Fix grayscaling of images not working on Qt < 5.5

src/calibre/utils/imageops/imageops.cpp

@@ -38,18 +38,20 @@ unsigned int read_border_row(const QImage &img, const unsigned int width, const
 	return ans;
 }
 
+#define ENSURE32(img) \
+	if (img.format() != QImage::Format_RGB32 && img.format() != QImage::Format_ARGB32) { \
+		img = img.convertToFormat(img.hasAlphaChannel() ? QImage::Format_ARGB32 : QImage::Format_RGB32); \
+		if (img.isNull()) { PyErr_NoMemory(); return NULL; } \
+	} \
+
 QImage* remove_borders(const QImage &image, double fuzz) {
 	int *buf = NULL;
 	QImage* ans = NULL, img = image, timg;
 	QTransform transpose;
-	transpose.rotate(90);
 	unsigned int width = img.width(), height = img.height();
 	unsigned int top_border = 0, bottom_border = 0, left_border = 0, right_border = 0;
 
-	if (img.format() != QImage::Format_RGB32 && img.format() != QImage::Format_ARGB32) {
+    ENSURE32(img)
-		img = img.convertToFormat(img.hasAlphaChannel() ? QImage::Format_ARGB32 : QImage::Format_RGB32);
-		if (img.isNull()) { PyErr_NoMemory(); return NULL; }
-	}
 	buf = new int[3*(MAX(width, height)+1)];
 	fuzz /= 255;
 
@@ -57,6 +59,7 @@ QImage* remove_borders(const QImage &image, double fuzz) {
 	if (top_border >= height - 1) goto end;
 	bottom_border = read_border_row(img, width, height, buf, fuzz, false);
 	if (bottom_border >= height - 1) goto end;
+	transpose.rotate(90);
 	timg = img.transformed(transpose);
 	if (timg.isNull()) { PyErr_NoMemory(); goto end; }
 	left_border = read_border_row(timg, height, width, buf, fuzz, true);
@@ -74,3 +77,20 @@ end:
 	if (!PyErr_Occurred()) ans = new QImage(img);
 	return ans;
 }
+
+QImage* grayscale(const QImage &image) {
+    QImage img = image, *ans = NULL;
+    QRgb *row = NULL, *pixel = NULL;
+    int r = 0, gray = 0, width = img.width(), height = img.height();
+
+    ENSURE32(img);
+    for (r = 0; r < height; r++) {
+		row = reinterpret_cast<QRgb*>(img.scanLine(r));
+        for (pixel = row; pixel < row + width; pixel++) {
+            gray = qGray(*pixel);
+            *pixel = QColor(gray, gray, gray).rgba();
+        }
+    }
+	if (!PyErr_Occurred()) ans = new QImage(img);
+	return ans;
+}

src/calibre/utils/imageops/imageops.h

@@ -11,4 +11,5 @@
 #include <Python.h>
 
 QImage* remove_borders(const QImage &image, double fuzz);
+QImage* grayscale(const QImage &image);
 

src/calibre/utils/imageops/imageops.sip

@@ -16,3 +16,11 @@ QImage* remove_borders(const QImage &image, double fuzz);
 			if (sipRes == NULL) return NULL;
 		} catch (std::bad_alloc) { PyErr_NoMemory(); return NULL; }
 %End
+
+QImage* grayscale(const QImage &image);
+%MethodCode
+		try {
+			sipRes = grayscale(*a0);
+			if (sipRes == NULL) return NULL;
+		} catch (std::bad_alloc) { PyErr_NoMemory(); return NULL; }
+%End

src/calibre/utils/img.py

@@ -120,6 +120,13 @@ def add_borders_to_image(img_data, left=0, top=0, right=0, bottom=0,
     img = add_borders(img, left=left, top=top, right=right, bottom=bottom, border_color=border_color)
     return image_to_data(img, fmt=fmt)
 
+def to_grayscale(img):
+    if hasattr(QImage, 'Format_Grayscale8'):
+        return img.convertToFormat(QImage.Format_Grayscale8)
+    if imageops is not None:
+        return imageops.grayscale(img)
+    return img
+
 def save_cover_data_to(data, path=None, bgcolor='#ffffff', resize_to=None, compression_quality=90, minify_to=None, grayscale=False):
     '''
     Saves image in data to path, in the format specified by the path
@@ -155,9 +162,9 @@ def save_cover_data_to(data, path=None, bgcolor='#ffffff', resize_to=None, compr
         changed = True
         img = blend_image(img, bgcolor)
     if grayscale:
-        changed = True
         if not img.allGray():
-            img = img.convertToFormat(QImage.Format_Grayscale8)
+            changed = True
+            img = to_grayscale(img)
     if path is None:
         return image_to_data(img, compression_quality, fmt) if changed else data
     with lopen(path, 'wb') as f: