Drawing a circle in fragment shader - filter

Question

I am a complete noob when it comes to creating shaders. Or better said, I just learned about it yesterday.
I am trying to create a really simple circle. I thouht I finally figured it out but it turns out to be to large. It should match the DisplayObject size where the filter is applied to.
The fragment shader:
precision mediump float;
varying vec2 vTextureCoord;
vec2 resolution = vec2(1.0, 1.0);
void main() {
vec2 uv = vTextureCoord.xy / resolution.xy;
uv -= 0.5;
uv.x *= resolution.x / resolution.y;
float r = 0.5;
float d = length(uv);
float c = smoothstep(d,d+0.003,r);
gl_FragColor = vec4(vec3(c,0.5,0.0),1.0);
}
Example using Pixi.js:
var app = new PIXI.Application();
document.body.appendChild(app.view);
var background = PIXI.Sprite.fromImage("required/assets/bkg-grass.jpg");
background.width = 200;
background.height = 200;
app.stage.addChild(background);
var vertexShader = `
attribute vec2 aVertexPosition;
attribute vec2 aTextureCoord;
uniform mat3 projectionMatrix;
varying vec2 vTextureCoord;
void main(void)
{
gl_Position = vec4((projectionMatrix * vec3(aVertexPosition, 1.0)).xy, 0.0, 1.0);
vTextureCoord = aTextureCoord;
}
`;
var fragShader = `
precision mediump float;
varying vec2 vTextureCoord;
vec2 resolution = vec2(1.0, 1.0);
void main() {
vec2 uv = vTextureCoord.xy / resolution.xy;
uv -= 0.5;
uv.x *= resolution.x / resolution.y;
float r = 0.5;
float d = length(uv);
float c = smoothstep(d,d+0.003,r);
gl_FragColor = vec4(vec3(c,0.5,0.),1.0);
}
`;
var filter = new PIXI.Filter(vertexShader, fragShader);
filter.padding = 0;
background.filters = [filter];
body { margin: 0; }
<script src="https://cdnjs.cloudflare.com/ajax/libs/pixi.js/4.5.2/pixi.js"></script>

Answer 1

Pixi.js's vTextureCoord do not go from 0 to 1.
From the docs
V4 filters differ from V3. You can't just add in the shader and assume that texture coordinates are in the [0,1] range.
...
Note: vTextureCoord multiplied by filterArea.xy is the real size of bounding box.
If you want to get the pixel coordinates, use uniform filterArea, it will be passed to the filter automatically.
uniform vec4 filterArea;
...
vec2 pixelCoord = vTextureCoord * filterArea.xy;
They are in pixels. That won't work if we want something like "fill the ellipse into a bounding box". So, lets pass dimensions too! PIXI doesnt do it automatically, we need a manual fix:
filter.apply = function(filterManager, input, output)
{
this.uniforms.dimensions[0] = input.sourceFrame.width
this.uniforms.dimensions[1] = input.sourceFrame.height
// draw the filter...
filterManager.applyFilter(this, input, output);
}
Lets combine it in shader!
uniform vec4 filterArea;
uniform vec2 dimensions;
...
vec2 pixelCoord = vTextureCoord * filterArea.xy;
vec2 normalizedCoord = pixelCoord / dimensions;
Here's your snippet updated.
var app = new PIXI.Application();
document.body.appendChild(app.view);
var background = PIXI.Sprite.fromImage("required/assets/bkg-grass.jpg");
background.width = 200;
background.height = 200;
app.stage.addChild(background);
var vertexShader = `
attribute vec2 aVertexPosition;
attribute vec2 aTextureCoord;
uniform mat3 projectionMatrix;
varying vec2 vTextureCoord;
void main(void)
{
gl_Position = vec4((projectionMatrix * vec3(aVertexPosition, 1.0)).xy, 0.0, 1.0);
vTextureCoord = aTextureCoord;
}
`;
var fragShader = `
precision mediump float;
varying vec2 vTextureCoord;
uniform vec2 dimensions;
uniform vec4 filterArea;
void main() {
vec2 pixelCoord = vTextureCoord * filterArea.xy;
vec2 uv = pixelCoord / dimensions;
uv -= 0.5;
float r = 0.5;
float d = length(uv);
float c = smoothstep(d,d+0.003,r);
gl_FragColor = vec4(vec3(c,0.5,0.),1.0);
}
`;
var filter = new PIXI.Filter(vertexShader, fragShader);
filter.apply = function(filterManager, input, output)
{
this.uniforms.dimensions[0] = input.sourceFrame.width
this.uniforms.dimensions[1] = input.sourceFrame.height
// draw the filter...
filterManager.applyFilter(this, input, output);
}
filter.padding = 0;
background.filters = [filter];
body { margin: 0; }
<script src="https://cdnjs.cloudflare.com/ajax/libs/pixi.js/4.5.2/pixi.js"></script>

Answer 2

It seems you've stumbled upon weird floating point precision problems: texture coordinates (vTextureCoord) in your fragment shader aren't strictly in (0, 1) range. Here's what I've got when I've added line gl_FragColor = vec4(vTextureCoord, 0, 1):
It seems good, but if we inspect it closely, lower right pixel should be (1, 1, 0), but it isn't:
The problem goes away if instead of setting size to 500 by 500 we use power-of-two size (say, 512 by 512), the problem goes away:
The other possible way to mitigate the problem would be to try to circumvent Pixi's code that computes projection matrix and provide your own that transforms smaller quad into desired screen position.