### The Key To Writing Shaders… Varies

Writing shaders has clicked for me today. By *shaders*, I mean: 3-D shaders, GLSL shaders, OpenGL shaders. They are little programs that draw things. With math. Right on the graphics card.

Being a newb, I can see what tripped me up. The tough thing about shaders is that they aren’t like other programs. They don’t just run once through and there’s your drawing.

Take this circle:

Well, here’s the shader that did that:

How could THAT POSSIBLY draw a cirCLE?!

### The Little Blue Fragments

Let’s start with something easy: why is it blue?

This line sets the color of a single pixel. Or—as the shader calls it—a single *fragment*.

In shaders, colors are represented by a series of four numbers—0.0 to 1.0 for red, green, blue and alpha. The vector of (0.3, 0.4, 0.7) is this blue color. In hex colors, we’re looking at #4C66B2.

**Our circle starts its life as a square.** I’ve already setup a 200 pixel by 200 pixel canvas. The shader will be painting this entire area.

Oh hey, I know: let’s just set the alpha to 1.0. This means the blue will not be transparent at all—it will be solid.

That paints our whole square:

So I am specifically talking about *fragment shaders* here.

In this fragment shader, we’re now saying, “Paint this pixel **solid** blue.” And it’s just doing that for every pixel in the square.

200 x 200 = 40,000.

This shader runs—not once—but 40,000 times!

### Now Let’s Zoom Out

I’ve setup a canvas that goes from (-1, -1) to (1, 1)—with (0, 0) in the middle—and I’ve told the fragment shader to paint this area.

Here we’ve set up four vertices—those are the points on the corners—going from -1 to 1 on each side. We’ll set up OpenGL to pass in these four coordinates—and the vertex shader will pass these on to our fragment shader. And we’re drawing the circle inside that box.

This all seems pretty easy, right? 🤷

### Explain This One

Ok, this time, we’re going to go with a dead simple fragment shader:

We’re going to pass in the color through a variable.

In order to do this, we need to write a *separate* vertex shader:

Not too bad, yeah? We have a variable **v_color** we’re using to pass the color. And we’re passing the color blue.

The **gl_Position** variable is passing a corner over—such as (-1, -1) or (1, -1). These were set up in my OpenGL code.

Our vertex shader runs once for every corner; our fragment shader runs for every pixel. Together, they draw the square:

Okie—now let’s make a subtle change. Let’s make it so one corner is blue. The (1, 1) upper right corner will be blue. And the rest will be green.

Our vertex shader could be written like this:

Ok, good—if the coordinate is (1, 1) then the color will be blue.

And—with that change—we get this:

That makes sense—the upper corner is certainly blue. **But how in the world did we get a gradient??**

Look at that vector shader again. Shouldn’t just the (1, 1) coordinate be blue? Why is the (0.99, 0.99) coordinate also blue??

### Like I Said: It Varies

How do we pass these coordinates to the fragment shader? We use **varying**.

The key to all of this is **varying**.

🙌 PLEASE NOTE: More modern OpenGL fragment shaders (such as those in OpenGL 3) use **in** and **out** rather than **varying**. They STILL vary, though!

Now, normally when you pass stuff around in a program, you use a variable.

You might say **a = 1**. Ok, so now **a** is one. And it’ll always be one. Until you change it. So **a = 911**. Now **a** is nine-eleven. Nice, you turned an innocent variable into a crisis. And it will stay that way.

But WOAH WOAH WOAH—that is NOT what **varying** is. When you pass something through **varying** it is not just a variable. It is now a range.

😳 A range??

A crazy, slidy, slippery range.

😳

I’m sorry, it’s true.

🙄 Don’t apologize, I’m sure there’s a reasonable explanation.

Oh, you’re right—I forgot—it’s awesome!! I apologize for apologizing.

Remember how the vertex shader ran once for each corner of the box? Here it is again, so you can look at it.

The vertex shader DOES color just the corners. Like this:

And then when those colors are passed through the **varying** variable—mind you, an ACTUAL varying variable!—the fragment shader sees a range from green to blue along the top x-axis and the right y-axis.

And because of **varying**, our little **v_color** is going to be different for every pixel. It’s going to be a color—a vec4, that is—somewhere between green and blue.

It’s going to go smoothly from green to blue, without your interference.

Why did it paint a solid blue earlier? Because all the corners were same. So—even though we used **varying**—all corners were the same, so all values in between were the same.

### Circle Time

Back to our circle. The trick here is: we are going to use—not the varying color—but the varying **position** to determine if we are inside or outside the circle.

You already understand the color. The vertex shader has given it to us.

What we turn our focus to now are these two built-in functions: **step** and **length**.

🙌 SPECIAL NOTE: You can browse a complete list of built-in functions at Shaderific.

The **length** function gives us the distance of our **position** from the origin. The origin is the center of our circle. So, in this case, that’s our radius.

Now, remember, we’re dealing with a **varying** variable here, so **r** is the distance of the *current pixel* we’re painting from the center.

The **length** function is also going to get rid of negative values here. The point (-1, -1) has a distance of 1.414, just like the point (1, 1) does.

The **step** function is going to take everything *above* 1.0 and make it *exactly* 1.0. And everything below 1.0 will be 0.0.

So, after **step**, the value will be 0.0 if we’re inside the circle. And 1.0 if we’re outside it.

See how the shader looks without the **step** function.

Now—if you want to play with these shaders (in a WebGL-compatible browser), you can do so here.
Just go to the upper-right corner and switch to **Cube**. Which will give you a 2-D plane.

(You can also *View Source* on this page—this page contains all of its shaders and renders them inline with the text.)

### A Few Things You May Still Wonder About

So, even though shaders look a lot like JavaScript or C, they actually have a few syntax differences that make the above codes possible.

Like take this line. How does it make the comparison here?

Vector math is built-in with shaders. So you can ask if two vectors are equal using the double-equals.

You can also add, multiply, subtract vectors and so on—just by using the operators.

However, I am comparing a **vec4 position** with the **vec2**. And I am using “swizzling”.

This special syntax lets me quickly grab the **x**, **y**, **z** or **a** parts of a vector.

Composing vectors also has a simplified syntax. Do you remember these lines from our final circle shader?

This could be simplified. You can use the **vec4** function to compose a vector from multiple vectors.

Here I am passing in a **vec3** and a **float**—which will be assembled, in order, to form a new **vec4**.
Similarly, I could pass in two **vec2** vectors.

As long as your arguments have four parts in total, you can build a **vec4** from them. Of course, this is
also true of **vec2**, **vec3** and any other type in the GL shader language.