Adding upstream version 1:7.0.4.upstream/1%7.0.4upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 35 insertions, 0 deletions

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+This is code for using the Skia library as a drawing library in VCL backends.
+See external/skia for info on the library itself.
+
+Environment variables:
+======================
+
+See README.vars in the toplevel vcl/ directory. Note that many backends do not
+use Skia. E.g. on Linux it is necessary to also use SAL_USE_VCLPLUGIN=gen .
+
+There are also GUI options for controlling whether Skia is enabled.
+
+Skia drawing methods:
+=====================
+
+Skia supports several methods to draw:
+- Raster - CPU-based drawing (here primarily used for debugging)
+- Vulkan - Vulkan-based GPU drawing, this is the default
+
+There are more (OpenGL, Metal on Mac, etc.), but (as of now) they are not supported by VCL.
+
+GrContext sharing:
+==================
+
+We use Skia's sk_app::WindowContext class for creating surfaces for windows, that class
+takes care of the internals. But of offscreen drawing, we need an instance of class
+GrContext. There is sk_app::WindowContext::getGrContext(), but each instance creates
+its own GrContext, and apparently it does not work to mix them. Which means that
+for offscreen drawing we would need to know which window (and only that window)
+the contents will be eventually painted to, which is not possible (it may not even
+be known at the time).
+
+To solve this problem we patch sk_app::WindowContext to create just one GrContext object
+and share it between instances. Additionally, using sk_app::WindowContext::SharedGrContext
+it is possible to share it also for offscreen drawing, including keeping proper reference
+count.