Efficient Anti-Aliasing

These days I did what I wanted to do for a long time, that is implement an efficient anti-aliasing for the rasterizer of the Escher peers (this rasterizer can be used in arbitrary other scenarios as well though). This was mainly triggered by OpenJDK’s need for a replacement for their encumbered rasterizer, and I hope that mine is probably a good fit. Let me outline how it works.

Before doing anti-aliasing, let me show how shapes are generally rendered to a raster in general:

Rendering a glyph

Basically, we scan the shape line by line, and at the intersection points we ‘turn on’ or ‘turn off’ the pixel at that intersection point. In order to do this efficiently, we first sort the edges of the shape according to their Y and X coordinates.

Ok, but what about anti-aliasing? Basically it’s the same, only that we scan each line more than once. Usually each line is scanned a -power-of-2 times, for example 8. Depending on how often we intersect the shape in a pixel, we can calculate a coverage value. This coverage value is then used as alpha value when filling the actual pixel.

The interesting part here is how the coverage values are managed.  A simple and naive approach is of course to store them in an array, one entry for each pixel. Now if we scan a line we proceed just like in non-AA rasterization and turn on (add 1) to the entry in this array that corresponds to the pixel when we ‘enter’ the shape, do the same for all pixel entries in between until we ‘leave’ the shape for that scanline. This is repeated for the same pixel line 2^N times. In the end we have an array that has the correct coverage values for each pixel.

Of course, iterating over the array and adding up the coverage entries isn’t exactly efficient. A better idea is to only store deltas. So, instead of adding 1 to all the pixel entries ‘inside’ the shape, we only add 1 at the entry pixel and substract 1 at the exit pixel. This is much better.

We must still iterate over the whole array though when we want to display it to the destination surface, to find consecutive chunks of equal pixels. Imagine that for a simple 600 pixels wide rectangle. Pretty bad. So what I did today and the last couple of days is design a datastructure that manages the coverage values in a much more efficient way. Basically, it is a simple linked list of deltas together with their X coordinates. This list is always sorted according to the X coordinates. Each time we scan a line, we insert one entry into that list for each intersection point. In order to find the insertion point, we do a linear search. A linear search? Haven’t we all learned that this is pretty inefficient. Yes. But here we make use of the fact that the scanline is always scanned in a linear fashion, from left to right, and that the list is always sorted. The data structure has a pointer to the last item that got inserted, and starting from there, it is not possible (or at least highly unlikely) that the next insertion point is more than 2 items away. When the next scanline inside the same Y pixel row is scanned, we ‘rewind’ the data structure to point to the first item again, and start searching there. Finally, when doing the actual rendering, we can simply iterate this list, and fill complete segments in one go. This can be quite efficiently implemented on most backends (for example, for a framebuffer it’s basically a simple memset operation when the color is opaque).

A nice side effect of this technique is that non-anti-aliased rendering can now be handled as a special case of anti-aliased rendering, with no performance penalty. Another little detail of the implementation is that the datastructure doesn’t throw away and re-allocate its entries, but keeps unused items for later reuse. This way, the scanline converter usually needs allocation, after some initial setup of its tables and structures.

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