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POOpenGL ignore object when blending?
 

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  1. POOpenGL ignore object when blending?
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    <p>I'm rendering a quad with an outline. I do this by rendering a quad of the required size in black, then rendering the "inner" quad smaller than the first. I also want this outlined quad to be transparent.</p> <p>My current attempt renders as so:</p> <p><a href="http://i39.tinypic.com/2eq993s.png" rel="nofollow noreferrer">http://i39.tinypic.com/2eq993s.png</a></p> <p>However the problem I have is that because I'm essentially rendering two quads on top of each other, the black "main" quad blends through to the smaller "inner" quad. I don't want this to happen as I want the blending to only blend the background and the "inner" quad colours.</p> <p>Other than rendering separate quads/triangles for the outline is there a way (ideally not using shaders as I'm using fixed pipeline) to make the blending ignore/not render the "main" quad when blending the "inner" quad?</p> <p>Relevant code:</p> <p>Blend mode:</p> <pre><code>glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA); glEnable( GL_BLEND ); </code></pre> <p>Rendering the box:</p> <pre><code>struct vertex { GLfloat x,y; }; void draw_box() { glPushMatrix(); glTranslatef( -3.0f, 1.12f, 1.0f ); // Draw a huge black quad const vertex vertices2[] = { { -1.0f, -1.0f }, { 0.0f, 0.0f }, { -1.0f, 0.0f }, { -1.0f, -1.0f }, { 0.0f, 0.0f }, { 0.0f, -1.0f }, }; glVertexPointer(2, GL_FLOAT, 0, vertices2); glColor4f(0.0f, 0.0f, 0.0f, 0.5f); glDrawArrays(GL_TRIANGLES, 0, 6); // Draw a smaller grey quad const GLfloat space = 0.04f; const vertex vertices3[] = { { -1.0f+space, -1.0f+space }, { 0.0f-space, 0.0f-space }, { -1.0f+space, 0.0f-space }, { -1.0f+space, -1.0f+space }, { 0.0f-space, 0.0f-space }, { 0.0f-space, -1.0f+space }, }; glVertexPointer(2, GL_FLOAT, 0, vertices3); glColor4f(0.5f, 0.5f, 0.5f, 0.5f); glDrawArrays(GL_TRIANGLES, 0, 6); glPopMatrix(); } </code></pre>
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