So in my render loop I am trying to draw a GL_POINT with a color. I have a shader that it runs through for matrix transformations. For some reason I can set the color, set the position, but not both. When I set the color the positions gl_Position gets messed up for some reason. I am sure this boils down to me not knowing much about GLSL. If somebody could help that would be great.

C code

internal void
Render(game_memory *Memory)
{
    game_state *GameState = (game_state *)Memory->PermanentStorage;

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glm::mat4 Model = glm::mat4(1.0f);
    glm::mat4 Projection = glm::ortho(0.0f, 1280.0f, 720.0f, 0.0f, 0.0f, 1.0f);

    GLint ModelUniform = glGetUniformLocation(GameState->ShaderProgram, "Model");
    Assert(ModelUniform != -1);
    glUniformMatrix4fv(ModelUniform, 1, GL_FALSE, glm::value_ptr(Model));

    GLint ProjectionUniform = glGetUniformLocation(GameState->ShaderProgram, "Projection");
    Assert(ProjectionUniform != -1);
    glUniformMatrix4fv(ProjectionUniform, 1, GL_FALSE, glm::value_ptr(Projection));

    r32 SinOfTime = sinf(GameState->ElapsedTime);
    r32 CosOfTime = cosf(GameState->ElapsedTime);

    v2 Location;
    Location.x = 500.0f + 100.0f * SinOfTime;
    Location.y = 500.0f + 100.0f * CosOfTime;

    glDisable(GL_TEXTURE_2D);
    glPointSize(10.0f);
    glBegin(GL_POINTS);
    glColor3f(fabs(SinOfTime), 0.0f, 0.0f);
    glVertex3f(Location.x, Location.y, 0.0f);
    glEnd();
    glEnable(GL_TEXTURE_2D);

    SDL_GL_SwapWindow(GameState->SdlWindow);
}

Additional code that sets the position attribute so I can use it.

    GLint PosAttrib = glGetAttribLocation(GameState->ShaderProgram, "position");
    glEnableVertexAttribArray(PosAttrib);
    glVertexAttribPointer(PosAttrib, 3, GL_FLOAT, GL_FALSE, 0, 0);

Fragment Shader

#version 130

in vec4 Color;

void main(void)
{
    gl_FragColor = Color;
}

Vertex Shader

#version 130

in vec3 position;
out vec4 Color;
uniform mat4 Model;
uniform mat4 Projection;

void main(void)
{
    Color = gl_Color; // If I comment this line out and not set the color the
                      // position sets fine. If its like this, color changes
                      // but the position sticks in the top left corner of 
                      // the screen.
    gl_Position = Projection * Model * vec4(position, 1.0);
}

You are mixing so many GL versions it's a little hard to track what all could be going wrong.

I'd advise explicitly declaring a colour attribute, rather than relying on the built-in gl_Color input - not all drivers properly support mixing and matching between generic and built-in attributes.

Because of the lack of up to date or concise guides out there I guess its no surprise my code looks like mixed GL versions. It would be nice to know what I am mixing and why its wrong. And I did as well try defining a color attribute with the same result.

In this case, I think the issue is that you are both providing a position through glVertexAttribPointer(), and via glVertex(). Or, in other words, you are providing 3 peices of vertex data (glVertexAttribPointer, glVertex, glColor), and only consuming 2.

I seem to have a misunderstanding as to how glColor and glVertex get passed into the shader perhaps. I thought I had to use glVertexAttribPointer to specify what I wanted to get passed to the shader.

EDIT:

Okay so I am starting to understand whats going on here. I got it working by removing the position input and doing. So if I understand correctly glVertex goes into gl_Vertex just as glColor goes into gl_Color?

void main(void)
{
    Color = gl_Color;
    gl_Position = Projection * Model * gl_Vertex;
}

What version of OpenGL do you want to use?

I am thinking 3.1. It is what I am telling SDL to initialize to and it seems like its what most people have. Is there any good reason to target 4.x? I don't have a set in stone target version simply because I am just exploring right now but I am open to suggestions. For right now I just want my own little sandbox where I can play around with 2D graphics using modern OpenGL and GLSL.

I am looking into drawing the points another way right now. Its not working but I think I am on the right track perhaps.

In the render loop

internal void
Render(game_memory *Memory)
{
    game_state *GameState = (game_state *)Memory->PermanentStorage;

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glm::mat4 Model = glm::mat4(1.0f);
    glm::mat4 Projection = glm::ortho(0.0f, 1280.0f, 720.0f, 0.0f, 0.0f, 1.0f);


    GLint ModelUniform = glGetUniformLocation(GameState->ShaderProgram, "Model");
    Assert(ModelUniform != -1);
    glUniformMatrix4fv(ModelUniform, 1, GL_FALSE, glm::value_ptr(Model));

    GLint ProjectionUniform = glGetUniformLocation(GameState->ShaderProgram, "Projection");
    Assert(ProjectionUniform != -1);
    glUniformMatrix4fv(ProjectionUniform, 1, GL_FALSE, glm::value_ptr(Projection));

    // I am perfectly aware allocating and unallocating new arrays each frame is slow
    // This is just test code so I can get an idea of what I am doing.
    GLuint VertexArrayObject;
    glGenVertexArrays(1, &VertexArrayObject);
    glBindVertexArray(VertexArrayObject);

    GLuint VertexBufferObject;
    glGenBuffers(1, &VertexBufferObject);

    GLfloat Point[] = 
    {
    //  Position        | Color
        0.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f
    };

    glBindBuffer(GL_ARRAY_BUFFER, VertexBufferObject);
    glBufferData(GL_ARRAY_BUFFER, sizeof(Point), Point, GL_STATIC_DRAW);

    GLuint ElementBufferObject;
    glGenBuffers(1, &ElementBufferObject);

    GLuint Elements[] = 
    {
        0
    };

    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ElementBufferObject);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(Elements), Elements, GL_STATIC_DRAW);

    glDrawElements(GL_POINTS, 1, GL_UNSIGNED_INT, 0);
    
    glDeleteBuffers(1, &VertexBufferObject);
    glDeleteBuffers(1, &ElementBufferObject);
    glDeleteVertexArrays(1, &VertexArrayObject);

    SDL_GL_SwapWindow(GameState->SdlWindow);
}

Code for setting the vertex properties

    // Before here is where I compile the shaders
    GLint PosAttrib = glGetAttribLocation(GameState->ShaderProgram, "position");
    glEnableVertexAttribArray(PosAttrib);
    glVertexAttribPointer(PosAttrib, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(GLfloat), 0);

    GLint ColAttrib = glGetAttribLocation(GameState->ShaderProgram, "color");
    glEnableVertexAttribArray(ColAttrib);
    glVertexAttribPointer(ColAttrib , 3, GL_FLOAT, GL_FALSE, 6 * sizeof(GLfloat), (void *)(3 * sizeof(GLfloat)));
    // After here is where I enter the game loop

Fragment Shader

#version 150

in vec4 Color;

void main(void)
{
    gl_FragColor = Color;
}

Vertex Shader

#version 150

in vec3 color;
in vec3 position;
out vec4 Color;
uniform mat4 Model;
uniform mat4 Projection;

void main(void)
{
    Color = vec4(color, 1.0f);
    gl_Position = Projection * Model * vec4(position, 1.0);
}

You need to call glVertexAttribPointer while the buffer is bound (i.e. right after glBindBuffer).

Ah, I see, thanks. That makes sense. Moved it right after the binds however still not drawing.

EDIT:
Silly me, I needed to set the point size

glPointSize(10.0f);