From 8e7b519b8d3ce5e13ee3c57e01a272b2598ddecb Mon Sep 17 00:00:00 2001
From: NiLuJe <ninuje@gmail.com>
Date: Fri, 24 May 2019 20:06:51 +0200
Subject: [PATCH] Inline the grayscaling pass in ordered_dither

Saving one QImage pixel-loop dance in the process...
---
 src/calibre/utils/imageops/ordered_dither.cpp | 10 ++++++++--
 src/calibre/utils/img.py                      |  8 ++++----
 2 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/src/calibre/utils/imageops/ordered_dither.cpp b/src/calibre/utils/imageops/ordered_dither.cpp
index 2fe3d29828..577fe1c181 100644
--- a/src/calibre/utils/imageops/ordered_dither.cpp
+++ b/src/calibre/utils/imageops/ordered_dither.cpp
@@ -109,13 +109,19 @@ QImage ordered_dither(const QImage &image) { // {{{
         if (img.isNull()) throw std::bad_alloc();
     }
 
+    const bool is_gray = img.isGrayscale();
+
     for (y = 0; y < height; y++) {
         const QRgb *src_row = reinterpret_cast<const QRgb*>(img.constScanLine(y));
         uint8_t *dst_row = dst.scanLine(y);
         for (x = 0; x < width; x++) {
             const QRgb pixel = *(src_row + x);
-            // We're running behind grayscale_image, so R = G = B
-            gray = qRed(pixel);
+            if (is_gray) {
+                // Grayscale and RGB32, so R = G = B
+                gray = qRed(pixel);
+            } else {
+                gray = qGray(pixel);
+            }
             dithered = dither_o8x8(x, y, gray);
             *(dst_row + x) = dithered;  // ... or palette.indexOf(dithered); for Indexed8
         }
diff --git a/src/calibre/utils/img.py b/src/calibre/utils/img.py
index 376d04d344..a929b09866 100644
--- a/src/calibre/utils/img.py
+++ b/src/calibre/utils/img.py
@@ -227,7 +227,7 @@ def save_cover_data_to(data, path=None, bgcolor='#ffffff', resize_to=None, compr
     if img.hasAlphaChannel():
         changed = True
         img = blend_image(img, bgcolor)
-    if grayscale:
+    if grayscale and not eink:
         if not img.allGray():
             changed = True
             img = grayscale_image(img)
@@ -464,13 +464,13 @@ def eink_dither_image(img):
     ''' Dither the source image down to the eInk palette of 16 shades of grey,
     using ImageMagick's OrderedDither algorithm.
 
-    NOTE: Expects input as a grayscale image in RGB32 pixel format (as returned by grayscale_image).
-          Running blend_image if the image has an alpha channel,
-          or grayscale_image if it's not already grayscaled is the caller's responsibility.
+    NOTE: No need to call grayscale_image first, as this will inline a grayscaling pass if need be.
 
     Returns a QImage in Grayscale8 pixel format.
     '''
     img = image_from_data(img)
+    if img.hasAlphaChannel():
+        img = blend_image(img)
     return imageops.ordered_dither(img)
 
 # }}}