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+---
+title: Plasmatron Shader
+layout: post
+categories:
+- shaders
+- webgl
+- demoscene
+- effects
+date: '2020-06-25'
+image: plasmatron.jpg
+---
+
+I've been interested in computer graphics for as long as I can remember. The
+reason I started programming at all was because I wanted to make things happen
+on the screen.
+
+Recently I've been playing around with fragment shaders. Fragment shaders are
+tiny programs that run on the GPU and compute the colour of individual pixels
+(fragments). They do this incredibly quickly because the GPU has a large number
+of simple processing cores, often numbering in the hundreds or thousands, to
+enable massive parallelisation.
+
+<!--more-->
+
+Shaders are written in a language called GLSL, or OpenGL Shader Language. It's
+pretty similar to C. Here is a simple plasma shader I wrote a few years ago:
+
+```glsl
+#ifdef GL_ES
+precision mediump float;
+#endif
+
+uniform vec2 u_resolution;
+uniform vec2 u_mouse;
+uniform float u_time;
+
+const float PI = 3.14159265;
+
+void main() {
+  float width = u_resolution.x;
+  float height = u_resolution.y;
+
+  float scale = (width + height) / 2.0;
+
+  float time = u_time * 0.3;
+
+  // calculate the centre of the circular sines
+  vec2 center = vec2((width / 2.0) + sin(time) * (width / 1.5),
+                     (height / 2.0) + cos(time) * (height / 1.5));
+
+  float distance = length(gl_FragCoord.xy - center);
+
+  // circular plasmas sines
+  float circ = (sin(distance / (scale / 7.6) + sin(time * 1.1) * 5.0) + 1.25)
+             + (sin(distance / (scale / 11.5) - sin(time * 1.1) * 6.0) + 1.25);
+
+  // x and y plasma sines
+  float xval = (sin(gl_FragCoord.x / (scale / 6.5) + sin(time * 1.1) * 4.5) + 1.25)
+             + (sin(gl_FragCoord.x / (scale / 9.2) - sin(time * 1.1) * 5.5) + 1.25);
+
+  float yval = (sin(gl_FragCoord.y / (scale / 6.8) + sin(time * 1.1) * 4.75) + 1.25)
+             + (sin(gl_FragCoord.y / (scale / 12.5) - sin(time * 1.1) * 5.75) + 1.25);
+
+  // add the values together for the pixel
+  float tval = circ + xval + yval / 3.0;
+
+  // work out the colour
+  vec3 color = vec3((cos(PI * tval / 4.0 + time * 3.0) + 1.0) / 2.0,
+                    (sin(PI * tval / 3.5 + time * 3.0) + 1.0) / 2.5,
+                    (sin(PI * tval / 2.0 + time * 3.0) + 2.0) / 8.0);
+
+  // set the fragment colour
+  gl_FragColor = vec4(color, 1.0);
+}
+```
+
+WebGL makes it possible to embed shaders directly into web pages, so I can show
+you the result of the code above in realtime!
+
+<figure>
+  <canvas class="glslCanvas shader" data-fragment-url="plasmatron.frag" width="800" height="450"></canvas>
+</figure>
+
+If you want to see the kind of things that are possible with fragment shaders,
+you should check out [Shadertoy](https://www.shadertoy.com/). There are some
+incredibly impressive creations on there!
+
+<script type="text/javascript" src="/js/glsl-canvas.js"></script>
diff --git a/content/blog/2020-06-25-plasmatron/plasmatron.frag b/content/blog/2020-06-25-plasmatron/plasmatron.frag
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+// Author: Daniel Barber
+// Title: Plasmatron
+
+#ifdef GL_ES
+precision mediump float;
+#endif
+
+uniform vec2 u_resolution;
+uniform vec2 u_mouse;
+uniform float u_time;
+
+const float PI = 3.14159265;
+
+void main() {
+    float width = u_resolution.x;
+    float height = u_resolution.y;
+
+    float scale = (width + height) / 2.0;
+
+    float time = u_time * 0.3;
+
+    // calculate the centre of the circular sines
+    vec2 center = vec2((width / 2.0) + sin(time) * (width / 1.5),
+                       (height / 2.0) + cos(time) * (height / 1.5));
+
+    float distance = length(gl_FragCoord.xy - center);
+
+    // circular plasmas sines
+    float circ = (sin(distance / (scale / 7.6) + sin(time * 1.1) * 5.0) + 1.25)
+               + (sin(distance / (scale / 11.5) - sin(time * 1.1) * 6.0) + 1.25);
+
+    // x and y plasma sines
+    float xval = (sin(gl_FragCoord.x / (scale / 6.5) + sin(time * 1.1) * 4.5) + 1.25)
+               + (sin(gl_FragCoord.x / (scale / 9.2) - sin(time * 1.1) * 5.5) + 1.25);
+
+    float yval = (sin(gl_FragCoord.y / (scale / 6.8) + sin(time * 1.1) * 4.75) + 1.25)
+               + (sin(gl_FragCoord.y / (scale / 12.5) - sin(time * 1.1) * 5.75) + 1.25);
+
+    // add the values together for the pixel
+    float tval = circ + xval + yval / 3.0;
+
+    // work out the colour
+    vec3 color = vec3((cos(PI * tval / 4.0 + time * 3.0) + 1.0) / 2.0,
+                      (sin(PI * tval / 3.5 + time * 3.0) + 1.0) / 2.5,
+                      (sin(PI * tval / 2.0 + time * 3.0) + 2.0) / 8.0);
+
+    // set the fragment colour
+    gl_FragColor = vec4(color, 1.0);
+}
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