# submap.frag

`ogre_media/materials/glsl120/submap.frag` is the GLSL 120 fragment shader that renders a submap slice texture into the scene.

```glsl
#version 120

varying vec2 out_submap_texture_coordinate;

uniform sampler2D u_submap;
uniform float u_alpha;

void main()
{
  vec2 texture_value = texture2D(u_submap, out_submap_texture_coordinate).rg;
  float value = u_alpha * texture_value.r;
  float alpha = u_alpha * texture_value.g;
  gl_FragColor = vec4(value, value, value, alpha);
}
```

The shader reads the red and green channels of the submap texture: the red channel carries the grayscale occupancy intensity, and the green channel carries the pre-multiplied alpha. The `u_alpha` uniform (set per-slice by `OgreSlice::SetAlpha`) scales both, and the result is written as a grayscale color with pre-multiplied alpha:

- `value = u_alpha * intensity`
- `alpha = u_alpha * alpha_channel`
- output `(value, value, value, alpha)`

This is what produces the transparent "X-ray" look when many submaps are composited with the `SBF_ONE, SBF_ONE_MINUS_SOURCE_ALPHA` blend mode configured in `ogre_slice.cpp`.