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OpenGL and realtime ColorMask

Started by greenpig83 Jul 28, 2012 at 1:43 PM 4 replies 2.4k views

Original Post

Jul 28, 2012 at 1:43 PM

hi, I'm just new to openGL (change from sdl to opengl cause of performance). I have a 2D game of units, and for team color (about 8 color). For each unit texture , I have a mask texture (of gray color), only for parts that need colors. So to apply unit color, I just draw unit texture, then draw mask texture with mask Color (for example remove red color, then green and blue make cian color...,red and blue make yellow...)

In DirectX, it's very easy :
long mask[]={0xFFFFFFFF,0xFFFFFF00,0xFFFF00FF,0xFF00FFFF,0xFFFF0000,0xFFCCCCCC,0xFF00FF00,0xFFFF8000};
gSprite->Draw(gTexture,&drawRect,NULL,&d3d,mask[color]);

In SDL, it's about the same :

SDL_Surface *src
long mask[]={0xFFFFFFFF,0xFFFFFF00,0xFFFF00FF,0xFF00FFFF,0xFF0000FF,0xFFCCCCCC,0xFF00FF00,0xFF0080FF};
src->format->Rmask=mask[color] & 0x00FF;
src->format->Gmask=mask[color] & 0x00FF00;
src->format->Bmask=mask[color] & 0x00FF0000;

So now I move to SDL_Opengl, I want to know the same technique to do it.
I found out : glColorMask (GLboolean red, GLboolean green, GLboolean blue, GLboolean alpha);
But it only work, if I set 1 color to true, if I set 2 color , it doesnt make yellow, or cian...

Thanks. I hope you understand the way I express my problem.

21st Century Moose

Jul 28, 2012 at 2:04 PM

glColorMask does work with 2 colours set to true. You've probably got a bug in the way you're interpreting the masks, so you'll need to post the code where you're setting up for and calling glColorMask.

I would add that using glColorMask is not going to be a great way of doing this though. You'll be better off using glTexEnv calls to combine your textures together (and best of all using shaders).

greenpig83

Jul 28, 2012 at 2:44 PM

the mask Texture I use is the texture of gray color (with 3 r,g,b value of the same), So that when I apply color Mask (00FFFF for example), only green and blue color left, so I make a new color. It's that simple.
So even if glColorMask works with 2 para set to true, It can only produce about 6 colors for me. That's not enough, I need about 8 ( like brown...)

The reason I want to use colorMask, is that I dont want to create a texture for each team color, If 4 player, then the memory need will be 4 times, that's not very optimized!
Thanks.

21st Century Moose

Jul 28, 2012 at 3:07 PM

You're making the classic mistake of assigning more importance to memory usage than it really has. There are clear cases where using extra memory can save having to do recalculations, and if that results in a performance benefit then it's worth doing. For the kind of mask textures you're using, and for the number of them you have, the memory usage is going to be absolutely tiny. In 1996 this would have been important; in 2012 where 1gb of video RAM is entry level, it's not.

I bet that using multiple textures here would give you much simpler run-time code too.

That aside, I wasn't even suggesting to use 4x the number of textures. What you need is to use multitexturing with the base texture in one texture unit, the mask texture in another, and blend in a fixed colour.

greenpig83

Jul 28, 2012 at 6:13 PM

Thanks mhagain! I'm getting very close to where I want!
First the problem that only 1 parameter work in glColorMask is that my current blendfunction is : glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
I'm not quite sure what this paras do. But it seems ok from begining, unit texture overlap background texture correctly, no blending effect happen!

After trying with glBlendFunc, and try glBlendFunc(GL_ONE,GL_ONE); I do notice that 2,3 parameter in glColorMask work now! I can create yellow (but a bit too bright, because it plus 2 color)!
So now i'm trying to find, how to get original Red,Blue,Green color. I mean src.red*0+dst.red*1 -> real red color (in Blending)! I tried GL_ZERO,GL_ONE but the result is not like I expected!

So thanks again, if u know anything that help me create really distintive colors aside from red,blue,green,gray... I really appreciate it!

21st Century Moose

Jul 28, 2012 at 7:50 PM

Hmmm, this is close to the point at which I'll need to bow out as this kind of fixed pipeline stuff is something that I seriously haven't done in many years; I also had more experience doing it in D3D than in GL so I can't give any kind of specific pointers for which GL calls to use, and overall it's something I'm seriously rusty at.

In general terms however, you're close but still going off in the wrong direction a little. You don't need either a colormask or a blendfunc to do this; you just need standard multitexturing. So it's a coupla glActiveTexture/glBindTexture/glTexEnv calls to set up, then just draw.

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