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 # Shaders
 > The dark magic – master this, and you rule the world
 
+## Definitions
+### GLSL
+GLSL is the shader language used for WebGL.
+
+### GLSL Fragment shaders
+Fragment shaders are responsible for giving each pixel their color.
+Fragment shaders are run on each pixel: they're not at all aware of their surrounding pixels. This is a challenging constraint at times, but also why they're so speedy.
+
+Fragment shaders also don't have much to use to find out where they are on the screen either. gl_FragCoord.xy will give you the exact position in pixels, but if you resize the screen the size of the object won't change to fit it.
+
+So we pass in a uniform value that contains the size, or resolution, of the screen. A uniform is a value passed in from JavaScript that is the same for every pixel in the shader. It's useful for giving the fragment shader some context to work with: for example, you might also pass in the time in seconds to animate the output.
+
+By dividing gl_FragCoord.xy by iResolution, we can get a value between 0 and 1 for the pixel's position on the screen:
+```
+vec2 p = gl_FragCoord.xy / iResolution;
+```
+
+### GLSL Vertex Shaders 
+
+### Swizzling
+Beyond being a great name — is a nice feature in GLSL for accessing the properties of a vector.
+You can get a single float from a vector using .r, .g, .b or .a. For example:
+
+```
+vec4(1, 2, 3, 4).r == 1.0
+vec4(1, 2, 3, 4).g == 2.0
+vec4(1, 2, 3, 4).b == 3.0
+vec4(1, 2, 3, 4).a == 4.0
+```
+
+But you can also create new vectors from combinations of their components like so:
+```
+vec4(1, 2, 3, 4).rb == vec3(1, 3)
+vec4(1, 2, 3, 4).rgg == vec3(1, 2, 2)
+vec4(1, 2, 3, 4).ggab == vec3(2, 2, 4, 3)
+```
+In addition to .rgba, you can also use .xyzw. These are equivalent, but if you're using the vector for a position instead of a color it's easy to reason about when using the latter.
+
+### Finding the midpoint
+The midpoint is just the average of two values, e.g. ```(x1 + x2) / 2```
+To calculate the midpoint of a vector, you just have to calculate the midpoint of each of its values:
+```
+vec2((p1.x + p2.x) / 2.0, (p1.y + p2.y) / 2.0);
+```
+*2.0 because there's two values, remember average calculations*
+
+That's a little verbose though. Can we make it shorter?
+
+### Piecewise Operations
+You can treat vectors a little like normal numbers: they can be added, multiplied, divided and subtracted just the same in GLSL!
+```
+p1 + p2 == vec2(p1.x + p2.x, p1.y + p2.y)
+p1 - p2 == vec2(p1.x - p2.x, p1.y - p2.y)
+p1 * p2 == vec2(p1.x * p2.x, p1.y * p2.y)
+p1 / p2 == vec2(p1.x / p2.x, p1.y / p2.y)
+```
+This is called a piecewise operation, because it is applied to each piece of the vector individually.
+
+You can even apply a piecewise operation to a vector using float, e.g.:
+```
+p1 + 1.0 == vec2(p1.x + 1.0, p2.x + 1.0)
+p1 - 1.0 == vec2(p1.x - 1.0, p2.x - 1.0)
+p1 * 5.0 == vec2(p1.x * 5.0, p2.x * 5.0)
+p1 / 5.0 == vec2(p1.x / 5.0, p2.x / 5.0)
+```
+
+So to get the midpoint without using vec2 or any swizzling:
+```
+vec2 midpoint(vec2 p1, vec2 p2) {
+  return p1 / 2.0 + p2 / 2.0;
+}
+```
+
+## Boolean Comparisons
+You can compare float values (but not vectors) using run-of-the-mill boolean operators, e.g.:
+```
+bool value = uv.x > 1.05;
+bool value = uv.x <= 0.0;
+bool value = uv.x != 0.5;
+bool value = uv.y != 0.0 && uv.x != 0.0;
+bool value = uv.y != 0.0 || uv.x != 0.0;
+```
+
+## Distance / length
+You can check if a point is in a circle by seeing if its distance from the circle's center is smaller than the circle's radius. In GLSL we measure distance using the length() function:
+```
+float d = length(p2 - p1)
+```
+Much more concise than it would be in JavaScript!
+All that's left to do is compare that distance to the radius and you should be able to draw that circle out to the screen.
+
+## Signed Distance Functions
+Yep, those words probably make a lot less sense when they're put together like that.
+
+To start: a **distance function** takes a point as input and returns the distance from a surface as output. In GLSL, you'd mark up a 2D Distance Function like so: ```float distanceFn(vec2 position);```
+
+A signed distance function (SDF) is very similar, but it returns a negative value when it's inside the surface. You can now very quickly draw out that shape in 2D by checking if the SDF's value is less than zero.
+
+
 # Links
 - [jam3-lesson-webgl-shader-intro](https://github.com/Jam3/jam3-lesson-webgl-shader-intro)
 - [jam3-lesson-webgl-shader-threejs](https://github.com/Jam3/jam3-lesson-webgl-shader-threejs)
 
 # Tutorials
 - https://github.com/stackgl/shader-school
+
+
+
+"While a CPU has 2–8 big cores, a GPU has hundreds or even thousands of small ones. This makes it great at running code in parallel: provided a thread doesn't need to know anything about its neighbours, you can run a whole bunch of them really quickly at the same time without waiting for the others to finish."
+
+Anki: Difference between CPU and GPU: