Why does anti-aliased text have slightly colored edges?

Discussion in 'Design and Graphics' started by Let's Sekuhara!, Feb 20, 2014.

  1. Let's Sekuhara! macrumors 6502

    Let's Sekuhara!

    Jun 30, 2008
    I've been curious for a long time and finally decided I have to know:
    Why does OS X (and possibly other platforms) add color data when anti-aliasing text?

    Screen shot of anti-aliased text after magnification:

    It doesn't bother me or anything. It's just I feel like I'm missing some possibly important knowledge about how digital graphics are processed and why coloring is desirable.

  2. slayerizer macrumors 6502a

    Nov 9, 2012
  3. gnasher729 macrumors P6


    Nov 25, 2005
    You most likely have an RGB display. When you think you see a square pixel on the screen, it is really three sub pixels, each a vertical stripe coloured Red, Green or Blue. If all three sub pixels (Red, Green and Blue) are turned off, you see a black pixel.

    Now if you look at the pixel to the left of it, it also has three sub pixels. If you turn off the Blue sub pixel in the left pixel, and the Red and Green sub pixels in the right pixel, you have three sub pixels turned off, and again you see a black pixel. But that sub pixel is positioned one third of a pixel to the right. So by playing with the colours, you can position black pixels horizontally with 1/3rd pixel resolution.

    You can't do that vertically, because the sub pixels are vertical stripes. You can't create black pixels that are 2/3rds or 4/3rds of a pixel wide, because if you turned off for example a red/green/blue/red sub pixel, you wouldn't see black but a bit of colour.

    And it doesn't work if you take the image and blow it up to a bigger size.

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