Rendering lines and squares

How to render lines in a MetaSL shader
If you haven't read the previous post, I recommend to do so as well. However, I will repeat some things: In a shader you can't ask the graphics engine to draw lines or triangles to the screen because the shader is executed already during that process.
Instead it has access to a variety of data that is passed to the shader. This data includes texture coordinates, normals, position in space and so on.
When rendering a pixel we need to determine whether we shall color the pixel because it is close to a line or whether we want to output the background color. So much for the theory. The simplest thing that we can do is to draw a straight line, let's say along the x-axis:

We can determine how far the pixel is away from it and apply a smoothstep function to generate a clamped transition value that fades from 0 to 1 within that range. This allows us to set the line width using a shader parameter:

I will let you figure out how that shader would look, it's really trivial. So next I want to extend the line drawing so that I am able to draw lines in any arbitrary angle. To define a line, we need 2 points, pt_a and pt_b.
There are plenty of explanations for calculating the distance between a point and a line, check out this link on Wolfram Mathworld, for example.

Using the function that calculates the distance of a point to the line, we can now draw lines with arbitrary slopes:

Check out the source code for the shader here

Rendering a wire/solid square in a MetaSL shader
Since we already covered how to calculate the distance from a line/circle, we are finally missing the squares and rectangles. Without further ado, the formula for the distance of a point from a rectangle is:

distance = max(abs(rect_center.x - coord.x) - rect_size.x,
           abs(rect_center.y - coord.y) - rect_size.y);

float2 rect_center - The center position of the rectangle

float2 coord - The coordinate to test against

float2 rect_size - The size of the rectangle

The debugger in mental mill allows you to take a look at the result of the distance function.
You can activate the debugger by holding down the "Ctrl" key and dragging with your mouse
in one of the debug variable viewports.

You can find the shader code for the rectangle here.