Dither colors in non-default low depth color path

This patch modifies the low color mode (slow) path
to dither the colors before flushing them to the
scanout buffer.  This improves the output quality
in those corner cases.

diff --git a/src/libply/ply-frame-buffer.c b/src/libply/ply-frame-buffer.c
index d80a6cdbcbb2a98f1fb8bbee582ba9b878161c5e..dd26646962ad823a307d3d9c5864438ebfaf16df 100644 (file)
--- a/src/libply/ply-frame-buffer.c
+++ b/src/libply/ply-frame-buffer.c
@@ -68,6 +68,10 @@ struct _ply_frame_buffer
   uint32_t bits_for_blue;
   uint32_t bits_for_alpha;
 
+  int32_t dither_red;
+  int32_t dither_green;
+  int32_t dither_blue;
+
   unsigned int bytes_per_pixel;
   unsigned int row_stride;
 
@@ -315,6 +319,10 @@ ply_frame_buffer_query_device (ply_frame_buffer_t *buffer)
   buffer->bytes_per_pixel = variable_screen_info.bits_per_pixel >> 3;
   buffer->row_stride = fixed_screen_info.line_length / buffer->bytes_per_pixel;
   buffer->size = buffer->area.height * buffer->row_stride * buffer->bytes_per_pixel;
+  
+  buffer->dither_red = 0;
+  buffer->dither_green = 0;
+  buffer->dither_blue = 0;
 
   if (buffer->bytes_per_pixel == 4 &&
       buffer->red_bit_position == 16 && buffer->bits_for_red == 8 &&
@@ -340,24 +348,37 @@ ply_frame_buffer_map_to_device (ply_frame_buffer_t *buffer)
   return buffer->map_address != MAP_FAILED;
 }
 
-__attribute__((__pure__))
 static inline uint_fast32_t 
 ply_frame_buffer_pixel_value_to_device_pixel_value (ply_frame_buffer_t *buffer,
                                                     uint32_t        pixel_value)
 {
   uint8_t r, g, b, a;
+  int orig_r, orig_g, orig_b, orig_a;
+  int i;
+  
+  orig_a = pixel_value >> 24; 
+  a = orig_a >> (8 - buffer->bits_for_alpha);
 
-  a = pixel_value >> 24; 
-  a >>= (8 - buffer->bits_for_alpha);
-
-  r = (pixel_value >> 16) & 0xff; 
-  r >>= (8 - buffer->bits_for_red);
+  orig_r = ((pixel_value >> 16) & 0xff) - buffer->dither_red; 
+  r = CLAMP(orig_r, 0, 255) >> (8 - buffer->bits_for_red);
 
-  g = (pixel_value >> 8) & 0xff; 
-  g >>= (8 - buffer->bits_for_green);
+  orig_g = ((pixel_value >> 8) & 0xff) - buffer->dither_green;
+  g = CLAMP(orig_g, 0, 255) >> (8 - buffer->bits_for_green);
 
-  b = pixel_value & 0xff; 
-  b >>= (8 - buffer->bits_for_blue);
+  orig_b = (pixel_value & 0xff) - buffer->dither_blue;
+  b = CLAMP(orig_b, 0, 255) >> (8 - buffer->bits_for_blue);
+  
+  uint8_t new_r = r << (8 - buffer->bits_for_red);
+  uint8_t new_g = g << (8 - buffer->bits_for_green);
+  uint8_t new_b = b << (8 - buffer->bits_for_blue);
+  for (i=buffer->bits_for_red;   i<8; i*=2) new_r |= new_r >> i;
+  for (i=buffer->bits_for_green; i<8; i*=2) new_g |= new_g >> i;
+  for (i=buffer->bits_for_blue;  i<8; i*=2) new_b |= new_b >> i;
+  
+  buffer->dither_red = new_r - orig_r;
+  buffer->dither_green = new_g - orig_g;
+  buffer->dither_blue = new_b - orig_b;
+  
 
   return ((a << buffer->alpha_bit_position)
           | (r << buffer->red_bit_position)