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

System.Drawing.Bitmap's Stride Parameter: A Deep Dive

The stride parameter in the System.Drawing.Bitmap constructor often causes confusion. This article explains why it must be a multiple of 4.

This requirement stems from older CPU architectures. For optimal performance, these CPUs processed bitmap data in 32-bit chunks. Alignment of each scan line's first byte to a 32-bit boundary (a multiple of 4) was crucial. Any misalignment necessitated extra CPU cycles for data reorganization.

Although modern CPUs favor cache line alignment, the multiple-of-4 stride constraint remains for backward compatibility.

Stride Calculation

The correct stride is calculated as follows:

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

Substitute the image's format and width to obtain the appropriate stride.

In Summary

Understanding the historical context of the stride restriction is key to efficiently using the System.Drawing.Bitmap constructor. A multiple-of-4 stride ensures compatibility and performance optimization across various architectures.

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