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Blending

Started by jonbell Jan 18, 2004 at 12:14 PM 2 replies 450+ views

Original Post

Jan 18, 2004 at 12:14 PM

Hi i am rendering some 2d images using my own format and am trying to blend out one colour. In my images all pixels with a value of 0,255,0 has its alpha set to 1 else it is 0, I am certain my data is correct. I then want to render the image with none of the 0,255,0 pixel showing. This is my code : glEnable(GL_TEXTURE_2D); glEnable(GL_BLEND); // blend out the colour mask (0,255,0) glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); // bind texture glBindTexture(GL_TEXTURE_2D, GetTexture()); glBegin(GL_QUADS); glTexCoord2f(0,1); glVertex2f( (float) x, (float) y); // tl glTexCoord2f(0,0); glVertex2f( (float) x, (float) y + iYRes); // bl glTexCoord2f(1,0); glVertex2f( (float) x + iXRes, (float) y + iYRes); // br glTexCoord2f(1,1); glVertex2f( (float) x + iXRes, (float) y); // tr glEnd(); glDisable(GL_BLEND); When i run this with blending i just get solid black quads (i am using an orthogonal projection to render in 2d if that matters). If i turn blending off my images render fine but with the solid quads and colour key showing. This code is rendering on my HUD and i have quite a bit of other 3D stuff going on before i get here so maybe i am doing something to the state machine previously? Any ideas? Cheers

jonbell

Author

jonbell

Jan 20, 2004 at 9:34 AM

Does anyone know how to solve this blending problem? From reading up i see that if the alpha value of a pixel is 1 then the lixel should not be drawn and if it is 0 then the pixel is not blended with the current pixel in the colour buffer. I am only using 0 and 1 so i *should* be able to mask out my pixels but it doesn''t work.

Do i need to do anything to the state machine other than enable blending and 2d texturing?

OrangyTang

335

OrangyTang

Jan 20, 2004 at 9:42 AM

Using alpha test would be a better idea:

glEnable(GL_ALPHA_TEST);
glAlphaFunc(GL_EQUAL, 0);

Monder

137

Monder

Jan 20, 2004 at 2:23 PM

If you have it set so that an alpha of 0 means you can''t see the pixel and an alpha of 1 means you can then your above code will work as long as you call glColor4f(1.0f, 1.0f, 1.0f, 1.0f); Though a better idea would be to do alpha testing as OrangyTang suggested (go look up glAlphaFunc in the blue book).

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