[ft-devel] [03/08] gamma correction issues with FreeType

[Werner LEMBERG]

>From Dave, replying to Octoploid:

  I agree that the text looks better with the reduced amount of
  darkening.

  I am able to make some conclusions from the screenshots. The reason
  your text is too dark is because your system is blending
  anti-aliased pixels in a non-linear color space.  To get proper
  results, you need to blend in a linear color space, or at least a
  reasonable approximation to a linear color space.  This is called
  linear blending, or linearized blending or gamma-corrected blending.

  This is consistent with your observation that you've noticed this on
  other fonts, too.  It is not a font problem, it is a system problem.
  And unfortunately, it is a common problem.  Linearized blending adds
  some complication and performance cost.  But it is especially
  important for anti-aliased text.

  I've attached an image that may help explain how this affects your
  text.  My screenshot shows a magnified image of "deal" from your
  screenshot.  Compare the vertical stems of 'd' and 'l'.  In most
  font designs, these stems should have the same width.  They do not
  have the same pixel alignment, but they should have the same
  apparent weight.  If I calculate the pixel values using gamma 1.0
  then I get equal values for the two stems.  That tells me the
  calculation is not using linearized blending.  This results in gray
  pixel values that are too dark.  If you view the results an a screen
  that is closer to the typical gamma of 2, then the 'l' appears much
  bolder than the 'd' because the two gray pixels add more weight than
  they should.