Creating a Cairo-Inspired Graphics Library for MetaTrader 5 (Part 4): Anti-Aliasing, Coverage and Compositing

Why this article was written

Part 3 filled shapes, and the fill was a yes-or-no decision: a pixel was either inside the shape or it was not. That is why every diagonal edge came out as a staircase, why slender elements could vanish completely, and why a semi-transparent fill painted over what was underneath instead of blending with it.

Part 4 replaces that yes-or-no test with a number. Once a pixel knows what fraction of it a shape covers, smooth edges and real transparency stop being two separate problems and become the same arithmetic.

What is inside

Coverage. The fraction of a pixel a shape occupies. Exact along x, by subtracting intervals; approximated along y, by sampling several horizontal sub-scanlines per row.

One compositing operator. CairoBlendOver multiplies coverage into the source alpha and blends. That single multiplication is what makes anti-aliasing and transparency one mechanism instead of two features working against each other.

The rasterizer, second version. A coverage accumulator in place of a paint-or-skip loop, an AddSpan() that credits partial pixels, and sub-scanline sampling instead of one sample per row.

A quality dial you can turn at runtime. CairoAaSamples takes 1, 2, 4, 8 or 16. The article measures what each setting costs, so the trade between quality and speed is a number you can look at rather than a feeling.

The details that bite. Straight versus premultiplied ARGB, un-premultiplication, and the epsilon handling that stops spans leaking a pixel at either end.

Six demo panels. Edge smoothing, the quality settings side by side, sub-pixel positioning, shallow edges, colour compositing, and coverage drawn as a gradient so you can see the number itself.

An honest note on scope

Coverage along y is sampled, not exact — that is precisely what CairoAaSamples controls, and why 16 samples costs more than 4. Fill rules and strokes are still ahead.

State of the library after this part:

Color.mqh      Part 1    the ARGB color type, uint 0xAARRGGBB
Surface.mqh    Part 1    a pixel buffer bound to one chart object
Path.mqh       Part 2    points, contours, MoveTo / LineTo / Close
Raster.mqh     Part 3    edges, winding, the first filled shape
Raster.mqh     Part 4    coverage, sub-scanlines, blend-over       <- today
Config.mqh     Part 4    CairoAaSamples, the runtime quality dial  <- today