read 2D texture pixel values

OpenGL OpenGL: Basic Coding
1

Hi,

would anyone know how I can sample/get the values of a specific pixel in a GL_TEXTURE_2D texture?

I found glGetTexImage but that copies the whole texture and I only want one pixel.

cheers
fred

2

Oh, i figured out I can do like this


glBindTexture (GL_TEXTURE_2D, tList[SAMPLE]);
glCopyTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 0, 0, 1, 1, 0);
glGetTexImage(GL_TEXTURE_2D, 0, GL_RGB, GL_FLOAT, &mouseRGB);

But now I still have the problem that the value returned clamps at 1.0.

Different thread really, I suppose, but do I have to use FBO’s to get unclamped values?

cheers
fred

3

With glCopyTexImage2D, pixels are copied from the color buffer or the color attachment in case of a fbo is bound (Except that I do not know what happens when you have multiple color attachments).
Anyway, AFAIK, color value in the application provided framebuffer are always in the [0 1] range.

Now, you can create a fbo and attach a texture with float point pixel format and read its values.

4

Hi, and thanks for your reply.
So it seems maybe I have to use FBO’s.
I am trying to set one up in wxWidgets, I hope it’ll work.

thanks again
fred

5

Someone pointed out that I should be able to do something like:

glClampColorARB(GL_CLAMP_VERTEX_COLOR_ARB, GL_FALSE);
glClampColorARB(GL_CLAMP_READ_COLOR_ARB, GL_FALSE);
glClampColorARB(GL_CLAMP_FRAGMENT_COLOR_ARB, GL_FALSE);

and then I wouldn’t need to use an FBO.

Problem is that glClampColorARB() does not seem to exist on Mac.
Which is what I am using.

cheers
fred

6

Interesting, did know this one. Check if your hardware supports the ARB_color_buffer_float extension.

7

I’m confused;

This suggests not,
http://lists.apple.com/archives/mac-opengl/2008/Apr/msg00051.html
http://developer.apple.com/graphicsimaging/opengl/extensions.html

This document
http://www.opengl.org/registry/doc/glspec21.20061201.pdf
says that OpenGl 2.1 supports it but only in the combination of

System: Mac OS X v10.5.0 “Leopard” and later
Renderers: Apple’s Software Renderer

and no graphics cards listed.

So I’m probably out of luck. :frowning:

cheers
fred

8

I’m also working on a Mac (8600M) and can confirm
that glClampColorARB doesn’t exist on it. I’m also
having problems with it clamping values when it is
not supposed to, according to the specs.

9

In my case I could solve it by using an FBO to render to a float texture. Also inside this FBO ‘bind area’ I could do my sampling of un-clamped float values using


glBindTexture (GL_TEXTURE_2D, tList[SAMPLE]);
glCopyTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA32F_ARB, mouseX, mouseY, 1, 1, 0);
glGetTexImage(GL_TEXTURE_2D, 0, GL_RGB, GL_FLOAT, &mouseRGB);

Good FBO tutorial here,
http://www.gamedev.net/reference/programming/features/fbo1/

cheers
fred