Textures on top of other textures when using VBOs

Question

I'm currently making a cube style game. With chunks being drawn with VBOs. I'd like to know if there is a way to create an overlay texture on top of the existing texture without the need to rebuild the VBO. This is required to show activity in a cube (think of Minecraft when destroying a block and the little cracks start spreading).

I believe these are called "decals" but I've not found how to use them with VBOs. So how do I draw decals on OpenGL VBO drawn cubes?

Answer 1

Try to render your VBO with two textures: one as the diffuse map (the block texture) and one showing the crack.

You can combine them in a shader:

// GLSL fragment shader

#version 130

uniform sampler2D texDiffuse;
uniform sampler2D texCrack;
uniform float uniCrackAmount; // between 0 and 1

in vec2 vertTexCoord;

out vec4 fragColor;

void main()
{
    vec4 color_diffuse = texture(texDiffuse, vertTexCoord);
    vec4 color_crack = texture(texCrack, vertTexCoord);

    fragColor = color_diffuse + (color_crack * uniCrackAmount);
}

Answer 2

For a non-shader solution you could draw in two batches. First of all you flag the cubes in the current scene that need the overlay texture - this can be done during your scene tree traversal or other scene setup. Then draw the regular cubes using your normal draw routine. Finally draw the overlay-textured cubes using multitexturing - you can experiment with a decal or interpolate TexEnv for this.

The VBO setup would just use the same set of texcoords for both texture units; something like this (not intended as copy-and-paste code):

glClientActiveTexture (GL_TEXTURE0);
glEnableClientState (GL_TEXTURE_COORD_POINTER);
glTexCoordPointer (2, GL_FLOAT, sizeof (myvertextype), (void *) 12);

glClientActiveTexture (GL_TEXTURE1);
glEnableClientState (GL_TEXTURE_COORD_POINTER);
glTexCoordPointer (2, GL_FLOAT, sizeof (myvertextype), (void *) 12);

// drawing stuff

glClientActiveTexture (GL_TEXTURE1);
glDisableClientState (GL_TEXTURE_COORD_POINTER);

glClientActiveTexture (GL_TEXTURE0);
glDisableClientState (GL_TEXTURE_COORD_POINTER);

But otherwise - yeah - the shader-based solution would be preferable. Aside from those ClientActiveTexture calls going away (which is always a good thing) you've got much better control over the type of blend you do between the two textures.