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-Somewhat of a middle layer between rendering subsystems and application cores.
+Drawing API that can specify what to draw via a kind of display list.
-(What are "rendering subsystems" and "application cores"?)
+Example of the DrawingLayer use is eg. in svx/source/xoutdev/xtabhtch.cxx:121.
+A stripped down version with extended comments:
-It offers a nice and easy API to render complex shapes. (Pure "marketing".)
+ // Create a hatch primitive (here a rectangle that will be filled with
+ // the appropriate hatching, but has no border).
+ // This will not draw it yet; it's so far only constructed to add it to a
+ // display list later.
+ const drawinglayer::primitive2d::Primitive2DReference aHatchPrimitive(
+ new drawinglayer::primitive2d::PolyPolygonHatchPrimitive2D(...));
+
+ // Create a rectangle around the hatch, to give it a border.
+ const drawinglayer::primitive2d::Primitive2DReference aBlackRectanglePrimitive(
+ new drawinglayer::primitive2d::PolygonHairlinePrimitive2D(...));
+
+ // Here we want to render to a virtual device (to later obtain the bitmap
+ // out of that), so prepare it.
+ VirtualDevice aVirtualDevice;
+
+ // Create processor and draw primitives, to get it ready for rendering.
+ boost::scoped_ptr<drawinglayer::processor2d::BaseProcessor2D> pProcessor2D(
+ drawinglayer::processor2d::createPixelProcessor2DFromOutputDevice(...));
+
+ if (pProcessor2D)
+ {
+ // Fill-in the display list.
+ drawinglayer::primitive2d::Primitive2DSequence aSequence(2);
+
+ aSequence[0] = aHatchPrimitive;
+ aSequence[1] = aBlackRectanglePrimitive;
+
+ // Render it to the virtual device.
+ pProcessor2D->process(aSequence);
+ pProcessor2D.reset();
+ }
+
+ // Obtain the bitmap that was rendered from the virtual device, to re-use
+ // it in the widget.
+ aRetval = aVirtualDevice.GetBitmap(Point(0, 0), aVirtualDevice.GetOutputSizePixel());
+
+== DrawingLayer glossary ==
+
+Primitives - classes that represent what should be drawn. It holds the data
+what to draw, but does not contain any kind of the rendering. Some of the
+primitives are 'Basic primitives', that are primitives that cannot be
+decomposed. The rest of the primitives can be decomposed to the basic
+primitives.
+
+Decomposition - a way how to break down the more complicated primitives into
+the basic primitives, and represent them via them; this logically makes the
+plain Primitive2DSequence display list a hierarchy.
+Eg. PolygonMarkerPrimitive2D can be decomposed to 2 hairlines
+PolyPolygonHairlinePrimitive2D's, each with different color.
+
+Processor - a class that goes through the hierarchy of the Primitives, and
+renders it some way. Various processors, like VclPixelProcessor2D (renders to
+the screen), VclMetafileProcessor2D (renders to the VCL metafile, eg. for
+printing), etc.
+
+== How to Implement a new Primitive ("something new to draw") ==
+
+* Create an ancestor of BasePrimitive2D
+ (or of its ancestor if it fits the purpose better)
+
+ * Assign it an ID [in drawinglayer_primitivetypes2d.hxx]
+
+ * Implement its decomposition
+ [virtual Primitive2DSequence create2DDecomposition(...)]
+
+* Extend the (various) processor(s)
+ If you need more than relying on just the decomposition
+
+== Where is DrawingLayer used ==
+
+* SdrObject(s) (rectangles, Circles, predefined shapes etc.)
+
+* Selections
+
+* Various smaller cases to 'just draw something'
+
+ * Draw to a virtual device, and use the resulting bitmap (like the example
+ above)
+
+* Custom widgets (like the Header / Footer indicator button)