Jump to content
  • Advertisement
Sign in to follow this  
ViperG

OpenGL Textures and Stencil Buffer [Solved]

This topic is 4797 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

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]

Share this post


Link to post
Share on other sites
Advertisement
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.

Share this post


Link to post
Share on other sites
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]

Share this post


Link to post
Share on other sites
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

Share this post


Link to post
Share on other sites
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.

Share this post


Link to post
Share on other sites
Quote:
Original post by ViperG
Do you mean use glalphafunc or do glcolormask?
I mean enabling alpha test and using glAlphaFunc. For example, do something like this...
//Set up alpha test so a fragment with alpha=0.0 fails
glEnable(GL_ALPHA_TEST);
glAlphaFunc(GL_GREATER, 0.0f);

//Set up stencil test to always increment fragment's stencil value if it
//passes stencil test
glEnable(GL_STENCIL_TEST);
glStencilFunc(GL_ALWAYS, 0, 1);
glStencilOp(GL_KEEP, GL_KEEP, GL_INC);

//Here you render your ship's sprite. Where ever there is an alpha value of
//0.0 in the ship's texture it will fail the alpha test and will not alter the
//stencil buffer. If the alpha value is greater than 0.0 it will pass the
//alpha test and increment the stencil buffer value. After this, the stencil
//buffer will contain 1s where the alpha test passed and 0s where it failed
//(assuming the stencil buffer had all 0s to begin with).

//Now we will set opengl up for rendering the scorch marks
//Alpha test will be the same since an alpha of 0.0 is the mask for the scorch
//mark

//Set stencil test to only pass where there are 1s and not modify the stencil
//buffer
glStencilFunc(GL_EQUAL, 1, 1);
glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);

//Here you render your scorch marks. We will only get past the stencil test
//if we pass the alpha test (alpha!=0.0) and the stencil test (stencil==1),
//then we will continue on down the pipeline and eventually render the
//fragment.




If you are rendering more than one ship at a time in the ship rendering spot and they overlap, you will get some fragments in the stencil buffer with values greater than 1. You can account for this with a different glStencilFunc.

The scorch marks may also mess up if you have overlapping ships. You may end up rendering scorch marks for one ship on top of another one. This should be handled by using the z-buffer and setting the depths appropriately (this would also be a better method of dealing with overlapping ships. The stencil buffer won't be incremented if the fragment passes the alpha test, but fails the depth test).

Hope that cleared things up.

EDIT: Beat by a lot...

Share this post


Link to post
Share on other sites
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.

Share this post


Link to post
Share on other sites
Sign in to follow this  

  • Advertisement
×

Important Information

By using GameDev.net, you agree to our community Guidelines, Terms of Use, and Privacy Policy.

We are the game development community.

Whether you are an indie, hobbyist, AAA developer, or just trying to learn, GameDev.net is the place for you to learn, share, and connect with the games industry. Learn more About Us or sign up!

Sign me up!