL_BGRA, GL_UNSIGNED_INT_8_8_8_8_REV( or xxx_REV)
  3. set opengl swap byte to 1
  4. read pixel with 2’s parameter setting

sample code:

read from frame buffer:

.... allocate memory for your buffer to contain the data read from opengl's frame buffer
void *data = calloc(bytePerRow,height);
.... create context object
GLenum packedPixelFormat = GL_UNSIGNED_SHORT_1_5_5_5_REV ;
GLenum type = GL_BGRA ;
CGLSetCurrentContext( m_pGLContextObj) ;
CGLSetFullScreen( m_pGLContextObj) ;
glReadPixels( 0, 0, width, height, type, packedPixelFormat , data);
...destory context object

bind with a Quartz bitmap context:

CGContextRef m_hdcWindow = CGBitmapContextCreate( data,m_nWindowX ,m_nWindowY, bitsPerComponent,bytesPerRow,
colorspace, kCGImageAlphaNoneSkipFirst | kCGBitmapByteOrder16Little) ;

for create and destory context object, please refer to previous post.



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