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9a14526 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 | # Mapbox GL Shaders
This repository contains GL shaders which are shared by [Mapbox GL JS](https://github.com/mapbox/mapbox-gl-js) and [Mapbox GL Native](https://github.com/mapbox/mapbox-gl-native).
## Pragmas
Some variables change type depending on their context:
- if the variable is the same for all features, we declare it as a `uniform`
- if the variable is different for each feature, we declare it as an `attribute` (in the vertex shader) and an accompanying `varying` (in both the vertex and fragment shaders).
- if the variable is different for each feature and a function of zoom, we declare several `attributes` and `uniforms` then calculate the value using interpolation
We abstract over this functionality using pragmas.
```glsl
#pragma mapbox: define highp vec4 color
main() {
#pragma mapbox: initialize highp vec4 color
...
gl_FragColor = color;
}
```
This program defines a variable within `main` called `color`, initialize the value of `color`, then sets `gl_FragColor` to the value of `color`.
Pragmas take the following form.
```glsl
#pragma mapbox: (define|initialize) (lowp|mediump|highp) (float|vec2|vec3|vec4) {name}
```
When using pragmas, the following requirements apply.
- all pragma-defined variables must have both `define` and `initialize` pragmas
- `define` pragmas must be in file scope
- `initialize` pragmas must be in function scope
- all pragma-defined variables defined and initialized in the fragment shader must also be defined and initialized in the vertex shader because `attribute`s are not accessible from the fragment shader
## Prelude
The `_prelude.fragment.glsl` and `_prelude.vertex.glsl` files are automatically included in all shaders by the compiler.
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