# OpenGL invert colors

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hi, I'm new in opengl and in this forum... is it possible to define a color that for the next shape drawed its color is the inverse of background color??? what I wanna do is: I have a list of objects. The user draw the objects whith the mouse(click and drag) e.g. a rectangle. What may I put on the mouseMove function?? I was intending to draw the rectangle inverting the colors of the background... at the next time the moseMove execute it draw again the rectangle at the previous position returning the background('erasing' the object) and then drawing in the new position. All this because it is expensive to draw all the list at mouseMove. can you help me?? Thanks SassÃ¡

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XOR is possible see the imagesubset part of gl added in opengl1.2

though
Quote:
 All this because it is expensive to draw all the list at mouseMove

most likely this is the quicker way

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This can be done with subtractive blending, so you'll have to use an extension for it:

Check out the specs for GL_EXT_blend_subtract at

http://oss.sgi.com/projects/ogl-sample/registry/EXT/blend_subtract.txt

Obviously, this means that your card/driver combo will have to support this extension. If you need to know if your card/driver will support this, use glGetString(GL_EXTENSIONS) and find out whether GL_EXT_blend_subtract is a part of this returned string.

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Quote:
 Original post by FReYThis can be done with subtractive blending, so you'll have to use an extension for it:Check out the specs for GL_EXT_blend_subtract athttp://oss.sgi.com/projects/ogl-sample/registry/EXT/blend_subtract.txtObviously, this means that your card/driver combo will have to support this extension. If you need to know if your card/driver will support this, use glGetString(GL_EXTENSIONS) and find out whether GL_EXT_blend_subtract is a part of this returned string.

yes, my card/driver support this extension.
Is BlendEquationEXT a function? if yes, do I need any aditional library to use it??

if you post a code or a link for one it would help a lot.

remembering, I'm new in opengl... so sorry

Thanks for the help

SassÃ¡

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Quote:
 Is BlendEquationEXT a function? if yes, do I need any aditional library to use it??

Yes, this is an OpenGL function, but you can either setup the function pointer yourself or have an external library to do it for you. I highly recommend 'GLee' by Benjamin_Bunny. You can find it at:
http://elf-stone.com/glee.php

Download the 'GLee/GLee4_00.zip' file. Open the zip file and include the source files in your project. Then in the initialization of your program simply call GLeeInit();

Quote:
 if you post a code or a link for one it would help a lot.

okay, I've personally never actually used this extension, but I think I can figure it out from the blend_minmax and the blend_subtract extension definitions.

The blend_minmax extension will give you the glBlendEquationEXT function. The blend_subtract extension allows you to change your blending equation from the form:
C' = (Cs * S) + (Cd * D)
to
C' = (Cs * S) - (Cd * D)

Using GLee you can easily query support for extensions and get hold of the functions required. However, you have to test if they're supported first.
You will want to do something like this:

if (GLEE_EXT_blend_subtract){  // subtractive blending is supported. Yay!  glBlendEquationEXT(GL_FUNC_SUBTRACT_EXT);}else{  // subtractive blending is NOT supported. don't even try!}// do your subtractive blending rendering here. Do your quad rendering, but make sure your quad is white. //This will give you 'white' - backgroundcolour which should be the inverse.glBlendFunc(GL_ONE, GL_ONE);glColor3f(1.0f, 1.0f, 1.0f);glBegin(GL_QUADS)...glEnd();// change back to additive blending. this is important otherwise it will screw up the rest of your blend-modesif (GLEE_EXT_blend_subtract){  // subtractive blending is supported. Yay!  glBlendEquationEXT(GL_FUNC_ADD_EXT);}

Seriously, get GLee. It saves *so* much time it's not even funny.
Hope that helped you.

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