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Ping-pong technique on GPU

Hello there. Here is a new tutorial, this time about ping-pong on the gpu. I’ve been wanting to write about it for sometime, finally it’s off my todo list. Let’s get down to business. Ping-pong technique is normally used with a shader that needs it’s result as a source parameter for it’s next iteration. This is usually used in the gpu as for now it is not possible to write a program’s result to itself, so, we’ll need another equal buffer to save the current result for how next step/iteration. That was too hard?

So imagine we have 2 image buffers Image1, Image2. Usually to change data from Image1, you would simply access it and write directly back to the same positioion. Now, when we’re talking about a shader fragment program we can’t simply do that. You may ask now, what’s the solution? Ping-pong it!

Here is what i’m talking about:

//
// Initialization
//
int W = 100;
int H = 100;
ImageArray = new int[2][W*H];
int CurrActiveBuffer = 0; // Current active buffer index
 
 
//
// Mainloop
//
for( int j=0; j<H; j++ )
{
	for( int i=0; i<W; i++ )
	{
		// ERROR! Write to same buffer. Not possible in gpu shader
		//Image1[i+j*W] = Image1[i+j*W] * 2;  // Mul by 2
 
		// Ping-pong version
		int src = CurrActiveBuffer; // Current active buffer (Input)
		int dest = 1-CurrActiveBuffer;  // Back buffer (Output)
		ImageArray[dest][i+j*W] = ImageArray[src][i+j*W] * 2;  // Mul by 2
	}
}
 
// Swap back and front buffers (read becomes write and vice-versa)
CurrActiveBuffer = 1-CurrActiveBuffer;    // CurrActiveBuffer ? 0 : 1;

As you can see we start with buffer 0. That’s where we get out data from (read) and write it to Buffer 1. Once the operation is done, we swap buffers every frame. This way we’ll be able to use last iteration’s data as input for the next iteration.

Now let’s put this in OpenGL language. I will be using a FrameBufferObject (FBO) and 2 textures here. The framebuffer will be holding to both textures as 2 Color Attachments. So let’s get coding:

//
// Initialization
//
int W = 100;
int H = 100;
int FboID;
int TexID[2];
int CurrActiveBuffer = 0;  // Current active buffer index
 
// Create the frambuffer
glGenFramebuffersEXT( 1, &FboID );
glBindFramebufferEXT( GL_FRAMEBUFFER_EXT, FboID );
 
// Create 2 textures for input/output.
glGenTextures( 2, TexID );
 
for( int i=0; i<2; i++ )
{
	glBindTexture( GL_TEXTURE_2D, TexID[i] );
	glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR );
	glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );
	glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE );
	glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE );
        // RGBA8 buffer
	glTexImage2D( GL_TEXTURE_2D, 0, GL_RGBA, W, H, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL );
	if( _hasMipmapping )  glGenerateMipmapEXT( GL_TEXTURE_2D );
}
 
// Now attach textures to FBO
int src = CurrActiveBuffer;
int dest = 1-CurrActiveBuffer;
glFramebufferTexture2DEXT( GL_FRAMEBUFFER_EXT, 
                           GL_COLOR_ATTACHMENT0_EXT, 
                           GL_TEXTURE_2D, TexID[src], 0 );
glFramebufferTexture2DEXT( GL_FRAMEBUFFER_EXT, 
                           GL_COLOR_ATTACHMENT1_EXT, 
                           GL_TEXTURE_2D, TexID[dest], 0 );
);
 
 
 
//
// Mainloop
//
int src = CurrActiveBuffer;
int dest = 1-CurrActiveBuffer;
 
FBO.Bind();
 
glDrawBuffer( dest );
 
glBindTexture( GL_TEXTURE_2D, TexID[src] );
ShaderProgram.SetTextureUniform( 0 );
RenderScene();
 
FBO.Unbind();
 
 
// Swap back and front buffers (read becomes write and vice-versa)
CurrActiveBuffer = 1-CurrActiveBuffer;    // CurrActiveBuffer ? 0 : 1;

Why would you want to ping-pong? Well for instance imagine you are doing a water effect in the gpu. You’ll need to access data from your previous buffer right? Using the CPU that would be trivial as you have the last frame’s memory buffer allocated somewhere which you can read/write access directly. That doesn’t really work on the gpu side that way (but we’re getting closer with CL, CUDA, DC)

Have fun.


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