What is the Stride Parameter in System.Drawing.Bitmap and Why Must It Be a Multiple of 4?

In-depth understanding of the "step size" in the System.Drawing.Bitmap constructor

When constructing a System.Drawing.Bitmap object, the "step" parameter represents the distance between the starting position of one row of scan lines and the starting position of the next row of scan lines in memory. It must meet a special condition: it must be a multiple of 4. This requirement stems from older CPU designs, which favored 32-bit data accesses and required scan lines to start on a 32-bit address boundary (i.e., a multiple of 4). If this alignment is not respected, there will be a performance penalty during data retrieval.

Although modern CPUs are less sensitive to address alignment, the step size is still kept as a multiple of 4 for backward compatibility. To account for situations where the image width may not exactly align with this requirement, you can use the following formula to determine the appropriate step size:

int bitsPerPixel = ((int)format & 0xff00) >> 8;
int bytesPerPixel = (bitsPerPixel + 7) / 8;
int stride = 4 * ((width * bytesPerPixel + 3) / 4);

By applying this formula, you can ensure that the step size is a multiple of 4 while using memory efficiently.

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