viewer_thumbnail.py

def _get_viewer_thumbnail(self):
    """Creat an overall thumbnail based on the layer thumbnails."""
    thumbnail = np.zeros(layers.Layer._thumbnail_shape, dtype=np.uint8)

    for layer in self.layers[::-1]:
        if layer.blending == 'translucent':
            f_dest = thumbnail[..., 3][..., None] / 255
            f_source = 1 - layer.thumbnail[..., 3][..., None] / 255
        elif layer.blending == 'additive':
            f_dest = thumbnail[..., 3][..., None] / 255
            f_source = 1
        elif layer.blending == 'opaque':
            f_dest = 0
            f_source = 1
        else:
            raise ValueError(f'Layer blending mode not recognized, got {layer.blending}')
        thumbnail = thumbnail * f_dest + layer.thumbnail * f_source
        thumbnail = thumbnail.astype(np.uint8)
    return thumbnail