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 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

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 lv_draw_rect_dsc_t, for lv_draw_line it's called 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 lv_disp_drv_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_drv_t * disp_drv, lv_draw_ctx_t * draw_ctx) Callback to initialize a draw_ctx

  • void (*draw_ctx_deinit)(struct _lv_disp_drv_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. sizeof(lv_draw_sw_ctx_t)

When you ignore these fields, LVGL will set default values for callbacks and size in lv_disp_drv_init() based on the configuration in lv_conf.h. lv_disp_drv_register() will allocate a draw_ctx based on draw_ctx_size and call draw_ctx_init() on it.

However, you can overwrite the callbacks and the size values before calling 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 lv_draw_ctx_t structure and sets the draw callbacks. It looks like this:

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 draw_ctx_init() like:

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 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. 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 lv_draw_sw_ctx_t:


/*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_drv_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 lv_disp_draw_init(&drv) you can assign the new draw_ctx_init callback and set draw_ctx_size to overwrite the defaults:

static lv_disp_drv_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:

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:

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:

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 lv_draw_ctx_t (instead of lv_draw_sw_ctx_t) and extend/initialize it as you wish.