Shaders: How to draw high fidelity graphics with just x and y coordinates

Obligatory "this painting is a mathematical formula, a big function on the x and y coordinates of each pixel" video from Iñigo Quilez. https://www.youtube.com/watch?v=8--5LwHRhjk

For anyone looking for some IDEs to tinker around with shaders:

* shadertoy - in-browser, the most popular and easiest to get started with

* Shadron - my personal preference due to ease of use and high capability, but a bit niche

* SHADERed - the UX can take a bit of getting used to, but it gets the job done

* KodeLife - heard of it, never tried it

That, I think, is the most unintuitive part about writing fragment shaders. The idea that you take a couple of coordinates and output a color. Compared to traditional drawing, as with a pen and paper, you have to think in reverse.

For example if you want to draw a square with a pen, you put your pen where the square is, draw the outlines, than fill it up, with a shader, for each pixel, you look at where you are, calculate where the pixel is relative to the square, and output the fill color if it is inside the square. If you want to draw another square to the right, with the pen, you move your pen to the right, but with the shader, you move the reference coordinates to the left. Another way to see it is that you don't manipulate objects, you manipulate the space around the objects.

Vertex shaders are more natural as the output is the position of your triangles, like the position of your pen should you be drawing on paper.

The pixel position has to be known, how else are you rasterizing something?

Just absolutely beautiful execution, and this is a metric ton of work. I love the diagrams and the scrollbar and the style in general.

There are a few tiny conceptual things that maybe could smooth out and improve the story. I’m a fan of keeping writing for newcomers simple and accessible and not getting lost in details trying to be a Wikipedia on the subject, so I don’t know how much it matters, take these as notes or just pedantic nerdy nitpicks that you can ignore.

Shaders predate the GPU, and they run perfectly fine on CPU, and they’re used for ray tracing, so summarizing them as GPU programs for raster doesn’t explain what they are at all. Similarly, titling this as using an “x y coordinate” misses the point of shading, which is at it’s most basic to figure out the color of a sample, if we’re talking fragment shaders. Vertex shaders are just unfortunately misnamed, they’re not ‘shading’ anything. In that sense, a shader is just a specific kind of callback function. In OpenGL you get a callback for each vertex, and a callback for each pixel, and the callback’s (shader’s) job is to produce the final value for that element, given whatever inputs you want. In 3d scenes, fragment shaders rarely use x y coordinates. They use material, incoming light direction, and outgoing camera direction to “shade” the surface, i.e., figure out the color.

The section on GPU is kind of missing the most important point about GPUs: the fact that neighboring threads share the same instruction. The important part is the SIMT/SIMD execution model. But shaders actually aren’t conceptually different from CPU programming, they are still (usually) a single-threaded programming model, not a SIMT programming model. They can be run in parallel, and shaders tend to do the same thing (same sequence of instructions) for every pixel, and that’s why they’re great and fast on the GPU, but they are programmed with the same sequential techniques we use on the CPU and they can be run sequentially on the CPU too, there are no special parallel programming techniques needed, nor a different mindset (as is suggested multiple times). The fact that you don’t need a different mindset in order to get massively parallel super fast execution is one of the reasons why shaders are so simple and elegant and effective.