Can you write a texture into the stencil buffer. Example: you have a textured quad that is drawing a texture that has it's own alpha mask. is there anyway to just draw the texture into the stencil buffer, the polygon information isn't important. Or any buffer besides the alpha buffer my goal is here is to blend a texture with another texture, only where the textures overlay. So you don't see the second texture normally, until the second texture begins to occupy the same pixels as the first texture, they begin to blend. Basicly 1 texture is an object, it has it's own alpha layer, and it uses it to mask it's self(aka don't draw black). think of this as a spaceship texture. the second texture is a damage decal, with it's own alpha layer, to mask out a design of damage(a black shape). This is similure to how most FPS games, draw bullet marks or scorch marks on walls. But what I want to do, is draw a decal on the ship, where it gets hit from a weapon. But the problem is that the decal can "hang" off the ship, and then blend into space(the space background). But I only want to see it blend with the ship, then ignore anything under the ship. It doesn't look bad since the background is black (star field) but when the ship flys over a planet, you can see all the dark decals hanging off the ship. anyone know if I can fix this problem? (yeah I know, if I had 3d ship models, that would solve my problem, but im using 2d textures, as my game is 2d using opengl) [Edited by - ViperG on September 4, 2005 6:01:31 PM]

Quote:Original post by Kalidor

Quote:Original post by ViperG
is there anyway to just draw the texture into the stencil buffer, the polygon information isn't important.

You can't really draw the texture (as in different colors/grayscale levels of it) into the stencil buffer. The alpha test comes before stencil test in the pipeline though, so you can use that combined with stencil so that all fragments which pass the alpha test will be written to the stencil buffer.

Do you mean use glalphafunc or do glcolormask? If anyone knows what he's talking about help me out please :D

[Edited by - ViperG on September 4, 2005 5:01:25 PM]

Probably alpha. for example, glColormask(0,0,0,0) doesn't prevent the geometry from being drawn into the stencil buffer, just from appearing onscreen

1. Draw ship as normal
2. Clear stencil buffer (or better, just clear the small area you're about to use) to a known value (probably 0).
3. Write a mask into the stencil buffer using your ship texture. Set the stencil to write 1s and always pass the stencil test. Use an alpha test as you would normally for your ship
For each decal:
4. Draw decal with whatever blending is needed. Turn stencil writes off and only pass the stencil test if the value already present is a 1 (ie. where your ship is).

Pretty standard stuff, the reflection tutorial on NeHe has a pretty decent starter guide to using the stencil buffer (where this uses a fixed poly rather than your alpha-tested sprite).

Edit: actually you can probably do 2 first and do 1 and 3 in one drawing op, assuming you're not doing any blending in the ship sprite.

sweet I got it working. nice nice. thanks ratings for both of you :D

I simply re-draw over in the stencil buffer, with gl_zero (only the affected area that was previously 1) so I don't get overlay damage decals ship to ship.