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Re: Draw multiple textures with additive alpha


On May 16, 2008, at 12:37 PM, Richard Salvatierra wrote:

I am drawing multiple textures during a transition from one image to another (slide show) 

// FIRST image with opacity = .4

glBlendMode(GL_SRC_ALPHA, GL_ZERO);

glColor4f(1.0f, 1.0f, 1.0f, opacity);
glBegin(GL_QUADS);            
glTexCoord2f(...);
glVertex2f  (...); etc.
...


glBlendMode(GL_SRC_ALPHA, GL_ONE);

// SECOND image with opacity = .6
glColor4f(1.0f, 1.0f, 1.0f, opacity);
glBegin(GL_QUADS);            
glTexCoord2f(...);
glVertex2f  (...); etc.

Try adding those modes.  The first will ensure that the background is effectively removed by the fist pass, leaving only .4 of the first image.     The second pass will then add .6 of the second plus the .4 from the first image, leaving you with an opaque blend of the two without any background.

-Ken

Kenneth Dyke, email@hidden (personal), email@hidden (work)
     Sr Engineer, Graphics and Imaging Team, Apple Computer, Inc.
   Java: The simple elegance of C++ combined with the blazing speed of SmallTalk.


 

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