When saving cover images don't re-encode the image data unless absolutely neccessary. This prevents information loss due to JPEG re-compression

src/calibre/utils/magick/__init__.py

@@ -158,7 +158,7 @@ class Image(_magick.Image): # {{{
             format = ext[1:]
         format = format.upper()
 
-        with open(path, 'wb') as f:
+        with lopen(path, 'wb') as f:
             f.write(self.export(format))
 
     def compose(self, img, left=0, top=0, operation='OverCompositeOp'):

src/calibre/utils/magick/draw.py

@@ -11,22 +11,57 @@ from calibre.utils.magick import Image, DrawingWand, create_canvas
 from calibre.constants import __appname__, __version__
 from calibre import fit_image
 
+def normalize_format_name(fmt):
+    fmt = fmt.lower()
+    if fmt == 'jpeg':
+        fmt = 'jpg'
+    return fmt
+
 def save_cover_data_to(data, path, bgcolor='#ffffff', resize_to=None,
-        return_data=False):
+        return_data=False, compression_quality=90):
     '''
     Saves image in data to path, in the format specified by the path
-    extension. Composes the image onto a blank canvas so as to
-    properly convert transparent images.
+    extension. Removes any transparency. If there is no transparency and no
+    resize and the input and output image formats are the same, no changes are
+    made.
+
+    :param compression_quality: The quality of the image after compression.
+        Number between 1 and 100. 1 means highest compression, 100 means no
+        compression (lossless).
+    :param bgcolor: The color for transparent pixels. Must be specified in hex.
+    :param resize_to: A tuple (width, height) or None for no resizing
+
     '''
+    changed = False
     img = Image()
     img.load(data)
+    orig_fmt = normalize_format_name(img.format)
+    fmt = os.path.splitext(path)[1]
+    fmt = normalize_format_name(fmt[1:])
+
     if resize_to is not None:
         img.size = (resize_to[0], resize_to[1])
-    canvas = create_canvas(img.size[0], img.size[1], bgcolor)
-    canvas.compose(img)
+        changed = True
+    if not hasattr(img, 'has_transparent_pixels') or img.has_transparent_pixels():
+        canvas = create_canvas(img.size[0], img.size[1], bgcolor)
+        canvas.compose(img)
+        img = canvas
+        changed = True
+    if not changed:
+        changed = fmt != orig_fmt
     if return_data:
-        return canvas.export(os.path.splitext(path)[1][1:])
-    canvas.save(path)
+        if changed:
+            if hasattr(img, 'set_compression_quality') and fmt == 'jpg':
+                img.set_compression_quality(compression_quality)
+            return img.export(fmt)
+        return data
+    if changed:
+        if hasattr(img, 'set_compression_quality') and fmt == 'jpg':
+            img.set_compression_quality(compression_quality)
+        img.save(path)
+    else:
+        with lopen(path, 'wb') as f:
+            f.write(data)
 
 def thumbnail(data, width=120, height=120, bgcolor='#ffffff', fmt='jpg'):
     img = Image()

src/calibre/utils/magick/magick.c

@@ -725,6 +725,49 @@ magick_Image_set_page(magick_Image *self, PyObject *args, PyObject *kwargs) {
 }
 // }}}
 
+// Image.set_compression_quality {{{
+
+static PyObject *
+magick_Image_set_compression_quality(magick_Image *self, PyObject *args, PyObject *kwargs) {
+    Py_ssize_t quality;
+    
+    if (!PyArg_ParseTuple(args, "n", &quality)) return NULL;
+
+    if (!MagickSetImageCompressionQuality(self->wand, quality)) return magick_set_exception(self->wand);
+
+    Py_RETURN_NONE;
+}
+// }}}
+
+// Image.has_transparent_pixels {{{
+
+static PyObject *
+magick_Image_has_transparent_pixels(magick_Image *self, PyObject *args, PyObject *kwargs) {
+    PixelIterator *pi = NULL;
+    PixelWand **pixels = NULL;
+    int found = 0;
+    size_t r, c, width, height;
+    double alpha;
+
+    height = MagickGetImageHeight(self->wand);
+    pi = NewPixelIterator(self->wand);
+
+    for (r = 0; r < height; r++) {
+        pixels = PixelGetNextIteratorRow(pi, &width);
+        for (c = 0; c < width; c++) {
+            alpha = PixelGetAlpha(pixels[c]);
+            if (alpha < 1.00) {
+                found = 1;
+                c = width; r = height;
+            }
+        }
+    }
+    pi = DestroyPixelIterator(pi);
+    if (found) Py_RETURN_TRUE;
+    Py_RETURN_FALSE;
+}
+// }}}
+
 // Image.normalize {{{
 
 static PyObject *
@@ -872,6 +915,14 @@ static PyMethodDef magick_Image_methods[] = {
      "set_page(width, height, x, y) \n\n Sets the page geometry of the image."
     },
 
+    {"set_compression_quality", (PyCFunction)magick_Image_set_compression_quality, METH_VARARGS,
+     "set_compression_quality(quality) \n\n Sets the compression quality when exporting the image."
+    },
+
+    {"has_transparent_pixels", (PyCFunction)magick_Image_has_transparent_pixels, METH_VARARGS,
+     "has_transparent_pixels() \n\n Returns True iff image has a (semi-) transparent pixel"
+    },
+
     {"thumbnail", (PyCFunction)magick_Image_thumbnail, METH_VARARGS,
      "thumbnail(width, height) \n\n Convert to a thumbnail of specified size."
     },