View Full Version : Bezier Surface + mask

03-18-2013, 04:14 PM
Hi, is it possible to generate a surface and make it visible only within a 2d mask (b/w texture) with glBlend I suppose ? Can it be repeated x times : {1 surface + 1 application mask} x times with non overlapping masks. Then, what would be the best solution go get a blending effect with a background image which goal would be to keep only the shades of this 2d texture and make them appears into the surfaced textures (blend or shader..??).

03-18-2013, 04:25 PM
If I understand you correctly you wish to create a mask from some geometry and use it to control texturing. If so yes you can but it requires a few steps.

step 1. Create an offset screen texture buffer
step 2. Clear this buffer to white
step 3 render your geometry in black to this buffer
step 4 render to screen with background image, foreground and mixing with the mask image ( you need a shader for this step)

03-18-2013, 05:07 PM

Thanks, well I've done a graphic it will be easier. The idea is to have a delimited zone MASK over a background picture (4) which is mainly a b/w picture with shades that I want to see on the final image. Texture (1) is deformed (2) thanks to a Bezier surface to simulate the volume of the background image (4). Then texture (3) (deformed, masked delimited) must be applied on top of (4) : shader or blend overlayer ? The only point would be to keep colors of (3) but get them darker from the shades of background image (4). I need an offset buffer I suppose so ? Shading (3) on (4) compels to have (3) beeing transparent ? I just manage to make the Bezier surface (2) and I'm trying to blend it on a black mask with glBlendFunc(GL_ONE_MINUS_DST_COLOR , GL_ZERO) on the scene but I get some black on the non masked parts and I have until now no texture buffer experience nor shader. Is it a long piece of work ?

03-18-2013, 09:56 PM
Is it a long piece of work
Off screen render buffer are easy to create and use. I find shaders save a lot of time when I want to do something clever or a bit different. They don't take that long to learn and there are excellent tutorials on the web.

For what you have shown I would use slightly different logic to what I just described.

step 1 Create an off screen texture buffer
step 2 Clear the buffer to a background colour
step 3 render your Bezier surface with texture to this buffer
step 4 create a texture for your mask with the same dimensions as the render texture
step 5 render to screen the background image, texture buffer from step 3, blended using the mask image as the blend amount. (the shader instruction is called "mix" search on "glsl mix")

03-27-2013, 03:31 AM
Thanks a lot, I'll try it and come back here later.