Simulated graphics bugs

Hi!

For a murder mystery in a party setting, I want to make a sort of broken laptop, which some broken or corrupted files (that are clues) and a glitchy screen. I was curious as to whether there was any way to intercept GNOME at the rendering level to apply a filter that would make the screen seem to be glitched out.

Thanks

Hello, it isn’t too hard to throw together a small shell extension to do a fullscreen shader, maybe borrowed from a shadertoy like this one: Video Glitch. Here is my 5 minute attempt to get it to work, it may not be perfect.

// SPDX-License-Identifier: MIT
// exported init

// shader source: https://www.shadertoy.com/view/XtK3W3

'use strict';

const { GLib, GObject, Shell } = imports.gi;

const GlitchEffect = new GObject.registerClass(
  {},
  class GlitchEffect extends Shell.GLSLEffect {
    vfunc_build_pipeline() {
      const SRC = `
//
// Description : Array and textureless GLSL 2D simplex noise function.
//      Author : Ian McEwan, Ashima Arts.
//  Maintainer : stegu
//     Lastmod : 20110822 (ijm)
//     License : Copyright (C) 2011 Ashima Arts. All rights reserved.
//               Distributed under the MIT License. See LICENSE file.
//               https://github.com/ashima/webgl-noise
//               https://github.com/stegu/webgl-noise
//

uniform sampler2D tex;
uniform float time;

vec3 mod289(vec3 x) {
  return x - floor(x * (1.0 / 289.0)) * 289.0;
}

vec2 mod289(vec2 x) {
  return x - floor(x * (1.0 / 289.0)) * 289.0;
}

vec3 permute(vec3 x) {
  return mod289(((x*34.0)+1.0)*x);
}

float snoise(vec2 v) {
  const vec4 C = vec4(0.211324865405187,  // (3.0-sqrt(3.0))/6.0
                      0.366025403784439,  // 0.5*(sqrt(3.0)-1.0)
                     -0.577350269189626,  // -1.0 + 2.0 * C.x
                      0.024390243902439); // 1.0 / 41.0
// First corner
  vec2 i  = floor(v + dot(v, C.yy) );
  vec2 x0 = v -   i + dot(i, C.xx);

// Other corners
  vec2 i1;
  //i1.x = step( x0.y, x0.x ); // x0.x > x0.y ? 1.0 : 0.0
  //i1.y = 1.0 - i1.x;
  i1 = (x0.x > x0.y) ? vec2(1.0, 0.0) : vec2(0.0, 1.0);
  // x0 = x0 - 0.0 + 0.0 * C.xx ;
  // x1 = x0 - i1 + 1.0 * C.xx ;
  // x2 = x0 - 1.0 + 2.0 * C.xx ;
  vec4 x12 = x0.xyxy + C.xxzz;
  x12.xy -= i1;

// Permutations
  i = mod289(i); // Avoid truncation effects in permutation
  vec3 p = permute( permute( i.y + vec3(0.0, i1.y, 1.0 ))
    + i.x + vec3(0.0, i1.x, 1.0 ));

  vec3 m = max(0.5 - vec3(dot(x0,x0), dot(x12.xy,x12.xy), dot(x12.zw,x12.zw)), 0.0);
  m = m*m ;
  m = m*m ;

// Gradients: 41 points uniformly over a line, mapped onto a diamond.
// The ring size 17*17 = 289 is close to a multiple of 41 (41*7 = 287)

  vec3 x = 2.0 * fract(p * C.www) - 1.0;
  vec3 h = abs(x) - 0.5;
  vec3 ox = floor(x + 0.5);
  vec3 a0 = x - ox;

// Normalise gradients implicitly by scaling m
// Approximation of: m *= inversesqrt( a0*a0 + h*h );
  m *= 1.79284291400159 - 0.85373472095314 * ( a0*a0 + h*h );

// Compute final noise value at P
  vec3 g;
  g.x  = a0.x  * x0.x  + h.x  * x0.y;
  g.yz = a0.yz * x12.xz + h.yz * x12.yw;
  return 130.0 * dot(m, g);
}

float rand(vec2 co) {
   return fract(sin(dot(co.xy,vec2(12.9898,78.233))) * 43758.5453);
}

void glitch() {
  vec2 uv = cogl_tex_coord_in[0].st;
  float t = time * 2.0;

  // Create large, incidental noise waves
  float noise = max(0.0, snoise(vec2(t, uv.y * 0.3)) - 0.3) * (1.0 / 0.7);

  // Offset by smaller, constant noise waves
  noise = noise + (snoise(vec2(t*10.0, uv.y * 2.4)) - 0.5) * 0.15;

  // Apply the noise as x displacement for every line
  float xpos = uv.x - noise * noise * 0.25;
  if (xpos < 0.0)
    xpos = -xpos;
  if (xpos > 1.0)
    xpos = 1.0 - xpos;
  cogl_color_out = texture2D(tex, vec2(xpos, uv.y));

  // Mix in some random interference for lines
  cogl_color_out.rgb = mix(cogl_color_out.rgb, vec3(rand(vec2(uv.y * t))), noise * 0.3).rgb;

  // Apply a line pattern every 4 pixels
  if (floor(mod(gl_FragCoord.y * 0.25, 2.0)) == 0.0)
  {
      cogl_color_out.rgb *= 1.0 - (0.15 * noise);
  }

  // Shift green/blue channels (using the red channel)
  cogl_color_out.g = mix(cogl_color_out.r, texture2D(tex, vec2(xpos + noise * 0.05, uv.y)).g, 0.25);
  cogl_color_out.b = mix(cogl_color_out.r, texture2D(tex, vec2(xpos - noise * 0.05, uv.y)).b, 0.25);
}
`;
    this.add_glsl_snippet(Shell.SnippetHook.FRAGMENT, SRC, 'glitch();', false);
  }


    vfunc_paint_target(...args) {
      const time = GLib.get_monotonic_time() / GLib.USEC_PER_SEC;

      this.set_uniform_float(this.get_uniform_location('tex'), 1, [0]);
      this.set_uniform_float(this.get_uniform_location('time'), 1, [time]);

      super.vfunc_paint_target(...args);
      this.queue_repaint();
    }
  }
);


class Extension {
    constructor() {
      this._glitch = null;
      this._actor = null;
    }

    enable() {
      this._actor = global.stage;
      this._glitch = new GlitchEffect();
      this._actor.add_effect(this._glitch);
    }

    disable() {
      if (this._actor) {
        this._actor.remove_effect(this._glitch);
        this._glitch = null;
        this._actor = null;
      }
    }
}

function init() {
    return new Extension();
}

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