A fast way to read data from the framebuffer

OpenGL OpenGL: Basic Coding
1

Hi,
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++)
{
   ...
   glReadPixels(...);
   ...
}

 
for(int i = 0; i < 256; i++)
{
   ...
   glBindTexture(...);
   glCopyTexImage2D(...);
   ReadTexture(...);
   ...
}

Who can explain the reasons?
Thank you.

2

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.

3

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.

http://oss.sgi.com/projects/ogl-sample/registry/EXT/pixel_buffer_object.txt

Nico