Drawing Graphics and Text

Rendering 2D shapes, text and images on paint surfaces. More...

Classes
class	Bitmap
	Off-screen image that can be painted. More...
class	ColorStop
	Color stop for a gradient brush. More...
class	ColorStops
	Ordered collection of gradient color stops. More...
class	Brush
	Fill color, gradient or texture. More...
class	Canvas
	Backend that executes drawing commands. More...
class	CompositionMode
	How new pixels combine with existing pixels. More...
class	Font
	Font family, size and style. More...
class	FontFace
	Resolved font family and style. More...
class	FontMetrics
	Ascent, descent and line height of a font. More...
class	FontProvider
	Backend that loads font files. More...
class	FontRegistry
	Registry of font files. More...
class	Paint
	Pen, brush, font and composition mode. More...
class	PaintContext
	Active painting session on a surface. More...
class	Painter
	Painter for a surface or a paint context. More...
class	PainterBase
	Drawing commands and paint state. More...
class	PaintSurface
	Target for drawing operations. More...
class	Path
	Sequence of lines and curves. More...
class	PathElement
	Read-only view of a path element. More...
class	PathIterator
	Forward iterator over path elements. More...
class	Pen
	Outline color, width and style. More...
class	Scaling
	Conversion between logical units and pixels. More...
class	TextMetrics
	Width and bounds of a text run. More...
class	Transform
	2D affine transform. More...

Enumerations
enum class	FillRule {   NonZero ,   EvenOdd }
	Determines how overlapping subpaths of a filled shape are painted. More...

Detailed Description

Drawing always targets a PaintSurface. The surface reports format, physical size, and Scaling, and it supplies the backend resources used to rasterize commands. Bitmap is the portable in-memory surface. After painting, Bitmap::image() returns the pixels so they can be copied, encoded, or drawn again. Other modules provide further surfaces: a Forms paint surface is the same abstraction on a window or control. Layouting and Painting documents when Forms paints; this chapter documents the drawing commands.

A PaintContext is an active session on a surface. It can install a default clip that every painter on that session intersects. Constructing a context attaches it to the surface; destroying it detaches it. The surface must outlive the context.

Painter is the type a caller constructs. It begins painting on a surface or on an existing context, through a constructor or begin(), and finish() ends the session. PainterBase is the command API: pens, brushes, fonts, transforms, clips, and the draw and fill operations. Painter does not own the surface or the context. One painter is bound to at most one target at a time.

Paint state is independent of geometry. Pen strokes outlines, Brush fills closed shapes, Font selects a typeface for text, and CompositionMode chooses source-copy or source-over blending. Paint bundles those four so the same state can be reused. setTransform() maps user coordinates before they reach the surface. setClip() restricts drawing to a rectangle. Coordinates passed to drawing commands are logical; the surface Scaling converts them to physical pixels.

Path stores moves, lines, and curves independently of a painter. A painter can stroke or fill its current path, or stroke or fill a path passed to drawPath() and fillPath(). Containment tests on the path use a FillRule and do not require a surface.

Text uses one font request for measurement and drawing. Font names a family, size, and style; it is not a loaded face. fontMetrics() returns ascent, descent, and line height for the current font. textMetrics() measures one string. drawText() draws that string at a baseline origin. Font::addFont() and Font::addFonts() register font files for later requests. Bitmap lists the families and faces the backend can resolve.

drawImage() and drawBitmap() composite image content onto the current target. The image and bitmap APIs remain the storage model; these operations only sample them.

Canvas is the backend that executes commands. Its constructor is protected. Ordinary callers do not create a canvas; the surface creates one while a painter is active.

The example paints into a bitmap, then reads the result as an image. The painter does not own the bitmap.

Pt::Gfx::Bitmap bitmap(Pt::Gfx::SizeF(320, 240));
Pt::Gfx::Painter painter(bitmap);
painter.setBrush(Pt::Gfx::Brush(Pt::Gfx::Color(255, 255, 255)));
painter.fillRect(Pt::Gfx::RectF(Pt::Gfx::SizeF(320, 240)));
painter.setPen(Pt::Gfx::Pen(Pt::Gfx::Color(0, 0, 0)));
painter.drawLine(Pt::Gfx::PointF(0, 0), Pt::Gfx::PointF(320, 240));
painter.setFont(Pt::Gfx::Font("Sans", 12));
painter.drawText(Pt::Gfx::PointF(10, 24), Pt::String("Hello"));
painter.finish();
const Pt::Gfx::Image& image = bitmap.image();

Enumeration Type Documentation

FillRule

enum class FillRule

strong

When a Path contains overlapping subpaths, FillRule controls which enclosed regions are considered inside and therefore filled.

Enumerator
NonZero	Fills regions enclosed by a non-zero net winding count. For each point, a horizontal ray is cast and the signed number of path crossings is counted. If the result is non-zero the point is inside. This is the SVG and PostScript default.
EvenOdd	Fills regions enclosed by an odd number of path crossings. For each point, a horizontal ray is cast and the total number of path crossings is counted. If the result is odd the point is inside. Also known as the alternating rule.