iPhone OpenGL ES Texture2D Masking
- by Robert Neagu
What's the best choice when trying to mask a texture
like ColorSplash or other apps like iSteam, etc?
I started learning OPENGL ES like... 4 days ago (I'm a total
rookie) and tried the following approach:
1) I created a colored texture2D, a grayscale version of the first
texture and a third texture2D called mask
2) I also created a texture2D for the brush... which is grayscale and
it's opaque (brush = black = 0,0,0,1 and surroundings = white =
1,1,1,1). My intention was to create an antialiased brush with smooth
edges but i'm fine with a normal one right now
3) I searched for masking techniques on the internet and found this
tutorial ZeusCMD - Design and Development Tutorials : OpenGL ES Programming Tutorials - Masking
about masking. The tutorial tells me to use blending to achieve
masking... first draw colored, then mask with
glBlendFunc(GL_DST_COLOR, GL_ZERO) and then grayscale with
glBlendFunc(GL_ONE, GL_ONE) ... and this gives me something close to
what i want... but not exactly what i want. The result is masked but
it's somehow overbright-ed
4) For drawing to the mask texture i used an extra frame buffer object (FBO)
I'm not really happy with the resulting image (overbright-ed picture)
nor with the speed achieved with this method. I think the normal way
was to draw directly to the grayscale (overlay) texture2D affecting
only it's alpha channel in the places where the brush hits. Is there a
fast way to achieve this? I have searched a lot and never got an
answer that's clear and understandable. Then, in the main draw loop I
could only draw the colored texture and then blend the grayscale ontop
with glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA).
I just want to learn to use OPENGL ES and it's driving me nuts because i can't get it to work properly. An advice, a link to a tutorial would be much appreciated.
