Question

I have an open GL quad that is rendered with a grayscale gradient. I would like to colorize it by applying a filter, something like:

If color = 0,0,0 then set color to 255,255,255
If color = 0,0,1 then set color to 255,255,254
etc, or some scheme I decide on.

Note the reason I do this in grayscale because the algorithm I’m using was designed to be drawn in grayscale and then colorized since the colors may not be known immediately.

This would be similar to the java LookupOp http://download.oracle.com/javase/6/docs/api/java/awt/image/LookupOp.html.

Is there a way to do this in openGL?

thanks,
Jeff

Answer 1

You could interpret those colours from the grayscale gradient as 1-D texture coordinates and then specify your look-up table as a 1-D texture. This seems to fit your situation.

Alternatively, you can use a fragment program (shader) to perform arbitrary colour transformations on individual pixels.

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Some more explanation: What is a texture? A texture, conceptually, is some kind of lookup function, with some additional logic on top.

A 2-D texture is something which for any pair of coordinates (s,t) or (x,y) in the range of [0,0] – [1,1] yields a specific colour (RGB, RGBA, L, whatever). Additionally it has some settings like warping or filtering.
Underneath, a texture is described by discrete data of a given “density” – perhaps 16×16, perhaps 256×512. The filtering process makes it possible to specify a colour for any real number between [0,0] and [1,1] (by mixing/interpolating neighbouring texels or just taking the nearest one).

A 1-D texture is identical, except that it maps just a single real value to a colour. Therefore, it can be thought of as a specific type of a “lookup table”. You can consider it equivalent to a 2-D texture based on a 1xN image.

If you have a grayscale gradient, you may render it directly by treating the gradient value as a colour – or you can treat it as texture coordinates (= indices in the lookup table) and using the 1-D texture for an arbitrary colour space transform.

You’d just need to translate the gradient values (from 0..255 range) to the [0..1] range of texture indices. I’d recommend something like out = (in+0.5)/256.0. The 0.5 makes for the half-texel offset as we want to point to the middle of a texel (a value inside a texture), not to a corner between 2 values.

To only have the exact RGB values from lookup table (= 1-D texture), also set the texture filters to GL_NEAREST.

BTW: Note that if you already need another texture to draw the gradient, then it gets a bit more complicated, because you’d want to treat the values received from one texture as coordinates for another texture – and I believe you’d need pixel shaders for that. Not that shaders are complicated or anything… they are extremely handy when you learn the basics.