slides-alpha: Adapt some values based on resolution

When I rendered some slides in 4k, some of the values/radius where
not appropriate. So make those scale with the image resolution

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>

slides-alpha.py

@@ -69,11 +69,13 @@ def slide_process_single(in_file, out_file, opts):
 		y[-opts.bottom_border:, :] = np.zeros([opts.bottom_border, y.shape[1]], dtype=np.float32)
 
 	# Expand outward
-	ye = scipy.signal.convolve2d(y, np.ones([5,5]), mode='same', boundary='fill', fillvalue=0)
+	r = np.max(y.shape) // 384
+	ye = scipy.signal.convolve2d(y, np.ones([r,r]), mode='same', boundary='fill', fillvalue=0)
 	ye = ye.clip(0.0, 1.0)
 
-	# Blue it
-	ye = scipy.ndimage.gaussian_filter(ye, 1.5, mode='nearest')
+	# Blur it
+	r = np.max(y.shape) / 1280
+	ye = scipy.ndimage.gaussian_filter(ye, r, mode='nearest')
 
 	# Recombine
 	y = (y + ye).clip(0.0, 1.0)