With LVGL, you don't need to draw anything manually. Just create objects (like buttons, labels, arc, etc), move and change them, and LVGL will refresh and redraw what is required.
However, it might be useful to have a basic understanding of how drawing happens in LVGL to add customization, make it easier to find bugs or just out of curiosity.
The basic concept is to not draw directly to the screen, but draw to an internal draw buffer first. When drawing (rendering) is ready, that buffer is copied to the screen.
1. It avoids flickering while the layers of the UI are drawn. For example, if LVGL drawn directly into the display, when drawing a *background + button + text*, each "stage" would be visible for a short time .
2. It's faster to modify a buffer in internal RAM and finally write one pixel only once than reading/writing the display directly on each pixel access.
one holds the current image to show on the display, and rendering happens to the other (inactive) frame buffer, and they are swapped when the rendering is finished.
The main difference is that with LVGL you don't have to store 2 frame buffers (which usually requires external RAM) but only smaller draw buffer(s) that can easily fit into the internal RAM too.
2. LVGL saves the changed object's old and new area into a buffer, called an *Invalid area buffer*. For optimization, in some cases, objects are not added to the buffer:
- Hidden objects are not added.
- Objects completely out of their parent are not added.
For example, if a button's label has changed, the library will see that it's enough to draw the button under the text, and that it's not required to draw the screen under the button too.
2.**Two buffers** - LVGL can immediately draw to the second buffer when the first is sent to `flush_cb` because the flushing should be done by DMA (or similar hardware) in the background.
3.**Double buffering** - `flush_cb` should only swap the address of the frame buffer.
1.**Prepare the draw descriptors** Create a draw descriptor from an object's styles (e.g. `lv_draw_rect_dsc_t`). This gives us the parameters for drawing, for example the colors, widths, opacity, fonts, radius, etc.
2.**Call the draw function** Call the draw function with the draw descriptor and some other parameters (e.g. `lv_draw_rect()`). It will render the primitive shape to the current draw buffer.
3.**Create masks** If the shape is very simple and doesn't require masks go to #5. Else create the required masks in the draw function. (e.g. a rounded rectangle mask)
4.**Calculate all the added mask** It creates 0..255 values into a *mask buffer* with the "shape" of the created masks.
E.g. in case of a "line mask" according to the parameters of the mask, keep one side of the buffer as it is (255 by default) and set the rest to 0 to indicate that this side should be removed.
5.**Blend a color or image** During blending masks (make some pixels transparent or opaque), blending modes (additive, subtractive, etc) and opacity are handled.
-`LV_DRAW_MASK_TYPE_RADIUS` Removes the inner or outer parts of a rectangle which can have radius. It's also used to create circles by setting the radius to large value (`LV_RADIUS_CIRCLE`)
Every mask type has a related paramter to describe the mask's data. The following paramater types exist:
-`lv_draw_mask_line_param_t`
-`lv_draw_mask_radius_param_t`
-`lv_draw_mask_angle_param_t`
-`lv_draw_mask_fade_param_t`
-`lv_draw_mask_map_param_t`
1. Initialize a mask parameter with `lv_draw_mask_<type>_init`. See `lv_draw_mask.h` for the whole API.
2. Add the mask parameter to the draw engine with `int16_t mask_id = lv_draw_mask_add(¶m, ptr)`. `ptr` can be any pointer to identify the mask, (`NULL` if unused).
3. Call the draw functions
4. Remove the mask from the draw engine with `lv_draw_mask_remove_id(mask_id)` of `lv_draw_mask_remove_custom(ptr)`.
5. Free the parameter with `lv_draw_mask_free_param(¶m)`.
A parameter can be added and removed any number of times but it needs to be freed when not required anymore.
`lv_draw_mask_add` saves only the pointer of the mask so the parameter needs to be valid while in use.
Some fields of these parameters can be modified to draw something else or any custom drawing can be added manually.
A good use case for it is the [Button matrix](/widgets/core/btnmatrix) widget. By default its buttons can be styled in different states but you can't style the buttons one by one.
However, an event is sent for every button and you can for example tell LVGL to use different colors on a specific button or to manually draw an image on some buttons.
`lv_event_get_clip_area(event)` can be used to get the current clip area. The clip area is required in draw functions to make them draw only on a limited area.
The actual drawing of the object happens in this event. E.g. a rectangle for a button is drawn here. First, the widgets' internal events are called to perform drawing and after that you can draw anything on top of them.
For example you can add a custom text or an image.
Called when the main drawing is finished. You can draw anything here as well and it's also good place to remove the masks created in `LV_EVENT_DRAW_MAIN_BEGIN`.
Post drawing events are called when all the children of an object are drawn. For example LVGL use the post drawing phase to draw the scrollbars because they should be above all the children.
When LVGL draws a part of an object (e.g. a slider's indicator, a table's cell or a button matrix's button) it sends events before and after drawing that part with some context of the drawing.
`lv_event_get_cover_area(event)` returns an pointer to an area to check and `lv_event_set_cover_res(event, res)` can be used to set one of these results:
-`LV_COVER_RES_COVER` the areas is fully covered by the object
-`LV_COVER_RES_NOT_COVER` the areas is not covered by the object
E.g. if a widget has > 0 radius it might not cover an area but you need to handle `radius` only if you will modify it and the widget can't know about it.
If you need to draw outside of a widget LVGL needs to know about it to provide the extra space for drawing.
Let's say you create an event the writes the current value of a slider above its knob. In this case LVGL needs to know that the slider's draw area should be larger with the size required for the text.
