Finally! We have been building skill for this moment! You learn most of the GLSL foundations. You know about types and functions. You practice your shaping equations over and over. Now is time to put all that together. You are up for this challenge! In this chapter we are going to finally learn how to draw simple shapes in a parallel procedural way.
Imagine we have a grid paper, like the one we used on math classes, and the homework is to draw a square. The paper size is 10x10 and the square suppose to be 8x8. What you will do?
You paint everything except the last and first rows and last and first column. Right? How this relates to shaders? Each little scare of our grid paper is a thread (a pixel). Each little square knows their position, like the coordinates of a chess board. In previous chapters he have mapped *x* and *y* to the *red* and *green* color channels, we learn that’s our field and space. A narrow two dimensional territory between 0.0 and 1.0. How we can use it to draw a centered square in the middle of our billboard?
* Sketch a peace of code that use ```if``` statements over our spacial field. The principles to do it is remarcable similar to how we think it on the grid paper scenario.
Well done! This is a great step and accomplish. And speaking about steps, how can we simplify this code that use ```if``` statements with ```step()``` functions? Take a look to the following code.
Here we are using ```step()``` to turn everything bellow 0.1 to black color (```vec3(0.0)```). That will make a line on the left and bottom of the canvas.
If we look close, on the previous code we repeat the structure for each axis (left and bottom). We can save some lines of code by passing to ```step()```directly two values and treating them in the same way with the same function. Check the following code.
But this rectangle is not centered, is in the top right corner. We need to “take out” equal peaces on both extremes on left-bottom and tight-top to obtain a centered square.
So, to repeat this on the top-right side we can invert the ```st``` gradient and repeat the same ```step()``` function. That way the ```vec2(0.0,0.0)``` will be on the top right corner. This is the digital equivalent of flipping the page.
Interesting right? Because we only know the coordinate position per pixel, our drawing methods are based on it. Drawing shapes is all about flipping and stretching this coordinate system.
* Experiment with the same code but using ```smoothstep()``` instead of ```step()```. Note that by changing values, you can go from blurred edges to elegant antialiased borders.
* How do you think we can move and place different rectangles in a same billboard? If you figurate out how, show of your skills making a composition of rectangles and colors that resemble a [Piet Mondrian](http://en.wikipedia.org/wiki/Piet_Mondrian) painting.
In the same way is easy to draw squares on grid paper, is relatively easy to draw rectangles with cartesians coordinates. But circles requires another approach. Some how we need to *treat* the spacial coordinates in a way that we can draw circles with just a ```step()``` functions.
Back to math class and the grid paper, we can remember drawing perfect circles by opening a compass at the desired radius, pressing one of the endings on the desired center of the circles and spinning the other end around.
Do this over a grid paper, and think. Each square on the grid is a pixel, we defenetyy can draw a circle by *asking* each pixel (or thread) if they are inside the area of the circle. Right? We can know that area by computing the distance to the center of the center of the circle.
There are several ways to calculate that. The easiest one is just using the ```distance()``` functions, which internally computes the ```length()``` of the difference between two points (in our case the fragment coordinate and the center of the canvas). The ```length()``` function is nothing but a shortcut of the [hypotenuse equation](http://en.wikipedia.org/wiki/Hypotenuse) that use square root (```sqrt()```) internally.
So far I just describe three functions (```distance()```, ```length()``` and ```sqrt()```) you can use. The following code contain this three functions and how to get the exactly same result with each one. Note that on it we are mapping the distance to the center of the billboard to the color brightness of the pixel.
The resultan *mapping* values between distance and brightness can be a really helpful technique. Note that, the closer the pixel is to the center the lower (darker) values it have. Values don't get to high because from the center ( ```vec2(0.5, 0.5)``` ) the maximum distance barely goes over 0.5. Contemplate this map and think:
Imagine the above example as an inverse altitude map. The darker the taller. The gradient show us the pattern of something shape similar to a cone view from above. Imagine your self on the top of that cone, under your foot you hold the tip of a ruled tape while the rest of it goes than the hill. Because you are in the center of the canvas, the ruler will mark "0.5" in the extreme. This will be constant in all your directions. Buy choosing where to cut horizontally the top of cone you will get a bigger or smaller circular surface.
Interesting right? We can combine this re-interpretation of space to make shapes based on the distance to 0. This technique is known as “distant field” and is use in different ways from font outlines to 3D graphics.
In terms of computational power ```sqrt()``` function (and all the once that depend on it) can be expensive. Here is another way to create a circular distance field by using ```dot()``` product.
Distance fields can be use to draw almost everything. Obviously the complex the shape is, the more complicated the equation will be, but it pays off; a convenient feature of this technique could be instant is smooth edges and out lines. Because the “topological” nature of them, sharp edges could be blended together producing smooth edges the more away you are from the center. Also because the shape is "traced" from the field is very simple to draw extra contours lines. Because of this features, distances field are usually use for font rendering.
Take a look to the following code and uncomment lines one by one. Note how we move the center of coordenates to the centrer and shrinked to the half. Now the values goes from -1 to 1.
We start by visualizing the distance field with the line 24. You can see the distance pattern repeating over and over like rings in a Zen garden. This rings show the ecuation of line 20 were we compute the distance to the position `.3,.3` in all four sign permutations. We do that by obtaining the
Like concentric rings on a Zen garde the distance field values on edges get smooth and rounder the further away they get from the center.
In the chapter about color we map the cartesian coordinates to polar coordinates by calculating the *radius* and *angles* of each pixel with the following formula:
In the previous examples we have use the radius (from the center) to draw a circles. Now, if we compute the angle from the pixel (threat) to the center, we can modulate the radius to achieve different shapes. How? Yes! Shaping functions!
Below you will find the same functions in the cartesian graph and in a polar coordinates shader example (between lines 21 and 26). Uncomment one by one the functions paying attention the relationship between one coordinate system and the other
As we saw at the beginning polar coordinates are very useful and particularly easy for mapping distance fields. As we saw, in the circle example, they just require to compute the radius using a `length()` function. On the previus sec already learn how to modulate that radius according to the angle using the `atan()` function.
We can combine both to construct shapes according to how many sides it have. Check the following code from [Andrew Baldwin](https://twitter.com/baldand) [one of his blog post](http://thndl.com/square-shaped-shaders.html)
Congratulations! You have made it through the rough part! Take a break and let this concepts sediment, drawing simple shapes on Processing is really easy but not here. In shader-land everything the way to thing on shapes is twisted and can be exhausting to adapt to this new paradigm of coding.
Now that you know how to draw shapes I'm sure new ideas will pop to your mind. In the following chapter we will learn more about how to move, rotate and scale them moving. This will allow you to compose them!
