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Co-authored-by: Kevin Schlosser <kdschlosser@users.noreply.github.com> Co-authored-by: Liam <30486941+liamHowatt@users.noreply.github.com>
82 lines
2.5 KiB
ReStructuredText
82 lines
2.5 KiB
ReStructuredText
.. _porting_draw:
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==========
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Custom GPU
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==========
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LVGL has a flexible and extendable draw pipeline. You can hook it to do
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some rendering with a GPU or even completely replace the built-in
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software renderer.
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Overview
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********
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Draw task
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---------
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When :cpp:expr:`lv_draw_rect`, :cpp:expr:`lv_draw_label` or similar functions are called
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LVGL creates a so called draw task.
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Draw unit
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---------
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The draw tasks are collected in a list and periodically dispatched to draw units. A
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draw unit can a CPU core, a GPU, just a new rendering library for certain or all draw tasks,
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or basically anything that can draw somehow.
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Draw task evaluation
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--------------------
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Different draw units might render slight different output for example for an image transformation or
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a gradient. If such a draw task were assigned to a different draw units, the screen might jitter a
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little bit. To resolve it each draw unit has an ``evaluate_cb`` which is called when a draw task is created.
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Based on the type and parameters of the draw task each draw unit can decide if it want to assign the
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draw task to itself. This way a certain type of draw task (e.g. rounded rectangle with horizontal
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gradient) will be always assigned to the same draw unit. It avoid the above mentioned issue of
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slight difference between draw units.
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Dispatching
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-----------
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While collecting draw tasks LVGL frequently tries to dispatch the collected draw tasks to the draw units.
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This handles via the ``dispatch_cb`` of the draw units.
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If a draw unit is busy with another draw task, it just returns. However, it is available it can take a draw task.
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:cpp:expr:`lv_draw_get_next_available_task(layer, previous_task, draw_unit_id)` is a useful helper function which
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returns an available draw task. "Available draw task" means that, all the draw tasks which should be drawn under a draw task
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are ready and it is assigned to the given draw unit.
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Layers
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------
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A layer is a buffer with a given area on which rendering happens. Each display has a "main" layer, but
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during rendering additional layers might be created internally to handle for example arbitrary widget transformations.
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Hierarchy of modules
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--------------------
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All these together looks like this
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- list of draw units
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- display(s)
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- layer(s): Each display has its own list of layers
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- draw tasks: Each layer has its own list of draw tasks
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References
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**********
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As a reference take a look at `lv_draw_sw.c <https://github.com/lvgl/lvgl/blob/master/src/draw/sw/lv_draw_sw.c>`__
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API
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***
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