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Barrel Distortion Shader

edited September 2017 in Shaders Posts: 15

I converted a simple barrel distortion shader from Shadertoy for use in Codea. Barrel distortion is commonly used to produce the curved look of a CRT monitor (see the attached image).

EDITED 09-20-17: The shader is now updated and embedded in code which allows you to adjust the distortion amount to produce a barrel or pincushion effect.

-- Barrel Distortion Shader

function setup()

    print("Set positive distortion for barrel effect, negative distortion for pincushion effect.")
    print("Adapted from")

    parameter.number("Distortion", -0.1, 0.5, 0.25)
    parameter.boolean("Original", false)

    myMesh = mesh()
    local rIdx = myMesh:addRect(0, 0, 0, 0)
    myMesh.texture = "Cargo Bot:Codea Icon"
    meshWidth, meshHeight = spriteSize(myMesh.texture)
    myMesh:setRect(rIdx, 0, 0, meshWidth, meshHeight)
    myMesh.shader = shader(barrelShader.vert, barrelShader.frag)
    myMesh.shader.resolution = 32.0
    myMesh.shader.size = 1.0


function draw()


    if Original then
            translate(WIDTH/2, HEIGHT/2)
        myMesh.shader.distortion = Distortion
            translate(WIDTH/2, HEIGHT/2)


barrelShader = {
vert = [[
    uniform mat4 modelViewProjection;
    attribute vec4 position;
    attribute vec4 color;
    attribute vec2 texCoord;

    varying lowp vec4 vColor;
    varying highp vec2 vTexCoord;

    void main()
        vColor = color;
        vTexCoord = texCoord;

        gl_Position = modelViewProjection * position;

frag = [[    
    // Adapted from

    precision highp float;

    uniform lowp sampler2D texture;

    varying lowp vec4 vColor;
    varying highp vec2 vTexCoord;

    uniform highp float distortion;

    void main()
        //Sample the texture at the interpolated coordinate
        lowp vec4 col = texture2D( texture, vTexCoord ) * vColor;

        //Set the output color to the texture color
        gl_FragColor = col;

        vec2 uv = vTexCoord;
        uv = uv * 2.0 - 1.0;
        float r = uv.x*uv.x + uv.y*uv.y;
        uv *= 1.6 + distortion * r + distortion * r * r;
        uv = 0.5 * (uv * 0.5 + 1.0);
        gl_FragColor = texture2D(texture, uv);


  • dave1707dave1707 Mod
    edited September 2017 Posts: 9,244

    Nice job. It's good to have more shader examples for learning how to use them. It's also a good idea to include the setup and draw functions so it's ready to run when copied and pasted into a project to see it.

  • @dave1707 I can add setup & draw to future code posts. I wasn't sure if it was better to include it or leave it out. Thanks!

  • dave1707dave1707 Mod
    Posts: 9,244

    @mindless Anytime I show an example of anything, I include everything so whoever looks at it can just copy it, paste it, and then run it. I guess I'm just lazy because I don't try anyone's code unless it's runnable and then I just run it to see what it does and then delete it.

  • I updated the shader code and included setup() and draw() functions along with some parameters. See the attached screenshot for an example.

  • dave1707dave1707 Mod
    Posts: 9,244

    Another nice shader example. I like that you added the parameter slider and the option to show the original Sprite.

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