Re: CILinearGradient not linear?
Re: CILinearGradient not linear?
- Subject: Re: CILinearGradient not linear?
- From: Christian Walther <email@hidden>
- Date: Tue, 30 May 2006 10:29:11 +0200
George Orthwein wrote:
The CTGradient looks almost identical to the Photoshop gradient but
the Core Image gradient differs quite a bit... showing more lighter
tones and then dropping to black very quickly at the end. The
midpoint is 32% black instead of 50%.
A wild guess without any knowledge about Core Image whatsoever (and
without having tried out your suggestions):
It seems that both gradients are actually "linear", they're just
linear to different quantities.
The CT and Photoshop gradients are probably linear in the pixel
values they send to video hardware - 0 for black, 255 for white, and
128 for the middle gray in the center. Incidentally, this is also
approximately linear in perceived brightness.
The CI gradient is probably linear in actual physical light
intensity. Try comparing this gradient with a checkerboard pattern of
white and black pixels (if you're on an LCD, on a CRT use alternating
white and black horizontal lines), you'll see that it approximately
matches the brightness in the middle of the gradient.
On a properly calibrated Mac display, the relationship is that
light_intensity = pixel_value ^ 1.8, with both quantities measured
from 0 to 1. In your example: 0.5 = 0.68 ^ 1.8, with 0.68 being the
complement of your "32% black".
If this sounds all greek to you, read up on "gamma", especially
<http://www.poynton.com/notes/colour_and_gamma/GammaFAQ.html>, or on
colorimetry in general. It's a wide and not completely trivial field,
but understanding it helps a lot if you're seriously interested in
computer graphics.
-Christian
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