# OpenGL Problem with Blending

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Hello, Reading all my books about OpenGL I could not find a solution to my blending problem. The following picture shows some details: For the written explanation: I try to render some grass with two crossed polygons textured with a blended bitmap using the IPicture code from NeHe (which generates an alpha channel as far as I understood). In my code there are the follwing lines used to switch alpha blending and the blend function for source and destination pixels. glEnable(GL_BLEND); glDisable(GL_LIGHTING); glColor4f(1.0f,1.0f,1.0f,alpha); glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); glBindTexture(GL_TEXTURE_2D,flora.texID); The problem is, that the blending effect is somehow dependent on the view angle. Some of the polygons more to the back are cut by the polygons in front of them. I have no clue why this effect happens. Does anyone of you have an idea, what the problem is, or what to be done to prevent it? Best regards XDigital

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Alpha Blending on OpenGL.org

The important thing to note is that each fragment is combined with the values in the framebuffer. So it cannot blend with fragments that are drawn after it which are also behind it (in terms of depth).

To solve this you have to sort your objects with transparency so they are in back-to-front order. Then when each fragment is blended, the grass that is behind it will already be in the framebuffer and thus be part of the blending equation.

Regards,
ViLiO

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Oh no.....
Sorting them should not be easy, I fear :(
Thanks for the quick answer however.
XDigital

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i've done this already and the solution is not blending, but alpha-testing. make alpha picture of the grass texture and draw with this code

glEnable ( GL_ALPHA_TEST );
glAlphaFunc ( GL_GREATER, 0.55f );

this should do the trick

indeed, sorting is a big nono :)

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Sorting them won't really help if they intersect in pairs, each one will be in front at some point in the picture.
Looks like a depth buffer problem to me - the quad drawn first fills in the depth buffer, so the second quad is not rendered when the first on is in front, even if its transparent. Try turning off depth buffer writes when drawing the grass:

glDepthMask(GL_FALSE);//draw grass hereglDepthMask(GL_TRUE);

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simply turning of the depthmask will give an unrealistc effect, you want things to be ocluded, to see that one grass element occludes another behind it. again, blending will not work, and the depthmask is insufficient. Alpha Testing does the trick by simply not displaying anything below a certain alpha value, this way, sharp borders are clipped around the grass textures.

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