UXGemGraphics

UXGemGraphics is the GEM realization of UXGraphics: the AES’s own VDI workstation, plus the theme.

#use <UXKit>            // or #import "UXGemGraphics.xc"

Thin on purpose

It is the thinnest of the seven vocabularies. A UXView that draws itself makes the same VDI calls that objc_draw makes for a G_BUTTON.

Custom views and stock widgets are therefore pixel-consistent by construction. There is no separate drawing path to drift and no palette to keep in step, so a custom control drawn beside a native one matches it.

Every other backend (GDI, CoreGraphics, cairo, Canvas2D) is a sibling of this class, implementing the same protocol over a different surface.

The coordinate contract

Incoming coordinates are bounds-relative; the bound origin makes them absolute:

g.bind(handle, theme, absRect);
g.fillRect(UXGeom.make(0, 0, 10, 10), pen);   // the view's own top-left

All seven vocabularies share this contract, so a drawRect body is written once. A view never knows where on screen it is.

GEM’s y grows down, the direction the whole protocol uses. The Mac shim flips to match; the other backends already agree.

It has theme art

hasThemeArt()      // true

GEM has a real theme with drawable slices, so drawTheme draws something here. The GDI and Cocoa vocabularies answer false, and the toolkit falls back to drawing the pieces itself.

Capability queries in this toolkit follow this pattern: ask what the backend can do, and have a neutral answer for when it cannot.

It is the one that does not stroke natively

strokesNatively()     // false — the only backend that says so

The VDI has no path stroker with joins and caps, so strokeNative is empty and UXPainter’s neutral stroker does all the work: a quad per segment, a fan per join, a polygon per cap.

Because GEM is the one backend where the neutral stroker is the only path, that stroker is exercised every time anything is stroked here. A regression in it shows up as a failing GEM test instead of going unnoticed.

On the other six backends, the neutral stroker still draws every arrowhead, because no platform has one.

Pens are indices

Colours are VDI pen indices rather than RGB, with fillRectRGB and fillPolygonRGB as the true-colour entry points. Pen 0 is white and pen 1 is ink. A default UXCharAttr uses 1 so that a default-constructed style is visible.

The other backends mirror the same index table, so a pen means the same colour everywhere.

Fonts

fontIdFor maps a family name to a VDI font id. A family the workstation does not have falls back instead of failing, so a document written on one machine stays legible on another.

Text measurement goes through the driver, not this class. Layout needs to measure text before drawTextFont draws it.