conversion/dithering.html

<html><head><title>High-speed Conversion of Floating Point Images to
8-bit</title></head><body bgcolor=black text=#c0c0c0 link=#80c0c0 vlink=#ffc0c0>

<h1>Dithering</h1>

The other thing my algorithim does is dithering. Dithering adds noise
to the floating point result before converting so that the resulting
image averages more accurately to the actual brightness.

<p>Here is a straight-forward result of converting a linear ramp to
the nearest sRGB byte value. In these examples I have exaggerated the
error by only using 32 levels instead of 256:

<p><img src=pics/nodither.gif>

<p>And here is the output of my algorithm (again if it is used to
produce only 32 levels instead of 256). It is a lot smoother, and in
the center you can see the dithering pattern:

<p><img src=pics/dither.gif>

<p>Dithering is done by error diffusion: the error in luminosity of
the output sRGB byte is added to the next pixel, so they all average
out to the correct luminosity. The diffusion is done in the sRGB
space, even though this is less accurate than in the linear space. The
reason I do it in the sRGB space is partly that a much more efficient
algorithim can be used and also that the linear one can introduce
large enough changes that some byte values are skipped completely,
resulting in less smooth patterns. Also at the scale of dithering (3
adjacent byte values) the sRGB plot is quite close to a straight line.