Day 4. Shader varyings
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Yesterday we learned how to render lines and triangles, so let's get started with the homework
How do we draw a rectangle if webgl can only render triangles? We should split a rectangle into two triangles
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Pretty simple, right?
Let's define the coordinates of triangle vertices
📄 src/webgl-hello-world.js
gl.uniform4fv(colorUniformLocation, [255, 0, 0, 255]);
const triangles = [
- 0, 0, // v1 (x, y)
- canvas.width / 2, canvas.height, // v2 (x, y)
- canvas.width, 0, // v3 (x, y)
+ // first triangle
+ 0, 150, // top left
+ 150, 150, // top right
+ 0, 0, // bottom left
+
+ // second triangle
+ 0, 0, // bottom left
+ 150, 150, // top right
+ 150, 0, // bottom right
];
const positionData = new Float32Array(triangles);
Great, we can render rectangles now!
Now let's draw a hexagon. This is somewhat harder to draw manually, so let's create a helper function
📄 src/webgl-hello-world.js
150, 0, // bottom right
];
+ function createHexagon(center, radius, segmentsCount) {
+
+ }
+
const positionData = new Float32Array(triangles);
const positionBuffer = gl.createBuffer(gl.ARRAY_BUFFER);
We need to iterate over (360 - segment angle) degrees with a step of a signle segment angle
📄 src/webgl-hello-world.js
gl.uniform2fv(resolutionUniformLocation, [canvas.width, canvas.height]);
gl.uniform4fv(colorUniformLocation, [255, 0, 0, 255]);
- const triangles = [
- // first triangle
- 0, 150, // top left
- 150, 150, // top right
- 0, 0, // bottom left
-
- // second triangle
- 0, 0, // bottom left
- 150, 150, // top right
- 150, 0, // bottom right
- ];
-
- function createHexagon(center, radius, segmentsCount) {
-
+ const triangles = [createHexagon()];
+
+ function createHexagon(centerX, centerY, radius, segmentsCount) {
+ const vertices = [];
+
+ for (let i = 0; i < Math.PI * 2; i += Math.PI * 2 / (segmentsCount - 1)) {
+
+ }
+
+ return vertices;
}
const positionData = new Float32Array(triangles);
And apply some simple school math
📄 src/webgl-hello-world.js
gl.uniform2fv(resolutionUniformLocation, [canvas.width, canvas.height]);
gl.uniform4fv(colorUniformLocation, [255, 0, 0, 255]);
- const triangles = [createHexagon()];
+ const triangles = createHexagon(canvas.width / 2, canvas.height / 2, canvas.height / 2, 6);
function createHexagon(centerX, centerY, radius, segmentsCount) {
const vertices = [];
+ const segmentAngle = Math.PI * 2 / (segmentsCount - 1);
- for (let i = 0; i < Math.PI * 2; i += Math.PI * 2 / (segmentsCount - 1)) {
-
+ for (let i = 0; i < Math.PI * 2; i += segmentAngle) {
+ const from = i;
+ const to = i + segmentAngle;
+
+ vertices.push(centerX, centerY);
+ vertices.push(centerX + Math.cos(from) * radius, centerY + Math.sin(from) * radius);
+ vertices.push(centerX + Math.cos(to) * radius, centerY + Math.sin(to) * radius);
}
return vertices;
Now how do we render circle?
Actually a circle can be built with the same function, we just need to increase the number of "segments"
📄 src/webgl-hello-world.js
gl.uniform2fv(resolutionUniformLocation, [canvas.width, canvas.height]);
gl.uniform4fv(colorUniformLocation, [255, 0, 0, 255]);
- const triangles = createHexagon(canvas.width / 2, canvas.height / 2, canvas.height / 2, 6);
+ const triangles = createHexagon(canvas.width / 2, canvas.height / 2, canvas.height / 2, 360);
function createHexagon(centerX, centerY, radius, segmentsCount) {
const vertices = [];
Varyings
Ok, what next? Let's add some color 🎨
As we already know, we can pass a color to a fragment shader via uniform
But that's not the only way.
Vertex shader can pass a varying to a fragment shader for each vertex, and the value will be interpolated
Sounds a bit complicated, let's see how it works
We need to define a varying in both vertex and fragment shaders.
Make sure type matches. If e.g. varying will be vec3 in vertex shader and vec4 in fragment shader – gl.linkProgram(program) will fail. You can check if program was successfully linked with gl.getProgramParameter(program, gl.LINK_STATUS) and if it is false – gl.getProgramInfoLog(program) to see what went wrang
📄 src/webgl-hello-world.js
attribute vec2 position;
uniform vec2 resolution;
+ varying vec4 vColor;
+
#define M_PI 3.1415926535897932384626433832795
void main() {
vec2 transformedPosition = position / resolution * 2.0 - 1.0;
gl_PointSize = 2.0;
gl_Position = vec4(transformedPosition, 0, 1);
+
+ vColor = vec4(255, 0, 0, 255);
}
`;
const fShaderSource = `
precision mediump float;
- uniform vec4 color;
+
+ varying vec4 vColor;
void main() {
- gl_FragColor = color / 255.0;
+ gl_FragColor = vColor / 255.0;
}
`;
const positionPointer = gl.getAttribLocation(program, 'position');
const resolutionUniformLocation = gl.getUniformLocation(program, 'resolution');
- const colorUniformLocation = gl.getUniformLocation(program, 'color');
gl.uniform2fv(resolutionUniformLocation, [canvas.width, canvas.height]);
- gl.uniform4fv(colorUniformLocation, [255, 0, 0, 255]);
const triangles = createHexagon(canvas.width / 2, canvas.height / 2, canvas.height / 2, 360);
Now let's try to colorize our circle based on gl_Position
📄 src/webgl-hello-world.js
gl_PointSize = 2.0;
gl_Position = vec4(transformedPosition, 0, 1);
- vColor = vec4(255, 0, 0, 255);
+ vColor = vec4((gl_Position.xy + 1.0 / 2.0) * 255.0, 0, 255);
}
`;
Looks cool, right?
But how do we pass some specific colors from js?
We need to create another attribute
📄 src/webgl-hello-world.js
const vShaderSource = `
attribute vec2 position;
+ attribute vec4 color;
uniform vec2 resolution;
varying vec4 vColor;
gl_PointSize = 2.0;
gl_Position = vec4(transformedPosition, 0, 1);
- vColor = vec4((gl_Position.xy + 1.0 / 2.0) * 255.0, 0, 255);
+ vColor = color;
}
`;
gl.useProgram(program);
- const positionPointer = gl.getAttribLocation(program, 'position');
+ const positionLocation = gl.getAttribLocation(program, 'position');
+ const colorLocation = gl.getAttribLocation(program, 'color');
+
const resolutionUniformLocation = gl.getUniformLocation(program, 'resolution');
gl.uniform2fv(resolutionUniformLocation, [canvas.width, canvas.height]);
const stride = 0;
const offset = 0;
- gl.enableVertexAttribArray(positionPointer);
- gl.vertexAttribPointer(positionPointer, attributeSize, type, nomralized, stride, offset);
+ gl.enableVertexAttribArray(positionLocation);
+ gl.vertexAttribPointer(positionLocation, attributeSize, type, nomralized, stride, offset);
gl.drawArrays(gl.TRIANGLES, 0, positionData.length / 2);
Setup buffer for this attribute
📄 src/webgl-hello-world.js
}
const positionData = new Float32Array(triangles);
+ const colorData = new Float32Array(colors);
const positionBuffer = gl.createBuffer(gl.ARRAY_BUFFER);
+ const colorBuffer = gl.createBuffer(gl.ARRAY_BUFFER);
+
+ gl.bindBuffer(gl.ARRAY_BUFFER, colorBuffer);
+ gl.bufferData(gl.ARRAY_BUFFER, colorData, gl.STATIC_DRAW);
gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);
gl.bufferData(gl.ARRAY_BUFFER, positionData, gl.STATIC_DRAW);
Fill buffer with data
📄 src/webgl-hello-world.js
gl.uniform2fv(resolutionUniformLocation, [canvas.width, canvas.height]);
const triangles = createHexagon(canvas.width / 2, canvas.height / 2, canvas.height / 2, 360);
+ const colors = fillWithColors(360);
function createHexagon(centerX, centerY, radius, segmentsCount) {
const vertices = [];
return vertices;
}
+ function fillWithColors(segmentsCount) {
+ const colors = [];
+
+ for (let i = 0; i < segmentsCount; i++) {
+ for (let j = 0; j < 3; j++) {
+ if (j == 0) { // vertex in center of circle
+ colors.push(0, 0, 0, 255);
+ } else {
+ colors.push(i / 360 * 255, 0, 0, 255);
+ }
+ }
+ }
+
+ return colors;
+ }
+
const positionData = new Float32Array(triangles);
const colorData = new Float32Array(colors);
And setup the attribute pointer (the way how attribute reads data from the buffer).
📄 src/webgl-hello-world.js
gl.enableVertexAttribArray(positionLocation);
gl.vertexAttribPointer(positionLocation, attributeSize, type, nomralized, stride, offset);
+ gl.bindBuffer(gl.ARRAY_BUFFER, colorBuffer);
+
+ gl.enableVertexAttribArray(colorLocation);
+ gl.vertexAttribPointer(colorLocation, 4, type, nomralized, stride, offset);
+
gl.drawArrays(gl.TRIANGLES, 0, positionData.length / 2);
Notice this gl.bindBuffer before attribute related calls. gl.vertexAttribPointer points attribute to a buffer which wa most recently bound, don't forget this step, this is a common mistake
Conclusion
We've learned another way to pass data to a fragment shader.
This is useful for per vertex colors and textures (we'll work with textures later)
Homework
Render a 7-gon and colorize each triangle with colors of rainbow 🌈
See you tomorrow 👋
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