gl-react/docs/examples/4.md

# Progress Indicator

This example shows a graphical component implemented in OpenGL that would not be easy to implement with raw RN components.

```html
<PieProgress width={256} height={180} progress={progress} />
```

![](4.gif)

> N.B. the pie is transparent and the background is black with 50% opacity (the colors are passed by props). That would easily allow to put this view on top of an image to show an image upload progress.

*explicit props:*

```html
<PieProgress
  width={256}
  height={180}
  progress={progress}
  colorInside={[0,0,0,0]}
  colorOutside={[0,0,0,0.5]}
  radius={0.4}
/>
```

## Implementation

```js
const React = require("react-native");
const GL = require("gl-react-native");

const shaders = GL.Shaders.create({
  pieProgress: {
    frag: `
precision mediump float;
varying vec2 uv;

uniform vec4 colorInside, colorOutside;
uniform float radius;
uniform float progress;
uniform vec2 dim;

const vec2 center = vec2(0.5);
const float PI = acos(-1.0);

void main () {
  vec2 norm = dim / min(dim.x, dim.y);
  vec2 p = uv * norm - (norm-1.0)/2.0;
  vec2 delta = p - center;
  float inside =
    step(length(delta), radius) *
    step((PI + atan(delta.y, -delta.x)) / (2.0 * PI), progress);
  gl_FragColor = mix(
    colorOutside,
    colorInside,
    inside
  );
}
    `
  }
});

class PieProgress extends React.Component {
  render () {
    const {
      width,
      height,
      progress,
      colorInside,
      colorOutside,
      radius
    } = this.props;
    return <GL.View
      width={width}
      height={height}
      shader={shaders.pieProgress}
      opaque={false}
      uniforms={{
        dim: [ width, height ],
        progress,
        colorInside,
        colorOutside,
        radius
      }}
    />;
  }
}
PieProgress.defaultProps = {
  colorInside: [0, 0, 0, 0],
  colorOutside: [0, 0, 0, 0.5],
  radius: 0.4
};
```

Note the usage of complex uniform types.


The JavaScript uniform:
```js
dim: [ width, height ]
```

is mapped to GLSL uniform:
```glsl
uniform vec2 dim;
```

where dim.x == width and dim.y == height.


Same mapping happens for the colors: `[1, 0.5, 0.2, 1]` mapped to a `vec4` will spread into {r,g,b,a}.

> Complex types in GLSL have "dynamic" properties:
vecN `v` can simultaneously be used using:

>```
v[0]  v[1]  v[2]  v[3]  // array notation
v.r   v.g   v.b   v.a   // color notation
v.x   v.y   v.z   v.w   // position notation
v.s   v.t   v.u   v.v   // coord notation
```