[SOLVED] Custom blend mode

Custom blend mode

I'm not really getting the glBlendFunc() function and how it works.

Is there a blend mode that lets destination black be black while overriding any other dst colour?

I have a black and white canvas containing colouring edges and shapes. I'd like to draw a brush anywhere over the canvas but not overriding the edges so they remain black once the image is fully coloured. Ideally, since the destination has some grey around the edges for smoothness, the grey shades would be tinted with the brush colour.

Thank you!

Solution

I believe what you need is glBlendFunc(GL_DST_COLOR, GL_ZERO).

To explain you how the blend function works: When blending you have 2 colours, source and destination. Source is the colour you are trying to apply (a textel, a solid colour...). Destination is the colour currently on the buffer. The 2 parameters in the glBlendFunc tell how to multiply the source and destination colour before adding them together to get the result colour.

So in the case of glBlendFunc(GL_DST_COLOR, GL_ZERO) and having a black and white destination buffer (as you described) will act like this:

if destination colour is white (1,1,1,1) and you will apply some colour for instance (.1, .2, .3, 1) the result will be R = (.1, .2, .3, 1)*(1,1,1,1) + (1,1,1,1)*(0,0,0,0) = (.1, .2, .3, 1)
if destination colour is black (0,0,0,0) and you will apply some colour for instance (.1, .2, .3, 1) the result will be R = (.1, .2, .3, 1)*(0,0,0,0) + (0,0,0,0)*(0,0,0,0) = (0, 0, 0, 0)
if destination colour is grey (.5,.5,.5,1.0) and you will apply some colour for instance (.1, .2, .3, 1) the result will be R = (.1, .2, .3, 1)*(.5,.5,.5,1.0) + (.5,.5,.5,1.0)*(0,0,0,0) = (.05, .1, .15, 1)

This case of yours is a bit specific but in most cases the alpha channel is only used. For instance most common blending is glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) which means the source colour will be multiplied with source alpha while the destination colour will be multiplied with 1.0-source alpha, so a white shape with alpha of .75 over a grey background RGB .5 (any alpha) will result in RGB all 1.0*.75 + .5*(1.0-.75) = .75 + .125 = .875.

Note that this procedure of yours might cost you if you will need some other blending type on the overlay. In that case you will need to look into stencil buffers. Another issue is you might not want to see the black and white part on places you do not draw the overlay. In this case you can simply use the destination alpha to do this blending for you: glColorMask can help you draw only to the alpha channel of your buffer where you can use alpha .0 for "black" areas and 1.0 for "white" areas, when drawing a shape over the buffer you would then use glBlendFunc(GL_DST_ALPHA, GL_ZERO).