At the beginning of this series, I introduced some of the different modes for drawing arrays in webgl. We touched on drawing points, lines, line stripes, line loops, and triangles. Let's take a look at the last two now, triangle strips and triangle fans. These are both very useful for drawing large, more complex surfaces in forms. In this lesson, we'll deal with triangle-strip, and triangle-fan in another lesson. Imagine we have a bunch of vertices laid out like so, right now they're rendered as points.
Just to recap here they are again as lines. Remember that gl.LINES take successive pairs of points and renders a single line with each pair. Then line strip, and line loop which is the same but looks back to the starting point, then we jump over to gl.TRIANGLES which takes groups of three points and renders a single triangle with each group. So we have one triangle with points 01, and 2, and a totally separate triangle with 3, 4, and 5, etc.
Now we hit TRIANGLE_FAN, this starts out the same as triangles, it takes the first three points, 01, and 2, and creates a triangle. Then it takes 1, 2, and 3, and creates the next triangle, then 2,3,4; 3,4,5, and so on. The result is that each successive triangle shares one edge with the last triangle. This allows you to build up surfaces with complex 2- and 3D contours. Let's see it in action. I'm working where we left off in a previous lesson.
This code has a pre-defined vertex array that I'll get rid of, but we'll keep all the code that parses the vertex colors and creates a perspective matrix. I'm going to keep an empty array here and set up a for loop that goes from 0to 2, incrementing by 01. In that, I'll push a point onto the array. The x will be i-1, this will wind up creating a series of points that go from -1 to +1 in 01 increments on the x-axis. Y will be -03, and z will be 0Now remember the way we have this set up now, we need to specify the RGB color right after each x, y, z coordinate, so I'll add those right in here, giving it a random red, green, and blue, and 1 for alpha. Now I'm going to copy that whole block and paste it in again. This will create another point at the same location, but this time I'm going to change y to +03. Now we're creating pairs of points from the left to the right.
To make this nice and dynamic, I'm going to go up and remove the initial vertex count hard-coded value, then come back down and set it to verticesLength/7, because each vertex is now defined by seven data points, this will calculate how many vertices are in the array without needing to manually change the value each time. We're good there. I'll jump down to the draw function and change mode to gl.POINTS, just so we can see what we created. Run it, and there's our array of points. Pretty close to what I do earlier.
Note that the perspective seems odd here, because points do not scale with perspective, point size that we gave it as absolute. Now with gl.LINES. This makes sense. gl.LINE_STRIP, yep. LINE_LOOP, OK. TRIANGLES, so far so good. Now into new territory, gl.TRIANGLE_STRIP, and we have one large surface. Note how all the random colors blend smoothly into each other. Now there's no need for all these vertices to be on the same plane, let's vary them by changing their z value.
I'll use a sine wave based on i. math.sin(i1002), and I'll do that for both pairs of points. Now we have a real somewhat complex mathematical 3D form, randomly colored, with perspective, spinning around in 3D space. Pretty cool if you ask me. We can do more. Rather than random colors, let's programmatically blend the colors. In the first point I'll set red to i/2 which will make a range from 0to 1 across the length of the form. For blue, I'll set to 1-(i/2) which will make it go from 1 to 0Green I'll set to 0For the second point, I'll do the exact same thing, but switch up which channels I'm applying each method to, and now we have a nice blend of colors going across the shape, pretty cool. But maybe this looks a little bit chunky. We can increase the resolution just by changing the increment in the for loop to 001. This will break that 0to 2 range into smaller steps making many more points, 10 times as many points. Now it's nice in smooth.
Note that because vertex count is dynamic, it all just works. So mess with these formulas and see what other kinds of shapes you can come up with.