View Full Version : Problem adjusting curves (contrast/brightness) on fragment shader

07-17-2007, 11:38 AM
I have some data in the range from roughly -3000 to 3000. What I want to do is load it into texture and display it applying some transformations to it (right now it's just a linear mapping).

I think all textures that you load are scaled and converted to [0..1] by openGL. I'm going by this diagram I found after digging through the specs: spec 1.1 (http://www.opengl.org/documentation/specs/version1.1/glspec1.1/node30.html#colorconvert) So scale my data to [0..1] myself and convert it into float myself before passing it as texture.

The problem is I seem to be loosing precision or level of detail as pixels with values that are close to each other get painted with the same value. If I decrease the range by which I scale my data to say -1000 to 1000 (clipping the rest) things seem better. However I thought float should have enough precision to handle a wider range.

Here's my vertex shader:

void main()
gl_TexCoord[0] = gl_MultiTexCoord0;
gl_Position = ftransform();
} Here's the fragment shader:

uniform sampler3D tex;
uniform float center;
uniform float window;

/* Texture is expected to be stored as luminance which means L goes to RGB values */
void main()
vec4 color =texture3D(tex,gl_TexCoord[0].stp);
//map colours to line y = (1.0/window)(x - x0)
//x0 = center - window/2
float x0 = center - window/2.0;
float maxX = window + x0;
float lum = color.r;
if (lum <= x0)
lum = 0.0;
else if (lum >= maxX)
lum = 1.0;
lum = (1.0/window) * ( lum - x0);
gl_FragColor = vec4(lum,lum,lum,1.0);
}Thanks for input on the issue.

07-17-2007, 12:22 PM
Never mind, found my bug. I specified internal format as GL_LUMINANCE. Changed to GL_LUMINANCE16 and all is well now.

So as I understand openGL does 2 mappings on the colour data? First is to go from your data to a float array clamped to [0..1] and then to internal format from [0..1] to whatever format?