============== Add custom GPU ============== LVGL has a flexible and extendable draw pipeline. You can hook it to do some rendering with a GPU or even completely replace the built-in software renderer. Draw context ************ The core structure of drawing is :cpp:type:`lv_draw_ctx_t`. It contains a pointer to a buffer where drawing should happen and a couple of callbacks to draw rectangles, texts, and other primitives. Fields ------ :cpp:type:`lv_draw_ctx_t` has the following fields: - ``void * buf`` Pointer to a buffer to draw into - ``lv_area_t * buf_area`` The position and size of ``buf`` (absolute coordinates) - ``const lv_area_t * clip_area`` The current clip area with absolute coordinates, always the same or smaller than ``buf_area``. All drawings should be clipped to this area. - ``void (*draw_rect)()`` Draw a rectangle with shadow, gradient, border, etc. - ``void (*draw_arc)()`` Draw an arc - ``void (*draw_img_decoded)()`` Draw an (A)RGB image that is already decoded by LVGL. - ``lv_res_t (*draw_img)()`` Draw an image before decoding it (it bypasses LVGL's internal image decoders) - ``void (*draw_letter)()`` Draw a letter - ``void (*draw_line)()`` Draw a line - ``void (*draw_polygon)()`` Draw a polygon - ``void (*draw_bg)()`` Replace the buffer with a rect without decoration like radius or borders. - ``void (*wait_for_finish)()`` Wait until all background operation are finished. (E.g. GPU operations) - ``void * user_data`` Custom user data for arbitrary purpose (For the sake of simplicity the parameters of the callbacks are not shown here.) All ``draw_*`` callbacks receive a pointer to the current ``draw_ctx`` as their first parameter. Among the other parameters there is a descriptor that tells what to draw, e.g. for ``draw_rect`` it's called :cpp:struct:`lv_draw_rect_dsc_t`, for :cpp:func:`lv_draw_line` it's called :cpp:struct:`lv_draw_line_dsc_t`, etc. To correctly render according to a ``draw_dsc`` you need to be familiar with the `Boxing model `__ of LVGL and the meanings of the fields. The name and meaning of the fields are identical to name and meaning of the `Style properties `__. Initialization -------------- The :cpp:type:`lv_disp_t` has 4 fields related to the draw context: - ``lv_draw_ctx_t * draw_ctx`` Pointer to the ``draw_ctx`` of this display - ``void (*draw_ctx_init)(struct _lv_disp_t * disp_drv, lv_draw_ctx_t * draw_ctx)`` Callback to initialize a ``draw_ctx`` - ``void (*draw_ctx_deinit)(struct _lv_disp_t * disp_drv, lv_draw_ctx_t * draw_ctx)`` Callback to de-initialize a ``draw_ctx`` - ``size_t draw_ctx_size`` Size of the draw context structure. E.g. :cpp:expr:`sizeof(lv_draw_sw_ctx_t)` When you ignore these fields, LVGL will set default values for callbacks and size in :cpp:func:`lv_disp_drv_init` based on the configuration in ``lv_conf.h``. :cpp:func:`lv_disp_drv_register` will allocate a ``draw_ctx`` based on ``draw_ctx_size`` and call :cpp:func:`draw_ctx_init` on it. However, you can overwrite the callbacks and the size values before calling :cpp:func:`lv_disp_drv_register`. It makes it possible to use your own ``draw_ctx`` with your own callbacks. Software renderer ***************** LVGL's built in software renderer extends the basic :cpp:type:`lv_draw_ctx_t` structure and sets the draw callbacks. It looks like this: .. code:: c typedef struct { /** Include the basic draw_ctx type*/ lv_draw_ctx_t base_draw; /** Blend a color or image to an area*/ void (*blend)(lv_draw_ctx_t * draw_ctx, const lv_draw_sw_blend_dsc_t * dsc); } lv_draw_sw_ctx_t; Set the draw callbacks in :cpp:func:`draw_ctx_init` like: .. code:: c draw_sw_ctx->base_draw.draw_rect = lv_draw_sw_rect; draw_sw_ctx->base_draw.draw_letter = lv_draw_sw_letter; ... Blend callback -------------- As you saw above the software renderer adds the ``blend`` callback field. It's a special callback related to how the software renderer works. All draw operations end up in the ``blend`` callback which can either fill an area or copy an image to an area by considering an optional mask. The :cpp:struct:`lv_draw_sw_blend_dsc_t` parameter describes what and how to blend. It has the following fields: - ``const lv_area_t * blend_area`` The area with absolute coordinates to draw on ``draw_ctx->buf``. If ``src_buf`` is set, it's the coordinates of the image to blend. - ``const lv_color_t * src_buf`` Pointer to an image to blend. If set, ``color`` is ignored. If not set fill ``blend_area`` with ``color`` - ``lv_color_t color`` Fill color. Used only if ``src_buf == NULL`` - ``lv_opa_t * mask_buf`` NULL if ignored, or an alpha mask to apply on ``blend_area`` - ``lv_draw_mask_res_t mask_res`` The result of the previous mask operation. (``LV_DRAW_MASK_RES_...``) - ``const lv_area_t * mask_area`` The area of ``mask_buf`` with absolute coordinates - ``lv_opa_t opa`` The overall opacity - ``lv_blend_mode_t blend_mode`` E.g. :cpp:enumerator:`LV_BLEND_MODE_ADDITIVE` Extend the software renderer **************************** New blend callback ------------------ Let's take a practical example: you would like to use your MCUs GPU for color fill operations only. As all draw callbacks call ``blend`` callback to fill an area in the end only the ``blend`` callback needs to be overwritten. First extend :cpp:struct:`lv_draw_sw_ctx_t`: .. code:: c /*We don't add new fields, so just for clarity add new type*/ typedef lv_draw_sw_ctx_t my_draw_ctx_t; void my_draw_ctx_init(lv_disp_t * drv, lv_draw_ctx_t * draw_ctx) { /*Initialize the parent type first */ lv_draw_sw_init_ctx(drv, draw_ctx); /*Change some callbacks*/ my_draw_ctx_t * my_draw_ctx = (my_draw_ctx_t *)draw_ctx; my_draw_ctx->blend = my_draw_blend; my_draw_ctx->base_draw.wait_for_finish = my_gpu_wait; } After calling :cpp:expr:`lv_disp_draw_init(&drv)` you can assign the new :cpp:func:`draw_ctx_init` callback and set ``draw_ctx_size`` to overwrite the defaults: .. code:: c static lv_disp_t drv; lv_disp_draw_init(&drv); drv->hor_res = my_hor_res; drv->ver_res = my_ver_res; drv->flush_cb = my_flush_cb; /*New draw ctx settings*/ drv->draw_ctx_init = my_draw_ctx_init; drv->draw_ctx_size = sizeof(my_draw_ctx_t); lv_disp_drv_register(&drv); This way when LVGL calls ``blend`` it will call ``my_draw_blend`` and we can do custom GPU operations. Here is a complete example: .. code:: c void my_draw_blend(lv_draw_ctx_t * draw_ctx, const lv_draw_sw_blend_dsc_t * dsc) { /*Let's get the blend area which is the intersection of the area to fill and the clip area.*/ lv_area_t blend_area; if(!_lv_area_intersect(&blend_area, dsc->blend_area, draw_ctx->clip_area)) return; /*Fully clipped, nothing to do*/ /*Fill only non masked, fully opaque, normal blended and not too small areas*/ if(dsc->src_buf == NULL && dsc->mask == NULL && dsc->opa >= LV_OPA_MAX && dsc->blend_mode == LV_BLEND_MODE_NORMAL && lv_area_get_size(&blend_area) > 100) { /*Got the first pixel on the buffer*/ lv_coord_t dest_stride = lv_area_get_width(draw_ctx->buf_area); /*Width of the destination buffer*/ lv_color_t * dest_buf = draw_ctx->buf; dest_buf += dest_stride * (blend_area.y1 - draw_ctx->buf_area->y1) + (blend_area.x1 - draw_ctx->buf_area->x1); /*Make the blend area relative to the buffer*/ lv_area_move(&blend_area, -draw_ctx->buf_area->x1, -draw_ctx->buf_area->y1); /*Call your custom gou fill function to fill blend_area, on dest_buf with dsc->color*/ my_gpu_fill(dest_buf, dest_stride, &blend_area, dsc->color); } /*Fallback: the GPU doesn't support these settings. Call the SW renderer.*/ else { lv_draw_sw_blend_basic(draw_ctx, dsc); } } The implementation of wait callback is much simpler: .. code:: c void my_gpu_wait(lv_draw_ctx_t * draw_ctx) { while(my_gpu_is_working()); /*Call SW renderer's wait callback too*/ lv_draw_sw_wait_for_finish(draw_ctx); } New rectangle drawer -------------------- If your MCU has a more powerful GPU that can draw e.g. rounded rectangles you can replace the original software drawer too. A custom ``draw_rect`` callback might look like this: .. code:: c void my_draw_rect(lv_draw_ctx_t * draw_ctx, const lv_draw_rect_dsc_t * dsc, const lv_area_t * coords) { if(lv_draw_mask_is_any(coords) == false && dsc->grad == NULL && dsc->bg_img_src == NULL && dsc->shadow_width == 0 && dsc->blend_mode = LV_BLEND_MODE_NORMAL) { /*Draw the background*/ my_bg_drawer(draw_ctx, coords, dsc->bg_color, dsc->radius); /*Draw the border if any*/ if(dsc->border_width) { my_border_drawer(draw_ctx, coords, dsc->border_width, dsc->border_color, dsc->border_opa) } /*Draw the outline if any*/ if(dsc->outline_width) { my_outline_drawer(draw_ctx, coords, dsc->outline_width, dsc->outline_color, dsc->outline_opa, dsc->outline_pad) } } /*Fallback*/ else { lv_draw_sw_rect(draw_ctx, dsc, coords); } } ``my_draw_rect`` can fully bypass the use of ``blend`` callback if needed. Fully custom draw engine ************************ For example if your MCU/MPU supports a powerful vector graphics engine you might use only that instead of LVGL's SW renderer. In this case, you need to base the renderer on the basic :cpp:type:`lv_draw_ctx_t` (instead of :cpp:struct:`lv_draw_sw_ctx_t`) and extend/initialize it as you wish. API ***