OpenGL 4 with OpenTK in C# Part 3: Passing data to shaders

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In this post we will look passing data from your application code to the shaders using OpenTK.

This is part 3 of my series on OpenGL4 with OpenTK.
For other posts in this series:
OpenGL 4 with OpenTK in C# Part 1: Initialize the GameWindow
OpenGL 4 with OpenTK in C# Part 2: Compiling shaders and linking them
OpenGL 4 with OpenTK in C# Part 3: Passing data to shaders
OpenGL 4 with OpenTK in C# Part 4: Refactoring and adding error handling
OpenGL 4 with OpenTK in C# Part 5: Buffers and Triangle
OpenGL 4 with OpenTK in C# Part 6: Rotations and Movement of objects
OpenGL 4 with OpenTK in C# Part 7: Vectors and Matrices
OpenGL 4 with OpenTK in C# Part 8: Drawing multiple objects
OpenGL 4 with OpenTK in C# Part 9: Texturing
OpenGL 4 with OpenTK in C# Part 10: Asteroid Invaders
Basic bullet movement patterns in Asteroid Invaders
OpenGL 4 with OpenTK in C# Part 11: Mipmap
OpenGL 4 with OpenTK in C# Part 12: Basic Moveable Camera
OpenGL 4 with OpenTK in C# Part 13: IcoSphere
OpenGL 4 with OpenTK in C# Part 14: Basic Text
OpenGL 4 with OpenTK in C# Part 15: Object picking by mouse

As stated in the previous post, I am in no way an expert in OpenGL. I write these posts as a way to learn, if you want to have all the theory behind why things are done in a certain way I recommend reading OpenGL SuperBible, Seventh Edition :)
This part will build upon the game window and shaders from the previous post.

Defining in parameters to our Vertex Shader

For starters a reminder of our OnRenderFrame method from the previous posts. Added _time to hold total time elapsed as OpenTK seems to be giving the delta time only in the FrameEventArgs.

private double _time;
protected override void OnRenderFrame(FrameEventArgs e)
{
    _time += e.Time;
    Title = $"{_title}: (Vsync: {VSync}) FPS: {1f / e.Time:0}";
    Color4 backColor;
    backColor.A = 1.0f;
    backColor.R = 0.1f;
    backColor.G = 0.1f;
    backColor.B = 0.3f;
    GL.ClearColor(backColor);
    GL.Clear(ClearBufferMask.ColorBufferBit | ClearBufferMask.DepthBufferBit);

    GL.UseProgram(_program);

    // add shader attributes here

    GL.DrawArrays(PrimitiveType.Points, 0, 1);
    GL.PointSize(10);
    SwapBuffers();
}

Float parameter

So, lets modify our vertex shader to take a parameter, time in this case and calculate a color and forward it to the fragment shader. I.e. out parameter of vertex shader matches the in parameter of fragment shader.

Vertex Shader

#version 440 core

layout (location = 0) in float time;
out vec4 frag_color;

void main(void)
{
 gl_Position = vec4( 0.25, -0.25,  0.5,  1.0);
 frag_color = vec4(sin(time) * 0.5 + 0.5, cos(time) * 0.5 + 0.5, 0.0, 0.0);
}

Fragment Shader

#version 440 core

in vec4 frag_color;
out vec4 color;

void main(void)
{
 color = frag_color;
}

And in our OnRenderFrame we add the following to pass along our float time to the shader.

    // add shader attributes here
    GL.VertexAttrib1(0, _time);

This should result in the vertex blending in color over time. Key here is that the first parameter of GL.VertexAttrib1 should match the location definition in our shader.

Vector4 parameter

Vertex Shader
Here we add a new in attribute, position that is a vec4. And we just set the gl_Position to whatever value is supplied by our game code.

#version 440 core

layout (location = 0) in float time;
layout (location = 1) in vec4 position;
out vec4 frag_color;

void main(void)
{
 gl_Position = position;
 frag_color = vec4(sin(time) * 0.5 + 0.5, cos(time) * 0.5 + 0.5, 0.0, 0.0);
}

And in our OnRenderFrame we add the following

    Vector4 position;
    position.X = (float)Math.Sin(_time) * 0.5f;
    position.Y = (float)Math.Cos(_time) * 0.5f;
    position.Z = 0.0f;
    position.W = 1.0f;
    GL.VertexAttrib4(1, position);

See that the location definition and the first parameter of the GL.VertexAttrib4 match.

This should give an output similar to the picture below (or watch at https://youtu.be/na_96BMN-EA), The dot should be moving on an elliptical path around the center of the window and it should still be changing colors as it moves.

There are a lot of overload versions for the GL.VertexAttrib function. I guess so that we can pass through all that we could possibly need. Just look around and see what overload to use that matches your needs. Head over to the OpenGL API documentation for a full list, and check your Intellisense to figure out what OpenTK overload to use.

Hope this helps someone out there :)
Thanks for reading. Here's another video of 2 of our cats fighting to lighten up your day.

All code provided as-is. This is copied from my own code-base, May need some additional programming to work. Use for whatever you want, how you want! If you find this helpful, please leave a comment or share a link on social media, not required but appreciated! :)
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