Question

This is a follow-up to this question.

I am working on a low level C app where I have to draw text. I have decided to store the font I want to use as an array (black and white, each char 128×256, perhaps), then I’d downscale it to the sizes I need with some algorithm (as grayscale, so I can have some crude font smoothing).

Note: this is a toy project, please disregard stuff like doing calculations at runtime or not.

Question is, which algorithm?

I looked up 2xSaI, but it’s rather complicated. I’d like something I can read the description for and work out the code myself (I am a beginner and have been coding in C/C++ for just under a year).

Suggestions, anyone?

Thanks for your time!

Edit: Please note, the input is B&W, the output should be smoothed grayscale

Answer 1

Figure out the rectangle in the source image that will correspond to a destination pixel. For example if your source image is 50×100 and your destination is 20×40, the upper left pixel in the destination corresponds to the rectangle from (0,0) to (2.2,2.2) in the source image. Now, do an area-average over those pixels:

• Area is 2.2 * 2.2 = 4.84. You’ll scale the result by 1/4.84.
• Pixels at (0,0), (0,1), (1,0), and (1,1) each weigh in at 1 unit.
• Pixels at (0,2), (1,2), (2,0), and (2,1) each weigh in at 0.2 unit (because the rectangle only covers 20% of them).
• The pixel at (2,2) weighs in at 0.04 (because the rectangle only covers 4% of it).
• The total weight is of course 4*1 + 4*0.2 + 0.04 = 4.84.

This one was easy because you started with source and destination pixels lined up evenly at the edge of the image. In general, you’ll have partial coverage at all 4 sides/4 corners of the sliding rectangle.

Don’t bother with algorithms other than area-averaging for downscaling. Most of them are plain wrong (they result in horrible aliasing, at least with a factor smaller than 1/2) and the ones that aren’t plain wrong are a good bit more painful to implement and probably won’t give you better results.