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+# Drawing API
+
+Drawing API that can specify what to draw via a kind of display list.
+
+Example of the DrawingLayer use is eg. in `svx/source/xoutdev/xtabhtch.cxx:121`.
+A stripped down version with extended comments:
+
+     // 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.
+     std::unique_ptr<drawinglayer::processor2d::BaseProcessor2D> pProcessor2D(
+         drawinglayer::processor2d::createPixelProcessor2DFromOutputDevice(...));
+
+     // 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)
+
+## Dumping DrawingLayer Primitives as XML
+
+For debugging purposes, it is possible to dump the drawinglayer primitives as
+an xml file. The drawinglayer xml dump can show possible problems with the
+rendering.
+
+For example, in `emfio/qa/cppunit/emf/EmfImportTest.cxx`, one can write:
+
+    Primitive2DSequence aSequence = parseEmf(u"emfio/qa/cppunit/wmf/data/stockobject.emf");
+    drawinglayer::Primitive2dXmlDump dumper;
+    Primitive2DContainer aContainer(aSequence);
+    dumper.dump(aContainer, "/tmp/drawyinglayer.xml");
+
+Then, after invoking `make CppunitTest_emfio_emf`, `/tmp/drawyinglayer.xml` will
+be the dump of the drawinglayer primitives used to draw the emf file in
+LibreOffice. The top level tag will be <primitive2D>.