Creating a Cairo-Inspired Graphics Library for MetaTrader 5 (Part 2): Points, Contours and the Path

Why this article was written

Part 1 ended with a triangle and a circle drawn by two separate functions. That approach does not scale: every new shape would need its own loops, its own bounds checking and its own edge handling, and none of that code would be reusable.

Part 2 removes that duplication by giving every shape the same description. Once geometry has one representation, a single fill routine in Part 3 can render all of them without knowing what it is drawing.

What is inside

The path model. Three levels, borrowed from Cairo: a point is a floating-point coordinate, a contour is a connected run of points, and a path is a set of contours. A ring is not a special shape — it is one path with an outer and an inner contour.

Path.mqh. The familiar drawing commands, implemented in MQL5:

• MoveTo — start a new contour
• LineTo — extend the current one
• Close — join the last point back to the first

Floating-point coordinates. Why anchors are stored as double and not int. Sub-pixel positions are what make smooth edges possible at all, so this decision in Part 2 is what Part 3's anti-aliasing depends on.

Bookkeeping. How contours are stored and grown, and how the path keeps track of its own bounds so the fill stage does not have to scan the whole surface.

An honest note on scope

By the end of Part 2 the library can describe geometry but cannot yet fill it. That is on purpose: filling is introduced in Part 3, once the thing it consumes has been properly defined.

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   <- today