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

Understanding the Stride Requirement in System.Drawing.Bitmap

The stride parameter within the System.Drawing.Bitmap constructor necessitates being a multiple of 4. This constraint originates from the architectural design of older CPUs.

Early CPU architectures optimized bitmap processing by reading pixels in 32-bit chunks, ensuring alignment with the beginning of each scanline. This alignment, a multiple of 4 bytes, was crucial for performance. Any misalignment resulted in significant performance penalties.

Although modern CPUs are less sensitive to this alignment issue, maintaining the stride-as-multiple-of-4 requirement ensures backward compatibility.

Calculating the Correct Stride

To prevent problems with incorrectly sized strides, dynamically calculate the necessary stride based on the image's format and width:

<code class="language-csharp">int bitsPerPixel = ((int)format & 0xff00) >> 8;
int bytesPerPixel = (bitsPerPixel + 7) / 8;
int stride = 4 * ((width * bytesPerPixel + 3) / 4);</code>

This calculation guarantees a stride that's a multiple of 4, ensuring optimal compatibility and performance across various CPU architectures.

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