Last month’s development recap. Not an over-productive month, but some interesting things have been achieved.

Configuration-file support is something that I reworked numerous times. I began with a JSON file, but I didn’t like to use a third-party library just for parsing a simple configuration file. So, I migrated to a Lua script… which made sense since Lua is the scripting language I’m using for the engine (courios that Sol, from which Lua descends, was created many years ago as a dynamic configuration language), but it over-complicated the engine code and gave no real benefit over non-interpreted configuration files. So, I settled on a basic and plain key-value format for which a custom parsed can be coded in a few lines. Over the last weeks, I reworked the file format once again, approaching something more similar to .ini files. This is a format that, despite being brain-dead simple, I always like to use because it just makes things done. No-fuss. Also, it’s interesting the fact that “regions” (called “contexts” in #tofuengine) are used to group related parameters. This goes along with a recent refactoring, where I refactored the code to use nameless structs as a way to (again) group related parameters and make the code clearer.

( I case you are asking, using nested nameless structs isn’t going to slow down execution times since the compiler is clever enough to optimize this kind of access as a single – cumulative – offsetting from the outer struct base address )

The engine API has been partially reworked, by making the Bank.blit() method overloading a tad clearer. Also, the canvas formal argument for Canvas, Bank, Font, and XForm object constructors is now mandatory (but it can also be changed dynamically). A Canvas.copy() method has been added (which copies a canvas region, ehm…), and the already existing Canvas.process() method has been generalized: a function is passed as the first actual argument, where the final pixel is computed. This feature can be seen as a somewhat “cheaper” variant of a (Lua-scripted) fragment-shader.

function Game:render(ratio)
  local canvas = Canvas:default()
  local width, height = canvas:size()


  -- Your canvas drawing code, here...

  canvas:process(function(x, y, current, new)
      -- You can calculate the resulting pixel at `< x, y >` as a function
      -- of its `current` and `new` value....
      return new -- ... or return the `new` value to perform a straight, but un-optimized, copy.
    0, 0, -- Target position.
    0, 0, width, height) -- Source rectangle.

( an interesting generalization would be to attach a similar “pseudo-shader” to a canvas and use it to filter every drawing operation )

As a minor optimization, an OpenGL’s vertex array object is used to perform the (one and only) texture draw operation, i.e. the one that moves the visual data from an offscreen texture to the frame-buffer. While this change doesn’t seem to boost performances significantly, it makes nonetheless the code cleaner.

At last the most time-consuming activity for this month worth of coding.

I (finally) fixed the roto-scaling algorithm for good. This has been bugged since the very beginning, as with a proper combination of the actual arguments, a pixel row and/or column would be missing due in the final result to rounding errors. Despite having pretty clear the origin of the issue, I wasn’t able to fix the optimized algorithm… so I had to opt for a different route.

For the sake of my mental health (and since this isn’t going to be the most significant part of the game engine), I went for a simpler/naive technique… but that handles rounding errors better! Yep, you guessed right: I’m (inverse) rotating by applying the rotation formula to each pixel individually.

The starting point was to compute the destination roto-scaled area. Instead of rotating the four (unscaled) sprite’s corners, I consider that, given the rotation anchor point, the sprite always resides inside a disc. The radius of the disc is equal to the distance between the anchor point and the farthest corner of the sprite rectangle, i.e. the magnitude of a vector with

  • the biggest horizontal distance between the anchor point and the rectangle left/right margin (as width), and
  • the biggest vertical distance between the anchor point and rectangle top/bottom margin (as height).

If the anchor point is set to a corner, for example, the radius is the full diagonal length of the sprite rectangle.

Then, we just iterate across the disc’s axis-aligned bounding-box (and we inverse roto-scale the pixels). As an optimization, we calculate the distance between each pixel and the disc’s center, to discard the pixels that lay outside the disc and save a useless operation (since roto-scaled pixels are tested for inclusion within the source texture and they would end being discarded eventually).

A nice feature of this approach is that horizontal and vertical flipping can be optimized because can be “fused” in the roto-scaling matrix (and no additional branches for offsets calculations are required in the inner loop).

Of course, crucial for this algorithm to properly work is considering the coordinates to be “sub-pixel” in nature, that is every point is placed at half-pixel (mid-center) position.

Speaking of which, a similar issue was present (albeit in a less prominent way) also in the scaling algorithm. We need to correct the calculations by considering the mid-center position of each pixel and use this formula for proper scaling coordinates:

x_s = round((x_r + 0.5) / S_x - 0.5) = floor((x_r + 0.5) / S_x)
y_s = round((y_r + 0.5) / S_y - 0.5) = floor((y_r + 0.5) / S_y)

In practical terms, we can rewrite the formula in a recurring fashion if we proceed by moving inside the resulting rectangle, and we increment by 1 / S_x and 1 / S_y steps.

These aren’t the best roto-scaling and scaling algorithms by any stretch, but for the moment being, I’m happy with the result. Perhaps I’ll go back and rewrite/optimize/fix this part of code once more, in the future… but that’s more than enough for a small 2D game engine. If I were to use prominently rotated and scaled sprites, I’ll write a different rendering system that fully exploits the GPU rather than being entirely software-based.