Add grayscale image conversion (#71)

Author: ThomasR

epdlib/Screen.py

@@ -666,18 +666,18 @@ def _full_writeEPD_hd(self, image):
     def _full_writeEPD_non_hd(self, image):
         '''wipe screen and write an image'''
         if self.screen_type == ScreenType.FOUR_GRAYS:
-            image_buffer = self.epd.getbuffer_4Gray(image)
+            image_buffer = self.epd.getbuffer_4Gray(Screen.image_to_4_grays(image))
         else:
             image_buffer = self.epd.getbuffer(image)
 
         try:
             if self.screen_type == ScreenType.FOUR_GRAYS:
-                logging.debug('one-bit display')
+                logging.debug('4 grayscale display')
                 self.epd.display_4Gray(image_buffer)
             elif self.screen_type == ScreenType.THREE_COLORS: # displays that require multiple images
                 logging.debug('bi-color display')
                 self.epd.display(image_buffer, self.buffer_no_image)
-            else: # 7 color displays
+            else: # 7 color or monochrome displays
                 logging.debug('seven-color or monochrome display')
                 self.epd.display(image_buffer)
 
@@ -771,6 +771,25 @@ def module_exit(self):
                 logging.warning(f'failed to sleep module: {e}')
                 raise ScreenError(e)
 
+    @staticmethod
+    def image_to_4_grays(image):
+        logging.debug('converting image to 4 grays')
+
+        '''
+        Waveshare displays do not render colors accurately.
+        For example, an input color of "#808080" looks more like "#a9aca3" on the ePaper.
+        Therefore, we convert our images to grayscale in 2 steps:
+        1. Convert colors to their actual displayed output
+        2. Update palette to reverse the error
+        '''
+        palette = Screen.colors2palette(constants.COLORS_4GRAY_NATURAL.values())
+        image_gs = Screen.reduce_palette(image, palette, True)
+
+        ws_palette = Screen.colors2palette(constants.COLORS_4GRAY_WS.values())
+        image_gs.putpalette(ws_palette)
+        return image_gs
+
+
 
 # + code_folding=[]
 def list_compatible_modules(print_modules=True, reasons=False):

epdlib/constants.py

@@ -23,6 +23,26 @@
     'ORANGE': (255, 128, 0)
 }
 
+COLORS_4GRAY_WS = {
+    'BLACK': (0, 0, 0),
+    'DARK_GRAY': (0x80, 0x80, 0x80),
+    'BRIGHT_GRAY': (0xc0, 0xc0, 0xc0),
+    'WHITE': (0xff, 0xff, 0xff)
+}
+
+'''
+"natural" display colors of 4 grayscale displays.
+Tweaking these may improve image rendering.
+The present values are based on experiments with a 2.7" e-Paper HAT,  
+using various photographs and graphics as test inputs.
+'''
+COLORS_4GRAY_NATURAL = {
+    'BLACK': (0x1b, 0x1b, 0x13),
+    'DARK_GRAY': (0x4d, 0x4d, 0x44),
+    'BRIGHT_GRAY': (0xa9, 0xac, 0xa3),
+    'WHITE': (0xeb, 0xea, 0xdf)
+}
+
 CLEAR_COLOR = 0xFF
 
 LAYOUT_DEFAULTS = {