pebble bitmap xor or mask? - pebble-sdk

Question

On the Pebble watch I am trying to overwrite a bitmap layer with text so the text is written white over black areas and black over white areas.
In other environments I would do this with an XOR operation, or create a mask and perform a couple of writes after masking out what I don't want overwritten.
I don't see an XOR graphics operator or a mask operator in the Pebble graphics library.
How can this be done?
I'm using C and CloudPebble.
Lynd

Answer 1

Here is a routine to do it. The text layer must be 'under' the graphic layer, then you use layer_set_update_proc() to set the update procedure for the graphic layer to the routine below.
static void bitmapLayerUpdate(struct Layer *layer, GContext *ctx){
GBitmap *framebuffer;
const GBitmap *graphic = bitmap_layer_get_bitmap((BitmapLayer *)layer);
int height;
uint32_t finalBits;
uint32_t *bfr, *bitmap;
framebuffer = graphics_capture_frame_buffer(ctx);
if (framebuffer == NULL){
APP_LOG(APP_LOG_LEVEL_DEBUG, "capture frame buffer failed!!");
} else {
// APP_LOG(APP_LOG_LEVEL_DEBUG, "capture frame buffer succeeded");
}
height = graphic->bounds.size.h;
for (int yindex =0; yindex < height; yindex++){
for ( unsigned int xindex = 0; xindex < (graphic->row_size_bytes); xindex+=4){
bfr = (uint32_t*)((framebuffer->addr)+(yindex * framebuffer->row_size_bytes)+xindex);
bitmap = (uint32_t*)((graphic->addr)+(yindex * graphic->row_size_bytes)+xindex);
finalBits = *bitmap ^ *bfr;
// APP_LOG(APP_LOG_LEVEL_DEBUG, "bfr: %0x, bitmsp: %0x, finalBits: %x", (unsigned int)bfr, (unsigned int)bitmap, finalBits );
*bfr = finalBits;
}
}
graphics_release_frame_buffer(ctx, framebuffer);
}