13.1.1 The Pixel Transfer Pipeline

 

The pixel transfer pipeline is the part of OpenGL most typically thought of in image processing applications. The pipeline is a configurable series of operations which are applied to each pixel during any command that moves pixels between the framebuffer, host memory, and texture memory, including:

• glDrawPixels()
• glReadPixels()
• glTexImage*D()
• glTexSubImage*D()
• glGetTexImage*D()
• glCopyPixels()
• glCopyTexImage*D()
• glCopyTexSubImage*D()

These operations move image data which falls into one of the following categories:

• Color index values
• Color values (RGBA, luminance, luminance/alpha, red, green, ...)
• Stencil buffer values
• Depth values

The ``pixel transfer pipeline'' processes each of these categories of data differently. For image processing, operations on color data are generally the most interesting. Before any operations are applied, source data in any color format (for example, GL_LUMINANCE) and type (for example, GL_UNSIGNED_BYTE) is converted into floating-point RGBA components. All color pixel transfer operations operate on images of this type and format. After the pixel transfer operations have been applied, the image is converted to its destination type and format.

Base OpenGL defines only a few pixel transfer operations, which are controlled using the glPixelTransfer() command. The operations are:

• GL_INDEX_SHIFT and GL_INDEX_OFFSET, which are applied only to color index images.
• Scale and bias values which are applied to each channel of RGBA images.
• Scale and bias values which are applied to depth values.
• Pixel maps, discussed in detail in Section 13.2.3.

The pixel transfer pipeline is the part of OpenGL that has grown the most through OpenGL extensions. Some of the more interesting extensions will be discussed in this section, including the vendors who support each extension in OpenGL 1.1 as of April 1998. Where possible, we will mention techniques to achieve equivalent results on systems that do not support the extension.