Pebble smartwatch SDK offers a very extensive graphics library. And if that is not enough – you can access graphics memory directly for pixel-precision manipulation. For example this code

GBitmap *fb = graphics_capture_frame_buffer_format(ctx, GBitmapFormat8Bit); uint8_t *fb_data = gbitmap_get_data(fb);

captures Pebble screen as a bitmap and consequently as raw uint8_t data you can manipulate. But the problem with this approach – you access the data as 1-dimensional array, which is very inconvinient when you’re dealing with 2-dimensional screen.

Fortunately due to magic of casting this problem can be addressed. Consider following addition to previous code:

#define WINDOW_WIDTH 144 uint8_t (*fb_matrix)[WINDOW_WIDTH] = (uint8_t (*)[WINDOW_WIDTH]) fb_data;

Using this, you can access screen data via familiar coordinates. For example if you need to set pixel at coordinates Y=120, X=60 to black color, all you have to do is

fb_matrix[120][60] = 0;

And now we can rewrite InverterLayer from previous post to a simpler form. First in layer callback we create bitmap matrix and call effect function:

static void effect_layer_update_proc(Layer *me, GContext* ctx) { // getting layer coordinates GRect layer_frame = layer_get_frame(me); //capturing framebuffer bitmap into 2-d matix GBitmap *fb = graphics_capture_frame_buffer_format(ctx, GBitmapFormat8Bit); uint8_t (*fb_matrix)[WINDOW_WIDTH] = (uint8_t (*)[WINDOW_WIDTH]) gbitmap_get_data(fb); //callig effect function effect_invert(fb_matrix, layer_frame); //releasing framebuffer graphics_release_frame_buffer(ctx, fb); }

And actual inverting function becomes much simpler as well, simple loop thru layer pixel, inverting colors:

// position: x,y,h,w of the layer void effect_invert(uint8_t (*fb_a)[WINDOW_WIDTH], GRect position) { for (int y = 0; y < position.size.h; y++) for (int x = 0; x < position.size.w; x++) fb_a[y + position.origin.y][x + position.origin.x] = ~fb_a[y + position.origin.y][x + position.origin.x]; }

You can see this approach in EffectLayer library.