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→Stateless vs stateful
* [[Platform/GFX/AzureConversion|Azure Conversion]]
== Stateless Stateful vs stateless == Many of the performance gains anticipated from Moz2D come about due to its mostly-stateless API. === Stateful APIs === The Thebes API wrapped the Cairo graphics library which uses a stateful context model much like Postscript or Quartz 2D. To draw content, you make individual API calls to set up various bits of state in a context object, followed by another API call to actually perform drawing. For example, to stroke a shape with a dashed line, you would typically set the color, set the line width, set the dashing style, start a new path, emit path segments, and finally draw the stroke—''all as separate API calls''. In Cairo, even drawing an image requires the caller to set the source surface, emit a rectangle path, and fill—at least 6 API calls. The overhead of setting up state can be justified when that state is re-used, but for CSS rendering this is typically not the case. Rather, for each item that is drawn fresh CSS values are fetched and re-set. Because the browser does not know what it is drawing, it is unable to group operations and re-use state effectively. On the other hand, the HTML5 canvas element has a stateful API but because its state-tracking differs from that of Cairo drawing state must be tracked twice: once for canvas and once in Cairo. A further complication specific to Cairo arises because internally Cairo uses a ''stateless'' surface API. On OS X, Cairo uses its Quartz backend for all drawing. However, when using Cairo, despite the fact that both Firefox and Safari ultimately use Quartz as their backend for drawing, Safari is faster than Firefox on some demos. We believe one reason this is the case is because Quartz is stateful. As a result, Cairo needs to convert from its stateful API to its internal, stateless surface API, then back to Quartz’s stateful API. The Moz2D API, despite being mostly-stateless, was designed to map onto stateful APIs like Quartz in a more efficient manner. === Moz2D: Mostly-stateless === Almost all the operations on an Moz2D DrawTarget (see [[#Draw targets]] below) take relevant state as parameters and do actual drawing. The only state carried by a DrawTarget is the destination surface itself plus a current transform and a current clip stack. We let the transform and clip state remain in the DrawTarget because those are the only pieces of state not constantly reset by CSS rendering. Our CSS rendering needs to always render under some given transform and clip and we don't want all our rendering code to have to pass those around all the time. Because of this, the Moz2D API is called ''mostly-stateless''.
== Learning Moz2D: Introducing the API ==
