Drawing Pipeline

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LVGL has a flexible and extendable drawing pipeline. You can hook it to do some rendering with a GPU or even completely replace the built-in software renderer.


LVGL 拥有一个灵活且可扩展的绘制管线。您可以通过挂钩将其与GPU一起渲染，甚至完全替换内置的软件渲染器。

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Overview

Draw task

When lv_draw_rect, lv_draw_label or similar functions are called LVGL creates a so called draw task.

Draw unit

The draw tasks are collected in a list and periodically dispatched to draw units. A draw unit can a CPU core, a GPU, just a new rendering library for certain or all draw tasks, or basically anything that can draw somehow.

Draw task evaluation

Different draw units might render slight different output for example for an image transformation or a gradient. If such a draw task were assigned to a different draw units, the screen might jitter a little bit. To resolve it each draw unit has an evaluate_cb which is called when a draw task is created. Based on the type and parameters of the draw task each draw unit can decide if it want to assign the draw task to itself. This way a certain type of draw task (e.g. rounded rectangle with horizontal gradient) will be always assigned to the same draw unit. It avoid the above mentioned issue of slight difference between draw units.

Dispatching

While collecting draw tasks LVGL frequently tries to dispatch the collected draw tasks to the draw units. This handles via the dispatch_cb of the draw units.

If a draw unit is busy with another draw task, it just returns. However, it is available it can take a draw task.

lv_draw_get_next_available_task(layer, previous_task, draw_unit_id) is a useful helper function which returns an available draw task. "Available draw task" means that, all the draw tasks which should be drawn under a draw task are ready and it is assigned to the given draw unit.

Layers

A layer is a buffer with a given area on which rendering happens. Each display has a "main" layer, but during rendering additional layers might be created internally to handle for example arbitrary widget transformations.

Hierarchy of modules

All these together looks like this

  • list of draw units

  • display(s)

    • layer(s): Each display has its own list of layers

      • draw tasks: Each layer has its own list of draw tasks