Why Must Bitmap Stride in C# Be a Multiple of 4?

C# Bitmap Constructor: Understanding the Stride Parameter

The System.Drawing.Bitmap constructor in C# offers fine-grained control over image creation. A key parameter, "stride," significantly impacts bitmap handling, but its 4-byte multiple constraint often puzzles developers.

The Reason Behind the 4-Byte Multiple Requirement

This restriction originates from older CPU architectures. For efficient bitmap processing, these CPUs favored 32-bit (4-byte) aligned memory reads. Each scanline (pixel row) needed to start at a memory address divisible by 4. This alignment optimized performance by reducing memory access operations.

Relevance in Modern Systems

Though modern CPUs are less sensitive to memory alignment, maintaining stride as a multiple of 4 ensures backward compatibility with legacy applications. Therefore, even in contemporary projects, this rule remains crucial.

Stride Calculation

Calculating the correct stride, especially for less common image formats, can be challenging. The following formula provides a reliable method:

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

Key Takeaway

Understanding the stride parameter in the System.Drawing.Bitmap constructor is vital for effective bitmap manipulation. By ensuring it's a multiple of 4, developers guarantee efficient code execution across various hardware generations.

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