OpenGL: Inaccurate Color?
I am trying to follow the directions on this page:
http://www.opengl.org/resources/faq/technical/color.htm
regarding rendering primitives with unique color
I checked the number of bits for each color and the result was 8 for each.
When called:
glColor3ui(0x80000000, 0, 0xFF000000);
and reading the pixel with glReadPixels () I get the color: 0xFFFE007F
which matches R=0x7F, G=0, B=0xFE
The bottom two bits of red and blue are wrong.
Why is this? I am using a brand new nVidia card on a Dell laptop with the most up to date drivers.
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It turned out that the FAQ is common.
glColor documentation
that: " Unsigned integer color components, if specified, map linearly to floating point values, so that the largest displayed value displays 1.0 (full intensity) and 0 maps to 0.0 (zero intensity)."
This actually suggests that in order to get the full white intensity I have to call:
glColor3ui(0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF);
but not
glColor3ui(0xFF000000, 0xFF000000, 0xFF000000);
as frequently asked questions.
And that explains why 0xFF000000 maps to 254.
I have filed a bug report for people presumably maintaining the FAQ
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Did you do that too?
In either event, you'll need to ensure that any state that could
affect the final color has been disabled. The following code will
accomplish this:
glDisable (GL_BLEND); glDisable (GL_DITHER);
glDisable (GL_FOG); glDisable (GL_LIGHTING);
glDisable (GL_TEXTURE_1D); glDisable (GL_TEXTURE_2D);
glDisable (GL_TEXTURE_3D); glShadeModel (GL_FLAT);
Also check if the glReadPixels buffer is 24/32 bit.
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