WebGL Month. Day 9. Image filters

Image filters

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Hey 👋 Welcome back to WebGL month

Yesterday we've learned how to use textures in webgl, so let's get advantage of that knowledge to build something fun.

Today we're going to explore how to implement simple image filters

Inverse

The very first and simple filter might be inverse all colors of the image.

How do we inverse colors?

Original values are in range [0..1]

If we subtract from each component 1 we'll get negative values, there's an abs function in glsl

You can also define other functions apart of void main in glsl pretty much like in C/C++, so let's create inverse function

📄 src/shaders/texture.f.glsl

  uniform sampler2D texture;
  uniform vec2 resolution;

+ vec4 inverse(vec4 color) {
+     return abs(vec4(color.rgb - 1.0, color.a));
+ }
+ 
  void main() {
      vec2 texCoord = gl_FragCoord.xy / resolution;
      gl_FragColor = texture2D(texture, texCoord);

and let's actually use it

📄 src/shaders/texture.f.glsl

  void main() {
      vec2 texCoord = gl_FragCoord.xy / resolution;
      gl_FragColor = texture2D(texture, texCoord);
+ 
+     gl_FragColor = inverse(gl_FragColor);
  }

Voila, we have an inverse filter with just 4 lines of code

Black and White

Let's think of how to implement black and white filter.

White color is vec4(1, 1, 1, 1)

Black is vec4(0, 0, 0, 1)

What are shades of gray? Aparently we need to add the same value to each color component.

So basically we need to calculate the "brightness" value of each component. In very naive implmentation we can just add all color components and divide by 3 (arithmetical mean).

Note: this is not the best approach, as different colors will give the same result (eg. vec3(0.5, 0, 0) and vec3(0, 0.5, 0), but in reality these colors have different "brightness", I'm just trying to keep these examples simple to understand)

Ok, let's try to implement this

📄 src/shaders/texture.f.glsl

      return abs(vec4(color.rgb - 1.0, color.a));
  }

+ vec4 blackAndWhite(vec4 color) {
+     return vec4(vec3(1.0, 1.0, 1.0) * (color.r + color.g + color.b) / 3.0, color.a);
+ }
+ 
  void main() {
      vec2 texCoord = gl_FragCoord.xy / resolution;
      gl_FragColor = texture2D(texture, texCoord);

-     gl_FragColor = inverse(gl_FragColor);
+     gl_FragColor = blackAndWhite(gl_FragColor);
  }

Whoa! Looks nice

Sepia

Ok, one more fancy effect is a "old-fashioned" photos with sepia filter.

Sepia is reddish-brown color. RGB values are 112, 66, 20

Let's define sepia function and color

📄 src/shaders/texture.f.glsl

      return vec4(vec3(1.0, 1.0, 1.0) * (color.r + color.g + color.b) / 3.0, color.a);
  }

+ vec4 sepia(vec4 color) {
+     vec3 sepiaColor = vec3(112, 66, 20) / 255.0;
+ }
+ 
  void main() {
      vec2 texCoord = gl_FragCoord.xy / resolution;
      gl_FragColor = texture2D(texture, texCoord);

A naive and simple implementation will be to interpolate original color with sepia color by a certain factor. There is a mix function for this

📄 src/shaders/texture.f.glsl

  vec4 sepia(vec4 color) {
      vec3 sepiaColor = vec3(112, 66, 20) / 255.0;
+     return vec4(
+         mix(color.rgb, sepiaColor, 0.4),
+         color.a
+     );
  }

  void main() {
      vec2 texCoord = gl_FragCoord.xy / resolution;
      gl_FragColor = texture2D(texture, texCoord);

-     gl_FragColor = blackAndWhite(gl_FragColor);
+     gl_FragColor = sepia(gl_FragColor);
  }

Result:

This should give you a better idea of what can be done in fragment shader.

Try to implement some other filters, like saturation or vibrance

See you tomorrow 👋

This is a series of blog posts related to WebGL. New post will be available every day

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Source code available here

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