improve apa102 display fcn (#296)

* improve apa102 display fcn

Actually for APA102 LEDs, the start-frame needs to be zeros of 32bit size.
For the end-frame n/2 bits needs to be written (n is size of apa102 strip.
This improvement reduces the bytes needed to be written resulting in notable increase of speed.

* fix tests

* fix edge case

* formatting

---------

Co-authored-by: Richard Hull <[email protected]>

luma/led_matrix/device.py

@@ -32,6 +32,7 @@
 # As before, as soon as the with block completes, the canvas buffer is flushed
 # to the device
 
+from math import ceil
 import luma.core.error
 import luma.led_matrix.const
 from luma.core.interface.serial import noop
@@ -420,13 +421,13 @@ def display(self, image):
         assert image.size == self.size
         self._last_image = image.copy()
 
-        # Send zeros to reset, then pixel values then zeros at end
+        # Send 32 zero-bits to reset, then pixel values then n/2 zero-bits at end
         sz = image.width * image.height * 4
-        buf = bytearray(sz * 3)
+        buf = bytearray(4 + sz + ceil(image.width * image.height / 8 / 2))
 
         m = self._mapping
         for idx, (r, g, b, a) in enumerate(image.getdata()):
-            offset = sz + m[idx] * 4
+            offset = 4 + m[idx] * 4
             brightness = (a >> 4) if a != 0xFF else self._brightness
             buf[offset] = (0xE0 | brightness)
             buf[offset + 1] = b

tests/test_apa102.py

@@ -3,21 +3,26 @@
 # Copyright (c) 2014-18 Richard Hull and contributors
 # See LICENSE.rst for details.
 
+from math import ceil
 from luma.led_matrix.device import apa102
 from luma.core.render import canvas
 
 from helpers import serial, setup_function  # noqa: F401
 
 
-def padding(n):
-    return [0] * n * 4
+def start_frame():
+    return [0] * 4
+
+
+def end_frame(n):
+    return [0] * ceil(n / 2 / 8)
 
 
 def test_init_cascaded():
     device = apa102(serial, cascaded=7)
     assert device.width == 7
     assert device.height == 1
-    serial.data.assert_called_with(padding(7) + [0xE0, 0, 0, 0] * 7 + padding(7))
+    serial.data.assert_called_with(start_frame() + [0xE0, 0, 0, 0] * 7 + end_frame(7))
 
 
 def test_hide():
@@ -40,7 +45,9 @@ def test_contrast():
         draw.rectangle(device.bounding_box, outline="red")
     serial.reset_mock()
     device.contrast(0x6B)
-    serial.data.assert_called_with(padding(6) + [0xE6, 0, 0, 0xFF] * 6 + padding(6))
+    serial.data.assert_called_with(
+        start_frame() + [0xE6, 0, 0, 0xFF] * 6 + end_frame(6)
+    )
 
 
 def test_display():
@@ -50,4 +57,6 @@ def test_display():
     with canvas(device) as draw:
         draw.rectangle(device.bounding_box, outline=(0x11, 0x22, 0x33, 0x44))
 
-    serial.data.assert_called_with(padding(4) + [0xE4, 0x33, 0x22, 0x11] * 4 + padding(4))
+    serial.data.assert_called_with(
+        start_frame() + [0xE4, 0x33, 0x22, 0x11] * 4 + end_frame(4)
+    )