When you lock a surface using the Direct3DSurface8.LockRect method, the method fills in aD3DLOCKED_RECT structure that contains the pitch of the surface and a pointer to the locked bits. The value in the Pitch 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 has 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 Direct3D 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, visualize the difference between width and pitch as shown in the following illustration.

Pitch values are only useful when you are directly accessing surface memory. For example, after calling the LockRect method, the pBits member of the associated D3DLOCKED_RECT structure contains the address of the top-left pixel of the locked area of the surface, and the Pitch 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.