From f4605dbbd165348362ea38b6313b0818196cc675 Mon Sep 17 00:00:00 2001
From: Jeff Carr <jcarr@wit.com>
Date: Mon, 19 Feb 2024 07:16:10 -0600
Subject: pulled out from my fork of upstream repo

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+# Shadertoy to Pixel shader conversion
+
+This will show you an example how you can take an example from [shadertoy.com](shadertoy.com) and use it with the Pixel Shader support.
+
+["Seascape" by Alexander Alekseev aka TDM - 2014](https://www.shadertoy.com/view/Ms2SD1) is a nice one. It's very impressive and doesn't use textures so that simplifies things for us. Let's try that one.
+
+Looking at the seascape.glsl example you can see **iResolution**, **iTime** and **iMouse** in there.
+
+These are commonly needed to be exposed because these things are coming from outside and needs to be updated. Any other variable you need to have changed/updated from code can be exposed like those.
+
+## Command line arguments
+
+```
+./seascape-shader -h   # will show you the command line options
+
+./seascape-shader -filename ./shaders/seascape.glsl    # Seascape
+./seascape-shader -filename ./shaders/planetfall.glsl  # Planet Fall demo
+```
+
+## Exposing variables
+
+How to expose variables like this?
+
+Well, first we need to figure out what type of variables they are. Looking at the shader, you can see **iResolution.x** in there. This tells you that it's not a simple type. In this case it's a **vec2***, a **Vector containing 2 values, x and y**. This makes sense since resolution is described by x and y. That is, width and height.
+
+And thus, we create our variable in Go with like so:
+```
+uResolution := mgl32.Vec2{float32(win.Bounds().W()), float32(win.Bounds().H())}
+```
+That is, to be of the type **mgl32.Vec2**. Here we create it by taking the window's width and height.
+
+**iTime** is just a float, and thus that is just created like so:
+```
+var uTime float32
+```
+
+For the **iMouse**, it's a **mgl32.Vec4**, a **Vector containing 4 variables**. We only use x and y for the mouse position here though.
+```
+var uMouse mgl32.Vec4
+```
+
+And finally, to make our variables available in the shader itself we use:
+```
+canvas.SetUniform(name string, value interface{})
+```
+
+We create a handy function to do this:
+
+``` go
+func EasyBindUniforms(c *pixelgl.Canvas, unifs ...interface{}) {
+	if len(unifs)%2 != 0 {
+		panic("needs to be divisable by 2")
+	}
+	for i := 0; i < len(unifs); i += 2 {
+
+		c.SetUniform(unifs[i+0].(string), unifs[i+1])
+	}
+}
+```
+and we call that function like so:
+
+``` go
+	EasyBindUniforms(canvas,
+		"uResolution", &uResolution,
+		"uTime", &uTime,
+		"uMouse", &uMouse,
+	)
+```
+
+## Updating shader source file
+
+We also need to do some updates to the shader file itself to match these variables. First thing would be to add the variables we exposed in Go.
+
+```
+uniform vec2 uResolution;
+uniform float uTime;
+uniform vec4 uMouse;
+```
+Then we just rename the variables to match.
+
+We also need to rename the main function itself, as the one used here is specific for use with shadertoy. For our shader, the entrypoint is main(). So we rename:
+```
+void mainImage( out vec4 fragColor, in vec2 fragCoord )
+```
+to
+```
+void main() {
+```
+
+
+Also, rename:
+```
+fragCoord
+```
+to
+```
+gl_FragCoord
+```
+because this is available globaly in the OpenGL space for the shader.
+
+We also need to add:
+```
+out vec4 fragColor;
+```
+
+to expose that.
+
+Lastly, we need to add:
+```
+#version 330 core
+```
+at the top to tell what version we require.
+
+
+## Using shader
+To use the shader in our canvas we do:
+```
+canvas.SetFragmentShader(fragSource string)
+```
+where fragSource is the fragment shader, not a path fo a file.
+
+#
+
+## Result converting shadertoy shader to use with Pixel
+
+Here is a diff of the changes:
+
+![code changes](shader_diffs.png "Code changes")
+
+
+And that is it. Running the program we should see this:
+
+![seascape animation](seascape.gif "Seascape animation")
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