How to draw OpenGL primitives (GL_POINT, GL_LINE, etc) using screen coordinates?

Question

I'm writing an OpenGL library that can draw points, lines, and rectangles using the screen coordinates. However, I do not know how to convert the screen coordinates to clip or camera coordinates. I am using modern OpenGL (vertex arrays and vertex buffers, as well as shaders).

This is basically what I'm working towards:

DrawPoint(10, 10, 5); // draws a point at pixel 10, 10 with a radius of 5

The same concept for drawing lines and rectangles.

Also, I'm not providing code because that isn't what I'm looking for, I'm looking for concepts and math.

Answer 1

What you probably want is an Orthogonal Projection Matrix. This code will go in your draw loop:

int width = getFramebufferWidth();
int height = getFramebufferHeight();
glm::mat4 mvp = glm::ortho(0, width, 0, height);

glUniformMatrix4fv(glGetUniformLocation(program, "mvp"), 1, false, glm::value_ptr(mvp));

glViewport(0, width, 0, height);
//Draw the Objects, clear the screen, whatever it is you need to do.

Then, in your Vertex Shader, you'll have something like this:

#version 330

layout(location = 0) in vec2 position;

uniform mat4 mvp;

void main() {
    gl_Position = mvp * vec4(position, 0, 1);
}

Then, when you specify something to be drawn at position <10, 10> , it'll be drawn at exactly that position.

This code uses GLM to build the matrix in question.