Hello, I am trying to use OpenGL for visualize 2D 16bit image data. I would like to change brightness (bias) and contrast (scale). In order to achieve this I can upload the texture repeatedly and use GL_RED_SCALE, GL_RED_BIAS, ... But I prefer not to upload the texture each time I want to change brightness. So I am trying to use the texture combiner functions to achieve this. To achieve this, I use RenderScene() { ... glColor3f(scale_val,scale_val,scale_val); GLfloat bias[4] = {bias_val, bias_val, bias_val, 1.0f}; glTexEnvfv(GL_TEXTURE_ENV, GL_TEXTURE_ENV_COLOR, bias); glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE); //For bias glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB, GL_SUBTRACT); glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB,GL_TEXTURE); glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB,GL_SRC_COLOR); glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_RGB,GL_CONSTANT); glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE2_RGB,GL_SRC_COLOR); //For scale glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB, GL_MODULATE); glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB,GL_PREVIOUS); glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB,GL_SRC_COLOR); glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_RGB,GL_PRIMARY_COLOR); glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE2_RGB,GL_SRC_COLOR); //After setting up the texture environment render quad glBegin(GL_QUADS) ... glEnd(); glutSwapBuffers(); } This works as long as my 0.0< scale < 1.0f Any help is much appreciated. -nsn

Whoops!!, I forgot to ask the question though

I am looking for way to use value of scale larger than 1.0.

I am not sure what you are trying to do, but if you want values outside of 0-1 you will need a floating point buffer setup instead of a fixed point.

Hi MARS_99

Thank you for your response.

What I am trying to do is if I have an image I(x,y) having RGB values

RGB(I(x,y)) = {R,G,B}

After I upload the data using glTexImage2D, I want to change RGB values
according to equation

RGBnew = {R,G,B}* scale + bias;

Note that openGL/GPU automatically clamps the new color values from (0 to 1) prior to display. This is effectively increasing the contrast.


Hi Ingrater,

If I wrote a shader then yes this operation is simple but I am hoping to use the fixed pipeline and hack this one operation.

The reason is that I am really using this to display a 3D volume, using the glBlendFunc in my bigger scheme. What I am doing is using texture env to scale and bias my data before my texture gets applied to the quad. Then draw each slice of 3D data as a quad. Then glBlendFunc is doing the blending to visualize my 3D volume.

Since I donot want to move the data from CPU memory to GPU memory repeatedly, I want to use something in the pipe after loading the data to GPU memory to scale and bias my data.

I read that I could use glCopyTexImage2D can be used, but I donot know how that would work.

Thank you soo much for any suggestions.

-nsn

I wanted to add the comment for the equation

RGBnew = {R,G,B}* scale + bias;

Here the problem I having is that scale is being used as a color and it is being clamped to the range (0,1) becaue it is represented as color.

Can I do something clever using alpha value. If so how can I do that.

effectively I am thinking if I can use alpha to scale my colors like

RGBnew = {R,G,B}/(alpha) + bias;

where my alpha = 1/scale. Then I have achieved what I want.

thanks,

nsn