Why must “stride” in the System.Drawing.Bitmap constructor be a multiple of 4?

by

This goes back to early CPU designs. The fastest way to crunch through the bits of the bitmap is by reading them 32-bits at a time, starting at the start of a scan line. That works best when the first byte of the scan line is aligned on a 32-bit address boundary. In other words, an address that’s a multiple of 4. On early CPUs, having that first byte mis-aligned would cost extra CPU cycles to read two 32-bit words from RAM and shuffle the bytes to create the 32-bit value. Ensuring each scan line starts at an aligned address (automatic if the stride is a multiple of 4) avoids that.

This isn’t a real concern anymore on modern CPUs, now alignment to the cache line boundary is much more important. Nevertheless, the multiple of 4 requirement for stride stuck around for appcompat reasons.

Btw, you can easily calculate the stride from the format and width with this:

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

More Related Contents:

  1. Property asking different amounts of constructors
  2. What is the best way to give a C# auto-property an initial value?
  3. Calling the base constructor in C#
  4. Load a WPF BitmapImage from a System.Drawing.Bitmap
  5. Can constructors be async?
  6. C# constructor execution order
  7. Fast work with Bitmaps in C#
  8. Is there a generic constructor with parameter constraint in C#?
  9. fast converting Bitmap to BitmapSource wpf
  10. C# “Parameter is not valid.” creating new bitmap
  11. Converting BitmapImage to Bitmap and vice versa
  12. When is a custom attribute’s constructor run?
  13. What’s the difference between Bitmap.Clone() and new Bitmap(Bitmap)?
  14. C# inheritance and default constructors
  15. What is the fastest way I can compare two equal-size bitmaps to determine whether they are identical?
  16. Why does the c# compiler emit Activator.CreateInstance when calling new in with a generic type with a new() constraint?
  17. How to save PictureBox.Image to file?
  18. .Net and Bitmap not automatically disposed by GC when there is no memory left
  19. Why can’t I create an abstract constructor on an abstract C# class?
  20. Combine two Images into one new Image
  21. Why are constructors not inherited in C#?
  22. C# static class constructor
  23. C# – Loading an indexed color image file correctly
  24. Is it possible to override a constructor in C#?
  25. C# constructors overloading
  26. How to instantiate an object with a private constructor in C#?
  27. C# – Convert WPF Image.source to a System.Drawing.Bitmap
  28. Drawing control to memory (Bitmap)
  29. How can I read image pixels’ values as RGB into 2d array?
  30. a constructor as a delegate – is it possible in C#?

Leave a Comment