Overview

OpenGL PBO

OpenGL ARB_pixel_buffer_object extension is very close toARB_vertex_buffer_object. It simply expands ARB_vertex_buffer_object extension in order to store not only vertex data but also pixel data into the buffer objects. This buffer object storing pixel data is called Pixel Buffer Object (PBO). ARB_pixel_buffer_object extension borrows all VBO framework and APIs, plus, adds 2 additional "target" tokens. These tokens assist the PBO memory manger (OpenGL driver) to determine the best location of the buffer object; system memory, AGP (shared memory) or video memory. Also, the target tokens clearly specify the bound PBO will be used in one of 2 different operations; GL_PIXEL_PACK_BUFFER_ARB to transfer pixel data to a PBO, or GL_PIXEL_UNPACK_BUFFER_ARB to transfer pixel data from PBO.

For example, glReadPixels() and glGetTexImage() are "pack" pixel operations, and glDrawPixels(), glTexImage2D() and glTexSubImage2D() are "unpack" operations. When a PBO is bound with GL_PIXEL_PACK_BUFFER_ARB token, glReadPixels() reads pixel data from a OpenGL framebuffer and write (pack) the data into the PBO. When a PBO is bound with GL_PIXEL_UNPACK_BUFFER_ARB token, glDrawPixels() reads (unpack) pixel data from the PBO and copy them to OpenGL framebuffer.

The main advantage of PBO are fast pixel data transfer to and from a graphics card through DMA (Direct Memory Access) without involing CPU cycles. And, the other advantage of PBO is asynchronous DMA transfer. Let's compare a conventional texture transfer method with using a Pixel Buffer Object. The left side of the following diagram is a conventional way to load texture data from an image source (image file or video stream). The source is first loaded into system memory, and then, copied from system memory to an OpenGL texture object with glTexImage2D(). These 2 transfer processes (load and copy) are all performed by CPU.

 
Texture loading without PBO

Texture loading with PBO

On the contrary in the right side diagram, the image source can be directly loaded into a PBO, which is controlled by OpenGL. CPU still involves to load the source to the PBO, but, not for transferring the pixel data from a PBO to a texture object. Instead, GPU (OpenGL driver) manages copying data from a PBO to a texture object. This means OpenGL performs a DMA transfer operation without wasting CPU cycles. Further, OpenGL can schedule an asynchronous DMA transfer for later execution. Therefore, glTexImage2D() returns immediately, and CPU can perform something else without waiting the pixel transfer is done.

There are 2 major PBO approaches to improve the performance of the pixel data transfer: streaming texture update and asynchronous read-back from the framebuffer.