OpenGL Programming Guide

Chapter 8Drawing Pixels, Bitmaps, Fonts, and Images

Storing, Transforming, and Mapping Pixels

Overview of the Pixel Modes

An image stored in memory has between one and four chunks of data, called elements, for each pixel in a rectangular portion of the screen. The data might consist of just the color index or the luminance (luminance is the possibly weighted sum of the red, green, and blue values), or it might consist of the red, green, blue, and alpha components for each pixel. The possible arrangements of pixel data, or formats, determine the number of elements stored for each pixel and their order.

Some elements (such as a color index or a stencil index) are integers, and others (such as the red, green, blue, and alpha components, or the depth component) are floating-point values, typically ranging between 0.0 and 1.0. Floating-point components are usually stored in your bitplanes in fixed-point with lower resolution than a full floating-point number would require (typically 8 bits are used for color components, for example). The exact number of bits used to represent the components depends on the particular hardware being used. Thus, it's often wasteful to store each component as a full 32-bit floating-point number, especially since images can easily contain a million pixels.

Elements can be stored in memory as various data types, ranging from 8-bit bytes to 32-bit integers or floating-point numbers. OpenGL explicitly defines the conversion of each component in each format to each of the possible data types. Keep in mind that you can lose data if you try to store a high-resolution component in a type represented by a small number of bits.

Image data is typically stored in processor memory in rectangular two- or three-dimensional arrays. Often, you want to display or store a subimage that corresponds to a subrectangle of the array. In addition, you might need to take into account that different machines have different byte-ordering conventions. Finally, some machines have hardware that is far more efficient at moving data to and from the framebuffer if the data is aligned on two-byte, four-byte, or eight-byte boundaries in processor memory. For such machines, you probably want to control the byte alignment. All the issues raised in this paragraph are controlled as pixel-storage modes; you specify these modes using the command glPixelStore*(), which you've seen used in a couple of example programs.

As image data is transferred from memory into the framebuffer, or from the framebuffer into memory, OpenGL can perform several operations on it. For example, the ranges of components can be altered - normally, the red component is between 0.0 and 1.0, but you might prefer to keep it in some other range, or perhaps the data you're using from a different graphics system stores the red component in a different range. You can even create maps to perform arbitrary conversion of color indices or color components during pixel transfer. Conversions such as these performed during the transfer of pixels to and from the framebuffer are called pixel-transfer modes. Not too surprisingly, they're controlled with the glPixelTransfer*() and glPixelMap*() commands.

Other modes that can be controlled include the framebuffer from which pixels are read, and any magnification that's to be performed on pixels as they are written to the framebuffer.

Finally, be aware that although the color, depth, and stencil buffers have many similarities, they don't behave identically, and a few of the modes have special cases for special buffers. All the mode details are covered in the sections that follow, including all the special cases.

OpenGL Programming Guide