Plotting Fractals in WebAssembly

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3: Basic WAT Implementation 4: Optimised WAT Implementation 5: Plotting a Julia Set
4.1 Check for Early Bailout 4.2: Modify the Render Loop  

4.2: Modify the Render Loop

In our previous implementation of the WAT function mandel_plot, we simply looped around every pixel in the image:

  1. Converting each pixel’s location to the corresponding coordinates on the complex plane, then
  2. Arbitrarily calling function escape_time_mj

Now, before calling function escape_time_mj, we must first check where the current pixel is located. If it lies within either the main cardioid or the period 2 bub, we can skip the call escape_time_mj and simply return the colour black.

The innermost if expression in function mandel_plot has now been extended to perform this additional test:

;; Store the current pixel's colour using the value returned from the following if expression
  (local.get $pixel_offset)
  (if (result i32)
    ;; Can we avoid running the escape-time algorithm?
    (call $early_bailout (local.get $x_coord) (local.get $y_coord))
    ;; Yup, so we know this pixel will be black
    (then (global.get $BLACK))
    ;; Nope, we can't bail out early
      (if (result i32)

        ;; Same coding as before


Just by the addition of this simple test, we have shortened the render time by a factor of about 6!

Optimised WAT Mandelbrot Set

Here’s an implementation of the optimised WAT coding