Re: Difficulty with the shaders tutorial

You might want to have a look at:

gl.glPixelTransferf(gl.GL_ALPHA_SCALE, scale)
gl.glPixelTransferf(gl.GL_ALPHA_BIAS, bias)


This will control how your texture is mapped from CPU to GPU memory.

Have a look at http://code.google.com/p/glumpy/source/browse/glumpy/image/texture.py, this is exactly what is done.



Nicolas



On Feb 21, 2012, at 20:10 , Derakon wrote:

> On Fri, Feb 17, 2012 at 1:44 PM, Derakon <[email protected]> wrote:
>> 
>> That did it! I have working false-color now; all that remains is to
>> integrate it into the rest of my program.
> 
> I've run into some implementation issues now that I'm not dealing with
> idealized data. My image data is using the GL_SHORT datatype (16-bit
> int). It can actually need anywhere from the full range to only 8 bits
> or so, and needs to display properly in all situations. I'm trying to
> deal with two things:
> 
> 1) I want the false coloration to be able to use the actual range in
> the data, as opposed to being locked to using 0 as the minimum and
> 2^15 - 1 as the maximum.
> 2) I want to be able to adjust the coloration on the fly, changing
> where the min and max are (which would mean that more pixels would be
> "off the end" of the spectrum). This is necessary to enhance contrast
> in many situations.
> 
> I figured this should be doable by modifying the fragment shader:
> 
>        uniform sampler2D texture;
>        uniform sampler1D color_lut;
>        uniform vec2 scaling;
>        void main() {
>            vec2 uv = gl_TexCoord[0].xy;
>            vec4 color = texture2D(texture, uv);
>            gl_FragColor = texture1D(color_lut,
>                min(.9999999, max(0, (color.a + scaling.x) * scaling.y)));
>        }
> 
> The "scaling" vec2 includes an offset and a scaling factor that should
> let me set the effective minimum (displayed as blue) and maximum
> (displayed as red) values in the texture.
> 
> I set scaling.x to 0 and scaling.y to
> (2 ** 16 - 1) / float(self.imageData.max() - self.imageData.min())
> That gives the following image:
> http://derakon.dyndns.org/~chriswei/temp2/auto.png
> 
> Then I applied the same scaling factor directly to the data before
> calling glTexSubImage2D, and left scaling as [0, 1], giving the
> following result:
> http://derakon.dyndns.org/~chriswei/temp2/manual.png
> 
> What I'm finding is that applying scaling in the shader gives much
> worse color resolution than if I manually scale the image -- auto.png
> only has two colors in it, while manual.png has many more. My values
> are also inverted for some reason, but I'm less worried about that.
> 
> What am I doing wrong?
> 
> -Chris
> 
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