GLSL offsetting multitexture

Question

How can I do something like this in GLSL shaders?

vec2 attribute texture_sky;
vec2 attribute texture_floor;

if(texture_sky) gl_position = position+Xoffset;
else gl_position = position; 

I want to move one texture over the other. Is it possible to do with the vertex shaders?

Answer 1

What I type up there is a pseudo code: not the actual code. More explanation; Let's say i have two textures(2 images bind as textures) overlap each other. I want to display one texture with X+0.5 displacement while the other remain constant. The problem I am facing is distinguishing two textures in the Shader code.

That is not something you can do using a vertex shader by itself.

You might apply an offset to the texture coordinates in a vertex shader, but you certainly would not change the vertex position. The whole idea of multi-texturing is to apply multiple textures at once, avoiding the need to draw two different copies of your polygon.

Before hardware had the capability of sampling multiple textures in a single pass (Riva TNT), you actually did have to draw each polygon multiple times and blend the results in order to apply multiple textures. These days you just use a fragment shader and call it a day because OpenGL 3.0 requires all hardware support a minimum of 16 simultaneous textures.

Very roughly, the psuedo-code would look like this:

  Vertex Shader:

#version 110

// Input texture coordinates (from vertex buffer)
attribute vec2 texture_sky;
attribute vec2 texture_floor;

// Output texture coordinates (to fragment shader)
varying vec2 sky_tc;
varying vec2 floor_tc;

// Your offset
uniform vec2 Xoffset;

void main (void) {
  sky_tc   = texture_sky + Xoffset;
  floor_tc = texture_floor;

  gl_Position = ...;
}

  Fragment Shader:

#version 110

// Input texture coordinates (from vertex shader)
varying vec2 sky_tc;
varying vec2 floor_tc;

// Samplers
uniform sampler2D sky_tex;
uniform sampler2D floor_tex;

void main (void) {
  vec4 sky   = texture2D (sky_tex,   sky_tc);
  vec4 floor = texture2D (floor_tex, floor_tc);

  //
  // You have to blend these, so pick one of the following...
  //

  // Additive Blending (GL_ONE, GL_ONE):
  gl_FragColor = sky + floor;

  // or

  // Alpha Blending (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA):
  gl_FragColor = mix (sky, floor, sky.a);
}