Masked Mirror Surfaces

If you're like me, you've envisioned a big city with lots of reflecting buildings. The surroundings all laid out to see, reflected in the glass of the windows. Well, sorry to say, with current hardware and the TGE's implementation of mirrors, this isn't possible without a LOT of tweaking. However, for that quick and dirty mirror (dirt is optional!) you can make these changes to get mirrors that are masked. It isn't a perfect mask, but it's close and you may find this little example useful in your implementation.

BACK UP YOUR CODE!! - Can't say this enough...

We only need to change mirrorSubObject.cc to get this to work. In the method MirrorSubObject::closePortal change the line that reads:

glEnable(GL_BLEND);
   [b]glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);[/b]

To read:

glEnable(GL_BLEND);
   [b]glBlendFunc(GL_SRC_COLOR, GL_ONE_MINUS_SRC_ALPHA);[/b]

Then scroll down a little and change the line (around 243):

glEnable(GL_TEXTURE_GEN_S);
   glTexGeni(GL_S, GL_TEXTURE_GEN_MODE, GL_OBJECT_LINEAR);
   glEnable(GL_TEXTURE_GEN_T);
   glTexGeni(GL_T, GL_TEXTURE_GEN_MODE, GL_OBJECT_LINEAR);
   [b]glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);[/b]

To read:

glEnable(GL_TEXTURE_GEN_S);
   glTexGeni(GL_S, GL_TEXTURE_GEN_MODE, GL_OBJECT_LINEAR);
   glEnable(GL_TEXTURE_GEN_T);
   glTexGeni(GL_T, GL_TEXTURE_GEN_MODE, GL_OBJECT_LINEAR);
   [b]glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_BLEND);[/b]

Finally, around line 249, change this line:

glActiveTextureARB(GL_TEXTURE0_ARB);

   glEnableClientState(GL_VERTEX_ARRAY);
   [b]glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);[/b]

To read:

glActiveTextureARB(GL_TEXTURE0_ARB);

   glEnableClientState(GL_VERTEX_ARRAY);
   [b]glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);[/b]

And that should do it.. Now any alpha in your textures will be applied to the mirror.

Here is a sample of what is possible:

Stencil buffer reflections are framerate-eaters!! And the actual image will be a little darker when it is displayed in the engine, due to the blending. But I think that you'll find this useful. Note that this change will modify ALL mirrors in your game, so if you already have some, any alpha textures will show up more vividly.

Enjoy,
Brett