OPENGL ES on iOS, glreadpixel doesn't work with BGRA format

On iOS device (I tried ipad1 and ipad2) glreadpixel works for the RGBA pixel format, but for BGRA pixel format it does not work. In开发者_C百科 the code below I use GL_IMPLEMENTATION_COLOR_READ_FORMAT_OES to get the native pixel format, which is GL_BGRA_EXT, but glgeterror returns GL_INVALID_ENUM. This contradicts the statement in the opengl doc http://www.khronos.org/opengles/sdk/1.1/docs/man/glReadPixels.xml which says GL_INVALID_ENUM is generated if format is not GL_RGBA or the value of GL_IMPLEMENTATION_COLOR_READ_FORMAT_OES.

Does anyone have any take on this?

GLint native_format;
GLint native_type;

glGetIntegerv(GL_IMPLEMENTATION_COLOR_READ_FORMAT_OES, &native_format);
glGetIntegerv(GL_IMPLEMENTATION_COLOR_READ_TYPE_OES, &native_type);

NSLog(@"native_format: 0x%04X", native_format);
NSLog(@"native_type: 0x%04X", native_type);

//screen size
CGSize s = [[CCDirector sharedDirector] winSize];
int tx = s.width;
int ty = s.height;

int bitsPerPixel=32;                

int bytesPerRow = (bitsPerPixel/8) * tx;
NSInteger myDataLength = bytesPerRow * ty;

GLubyte *buffer = malloc(sizeof(GLubyte)*myDataLength);

[target begin];
glReadPixels(0,0,tx,ty,native_format,native_type, buffer);
NSLog(@"gl get error %d", glGetError());
[target end];