lvgl/examples/widgets/canvas/lv_example_canvas_2.c

#include "../../lv_examples.h"
#if LV_USE_CANVAS && LV_BUILD_EXAMPLES

#define CANVAS_WIDTH  80
#define CANVAS_HEIGHT  40

/**
 * Create a transparent canvas with transparency
 */
void lv_example_canvas_2(void)
{
    lv_obj_set_style_bg_color(lv_screen_active(), lv_palette_lighten(LV_PALETTE_RED, 5), 0);

    /*Create a buffer for the canvas*/
    static uint8_t cbuf[CANVAS_WIDTH * CANVAS_HEIGHT * 4];

    /*Create a canvas and initialize its palette*/
    lv_obj_t * canvas = lv_canvas_create(lv_screen_active());
    lv_canvas_set_buffer(canvas, cbuf, CANVAS_WIDTH, CANVAS_HEIGHT, LV_COLOR_FORMAT_ARGB8888);
    lv_obj_center(canvas);

    /*Red background (There is no dedicated alpha channel in indexed images so LV_OPA_COVER is ignored)*/
    lv_canvas_fill_bg(canvas, lv_palette_main(LV_PALETTE_BLUE), LV_OPA_COVER);

    /*Create hole on the canvas*/
    uint32_t x;
    uint32_t y;
    for(y = 10; y < 20; y++) {
        for(x = 5; x < 75; x++) {
            lv_canvas_set_px(canvas, x, y, lv_palette_main(LV_PALETTE_BLUE), LV_OPA_50);
        }
    }

    for(y = 20; y < 30; y++) {
        for(x = 5; x < 75; x++) {
            lv_canvas_set_px(canvas, x, y, lv_palette_main(LV_PALETTE_BLUE), LV_OPA_20);
        }
    }

    for(y = 30; y < 40; y++) {
        for(x = 5; x < 75; x++) {
            lv_canvas_set_px(canvas, x, y, lv_palette_main(LV_PALETTE_BLUE), LV_OPA_0);
        }
    }
}
#endif