A fast way to read data from the framebuffer

I found a fast way to read data from the framebuffer. It’s faster than read data using glReadPixels directly.
First we draw the geometry into the buffer. Then a texture is binded. At last using the following code to read the data from the texture:

void ReadTexture(GLenum target, GLuint texture, float* pImage, GLenum format = GL_RGB, 
GLint level = 0, char *argstring = NULL)
	int w, h;
	int prevTexBind;
	glGetIntegerv(target, &prevTexBind);
	glBindTexture(target, texture);
	glGetTexLevelParameteriv(target, 0, GL_TEXTURE_WIDTH, &w);
	glGetTexLevelParameteriv(target, 0, GL_TEXTURE_HEIGHT, &h);
	glGetTexImage(target, level, format, GL_FLOAT, pImage);
	std::string s = (NULL == argstring) ? format2string(format) : argstring;
        glBindTexture(target, prevTexBind);

I don’t know why it’s faster than glReadPixels, but I tested it using the following codes.

for(int i = 0; i < 256; i++)
for(int i = 0; i < 256; i++)

Who can explain the reasons?
Thank you.

I have not tested or benchmarked it, but the same approach has been suggested by John Stauffer, Eng Mgr of Apple’s GL team. So it must be fast on some hardware.

IIRC readpixels stops any opengl processing until it’s finished.

Did you try the asynchronous readpixels with PBO?
You’ll need the GL_EXT_pixel_buffer_object extension though.