Drawing lines in 3D

A line is just a connection between two points. As such, rendering a line is not too different from rendering two points. And because you already know how to do that, lets dive right in:

GL.Begin(PrimitiveType.Lines);
    GL.Vertex3(-2.0f, -1.0f, 0.0f);
    GL.Vertex3(2.0f, 1.0f, 0.0f);
GL.End();

This time you start off by passing Lines as the primitive type to Begin / End so that OpenGL knows that for every two vertices it needs to draw one line.

Just like with points, you can draw multiple lines within the Begin / End calls. If you wanted to render wo lines, you would need to supply four vertices.

As with points, OpenGL allows you to change several parameters of the state machine that affect how your line is drawn. In addition to setting the line width and setting anti-aliasing you can also specify a stipple pattern.

Line Width

The default line width is 1.0. To find out the currently selected line with pass GetPName.LineWidth to GL.GetFloat. To change the line width, call the GL.LineWidth function:

void GL.LineWidth(float);

Anti aliasing lines

Anti Aliasing points works almost the same as it does for lines. You can toggle it by passing EnableCap.LineSmooth to GL.Enable and GL.Disable. You can check if it's turned on by passing the same enum value to GL.IsEnabled

Again, as with points, when using anti-aliasing OpenGL implementations are only required to support it for line widths of 1.0f.

Stipple pattern

You can specify a stipple pattern with which to draw lines. The stipple pattern specifies a mask that will determine which portions of the line get drawn, therefore it can be used to create dahsed lines. Before specifying a pattern, you must enable stipple-ing with EnableCaps.LineStipple. The you set the stipple with the following function:

void GL.Stipple(int factor, short pattern);

The factor paramater defaults to 1. It is capped to be between 1 and 256. It's used to specify how many times each bit in the pattern is repeated before moving on to the next bit.

The pattern paramater is used to specify a 16-bit pattern. Any bits that are set will result in the coresponding pixels getting drawn. If a bit is not set, it's pixel is not drawn. Something to be aware of, these bits are applied in reverse, so the low-order bit effects the left most pixel.

Here is an example of a bitmask, and how it corelates to rendering:

The following code enables linte stippling and then specifies a pattern of alternating dashes and dots:

GL.Enable(EnableCaps.LineStipple);
short pattern = Convert.ToInt16("FAFA", 16);
// Convert.ToInt16 converts a hex string into a short. 
// We are using the below hex:
// 0xFAFA OR 1111 1010 1111 1010

// Draw the pattern as 0101 1111 0101 1111
// because remember, it's reversed, low bit first.
GL.LineStipple(2, pattern);

You can determine the currently selected stipple pattern with:

int factor = GL.GetInteger(LineStippleRepeat);
short pattern =  (short)GL.GetInteger(GetPName.LineStipplePattern)

Example

Follow along with this example

// Draw a series of lines that increase in width
float lineWidth = 0.5f;
for (float lineY = 1.0f; lineY > -1.0f; lineY -= 0.25f) {
    GL.LineWidth(lineWidth);

    GL.Begin(PrimitiveType.Lines);
        GL.Vertex3(-0.9f,  lineY, 0.0f);
        GL.Vertex3(-0.1f,  lineY, 0.0f);
    GL.End();

    lineWidth += 1.0f;
}

// Deaw a series of lines with stippline
lineWidth = 0.5f;
GL.Enable(EnableCap.LineStipple);
short pattern = Convert.ToInt16("AAAA", 16);
GL.LineStipple(2, pattern);

for (float lineY = 1.0f; lineY > -1.0f; lineY -= 0.25f) {
    GL.LineWidth(lineWidth);

    GL.Begin(PrimitiveType.Lines);
    GL.Vertex3(0.1f, lineY, 0.0f);
    GL.Vertex3(0.9f, lineY, 0.0f);
    GL.End();

    lineWidth += 1.0f;
}

// Try taking this out, see what happens!
GL.Disable(EnableCap.LineStipple);

In the end, what happens if you take out that GL.Disable? The short of it is, nothing automatically resets the OpenGL state machine at the end of the frame. If you enable something, it will stay enabled (across frames & draw calls) until YOU disbale it!

The bolded sentance above is very important. More often than not when you have a bug it's because you set the state machine into some state, and didn't set it back to default afterwards