Although the terms width and pitch are discussed casually, they have very important (and distinctly different) meanings. As a result, you should understand the meanings for each, and how to interpret the values that DirectDraw uses to describe them.
DirectDraw uses the DDSURFACEDESC2 structure to carry information describing a surface. Among other things, this structure is defined to contain information about a surface's dimensions, as well as how those dimensions are represented in memory. The structure uses the dwHeight and dwWidth members to describe the logical dimensions of the surface. Both of these members are measured in pixels. Therefore, the dwHeight and dwWidth values for a 640×480 surface are the same whether it is an 8-bit palettized surface or a 24-bit RGB surface.
The DDSURFACEDESC2 structure contains information about how a surface is represented in memory through the lPitch member. The value in the lPitch member describes the surface's memory pitch (also called stride). Pitch is the distance, in bytes, between two memory addresses that represent the beginning of one bitmap line and the beginning of the next bitmap line. Because pitch is measured in bytes rather than pixels, a 640×480×8 surface will have a very different pitch value than a surface with the same dimensions but a different pixel format. Additionally, the pitch value sometimes reflects bytes that DirectDraw has reserved as a cache, so it is not safe to assume that pitch is simply the width multiplied by the number of bytes per pixel. Rather, you could visualize the difference between width and pitch as shown in the following illustration.
In this figure, the front buffer and back buffer are both 640×480×8, and the cache is 384×480×8.
Pitch values are useful when you are directly accessing surface memory. For example, after calling the IDirectDrawSurface4::Lock method, the lpSurface member of the associated DDSURFACEDESC2 structure contains the address of the top-left pixel of the locked area of the surface, and the lPitch member is the surface pitch. You access pixels horizontally by incrementing or decrementing the surface pointer by the number of bytes per pixel, and you move up or down by adding the pitch value to, or subtracting it from, the current surface pointer.
When accessing surfaces directly, take care to stay within the memory allocated for the dimensions of the surface and stay out of any memory reserved for cache. Additionally, when you lock only a portion of a surface, you must stay within the rectangle you specify when locking the surface. Failing to follow these guidelines will have unpredictable results. When rendering directly into surface memory, always use the pitch returned by the Lock method (or the IDirectDrawSurface4::GetDC method). Do not assume a pitch based solely on the display mode. If your application works on some display adapters but looks garbled on others, this may be the cause of your problem.
For more information, see Accessing Surface Memory Directly.