Home > backend, GSOC2015, kivy, matplotlib > Writing a matplotlib renderer for Kivy

Writing a matplotlib renderer for Kivy

A Renderer is a class that knows all the graphics instructions to draw in a canvas. The first implementation involved to use the canonical agg renderer to get a figure which we could texture in a rectangle, this is fully explained in a previous post. The objective of my implementation is to have a backend implemented using kivy graphics instructions. The RendererBase class defines all the required methods to do so. RendererKivy extends from it and implements draw_path, other 5 different methods can be implemented but the most important is draw_path.

In this method all the required information to draw on the canvas is provided however there is another class as well required to change the style of elements rendered. This class is GraphicsContext which is a middle class between a renderer and an Artist. Artist is one of the layers in the hierarchical architecture of matplotlib, the other two layers are backend and scripting. Basically, everything that can be drawn in a canvas is an Artist object such as Axis, Tick, Figure, etc. A GraphicsContextKivy is defined which extends from GraphicsContextBase and specifically translates matplotlib context instructions such as line width, line style, caps, joints, colors, etc. into the equivalent in Kivy. As can be seen in the figure below the lines are dashed and the foreground of the canvas was changed.

backend kivy

At the moment there are three issues: the first one is that kivy implements caps for lines {‘square’, ’round’, ‘none’} and matplotlib {‘butt’, ‘projecting’, ’round’}. the second issue is that dashes in kivy are supported depending on the line width and finally, kivy does not support dynamic values for dash offset and dash length. The renderer, receives this object GraphicsContext and applies such styles defined there into kivy context and vertex instructions. As can be seen in the figure a very simple drawing of the information received in the draw_path is performed. Basically lines are created with all the vertex received but a path is more complex than that and represents a series of possibly disconnected, possibly closed, line and curve segments. The next post will be about this class Path and which kivy instructions are being used to implement the renderer.

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