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Gaussian filter in OpenGL/WebGL

This sample source code below shows how to implement Gaussian blur filter in OpenGL (WebGL) shading language. This is a popular algorithm in computer graphics that results in a blurry image.

Source Code

The GL shader code below implements a separable convolution that we can use for a Gaussian filter image blur effect. The GSLS code convolve a one-dimensional kernel with coefficients once row-wise and once column-wise.

precision mediump float;

// our texture
uniform sampler2D u_image;

#define KERNEL_SIZE 15
uniform vec2 u_textureSize;
uniform int u_direction;
uniform float u_kernel[KERNEL_SIZE];
void main() {
	vec2 textCoord = gl_FragCoord.xy / u_textureSize;
	vec2 onePixel = ((u_direction == 0) ? vec2(1.0, 0.0) : vec2(0.0, 1.0)) / u_textureSize;
	vec4 meanColor = vec4(0);
	int ms = KERNEL_SIZE / 2;
	for (int i = 0; i < KERNEL_SIZE; i++)
	{
		meanColor += texture2D(u_image, textCoord  + onePixel*vec2(i - ms))*u_kernel[i];
	}
	gl_FragColor = meanColor;
}

The code accepts the following shader variables:

  • u_image – this is our source image
  • u_kernel – the filtering Gaussian kernel as one-dimension floating point array
  • u_direction – the filter direction – horizontal or vertical

How it works?

The algorithm applies a smoothing effect to images using a Gaussian filter.

Let’s say we have an image source ready as an OpenGL/WebGL texture. The following steps then briefly describe how the algorithm works:

  • Initially we must calculate a Gaussian kernel
  • We then assign it to the uniform variable u_kernel
  • Set the uniform parameter u_direction – rows or columns
  • Execute the shader program for both directions

Example Results

The following picture depicts an example output from the source code:

An example picture of a Gaussian blur filter
Gaussian blur result with GLSL code

Bonus Tip

In case you are looking for a pure Java Script solution then you may take a look at the Gaussian blur in plain JS. You can also take a look at how to apply various filters via SVG.

Try it Online

You can explore the above algorithm in our free online image processing app.

Image blur effect with Gaussian filter

WEB APPLET

See how it works in the browser!


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