OpenGL does not draw the exact color I specified!

OpenGL Development JOGL Development
#1

Hi!

I draw a simple quad with the color 128, 128, 128 (RGB) with orth. projection. Blending and lights are disabled and I use the shade model GL_FLAT.

My problem is, that the rendered quad has the color 130, 132, 130 instead of 128, 128, 128…

What do I have to switch on/off to get my rectangle rendered with the color 128, 128, 128?

Thanks!

#2

How do you specify your colors? Could it be the conversion to floating point 0…1 color that looses some precision?

#3

It’s pretty much like


int red = 128;
int green = 128;
int blue = 128;

gl.glColor3d((double)red/255.0,
                (double)green/255.0,
                (double)blue/255.0);

I think the conversion is not the problem. A conversion mistake does not explain why the value for green is different to the red and blue color values. A conversion mistake would result in three equal values.

I should maybe add here, that I determined the rendered color values by taking a screenshot. I wrote the color values in the 0…255 notation to make my post a bit more convenient to read :slight_smile:

#4

Maybe you don’t get a 24 bit backbuffer. Instead getting a 16 bit 5,6,5 format. That would explain why green is different from red and blue.

#5

Use the following code to print out your frame buffer’s bit depths:


            int[] parm = new int[1];
            
            int r,g,b,a,d,s;
            
            gl.glGetIntegerv(GL.GL_RED_BITS,     parm); r = parm[0];
            gl.glGetIntegerv(GL.GL_GREEN_BITS,   parm); g = parm[0];
            gl.glGetIntegerv(GL.GL_BLUE_BITS,    parm); b = parm[0];
            gl.glGetIntegerv(GL.GL_ALPHA_BITS,   parm); a = parm[0];
            gl.glGetIntegerv(GL.GL_DEPTH_BITS,   parm); d = parm[0];
            gl.glGetIntegerv(GL.GL_STENCIL_BITS, parm); s = parm[0];
            
            System.out.println(  "R=" + r + 
                               ", G=" + g +
                               ", B=" + b +
                               ", A=" + a +
                               ", D=" + d +
                               ", S=" + s);
#6

Wow! Of course! I’m such an idiot! I accidentally set up my graphics cards to use only 16 bits color depth a few days ago. I just turned it back to 32 bits and BANG it works fine. (I’m missing a smiley here for self-punishment).

Thank you very much! You rescued my computer graphics grade :slight_smile: