Add support for HiDPI images

This works by creating corresponding hidpi image sizes on startup and
letting the machinery generate high resolution images from the source
images (no way to use supplied images). However since browsers expect
exact image dimension multiples for the particular scale factor,
a reference image size (scale factor 1.0x) must be read first, then
cropped to match reference aspect ratio and resized to exact dimensions.
That way pixel-perfect results can be achieved for the chosen scale
factor.

TODO: the CSS background-image: image-set() tags are not supported
on Firefox.
TODO: try the 1.5x scale factor

1 files changed, 25 insertions, 3 deletions

diff --git a/src/jpeg-utils.cpp b/src/jpeg-utils.cpp
index aaf59fc..d7134c2 100644
--- a/src/jpeg-utils.cpp
+++ b/src/jpeg-utils.cpp
@@ -507,6 +507,7 @@ resize_image (const gchar *src, const gchar *dst,
               int quality,
               gboolean thumbnail,
               gboolean autorotate,
+              gboolean hidpi_strict_dimensions,
               ExifData *exif,
               gchar *resize_opts)
 {
@@ -539,7 +540,7 @@ resize_image (const gchar *src, const gchar *dst,
     autorotate_image (magick_wand);
 
   /*  Don't resize if smaller than desired size  */
-  if (MagickGetImageWidth (magick_wand) > size_x || MagickGetImageHeight (magick_wand) > size_y)
+  if (hidpi_strict_dimensions || MagickGetImageWidth (magick_wand) > size_x || MagickGetImageHeight (magick_wand) > size_y)
   {
     /*  Prepare image before resizing  */
     if (thumbnail) {
@@ -587,8 +588,29 @@ resize_image (const gchar *src, const gchar *dst,
       }
     }
 
+    /*  Shave the source image to match exact dimensions after resize  */
+    if (hidpi_strict_dimensions && (! thumbnail || exif->thumbnail_crop_style == CROP_STYLE_NORMAL)) {
+      w = MagickGetImageWidth (magick_wand);
+      h = MagickGetImageHeight (magick_wand);
+      source_aspect = (double) w / (double) h;
+      target_aspect = (double) size_x / (double) size_y;
+      if (source_aspect != target_aspect) {
+        if (target_aspect >= source_aspect) {
+          new_w = w;
+          new_h = lround ((double) w / target_aspect);
+        } else {
+          new_w = lround ((double) h * target_aspect);
+          new_h = h;
+        }
+        MagickCropImage (magick_wand, new_w, new_h, (w - new_w) / 2, (h - new_h) / 2);
+        g_warn_if_fail (MagickGetImageWidth (magick_wand) == new_w);
+        g_warn_if_fail (MagickGetImageHeight (magick_wand) == new_h);
+      }
+    }
+
     if (resize_opts == NULL) {
       /*  Perform internal resizing  */
+      /*  Note: MagickResizeImage() does no aspect correction, stretching the image to the required dimensions  */
       if (thumbnail) {
         MagickThumbnailImage (magick_wand, size_x, size_y);
       } else {
@@ -682,8 +704,8 @@ get_image_sizes (const gchar *img,
   ExceptionType severity;
   gchar *description;
 
-  *width = -1;
-  *height = -1;
+  *width = 0;
+  *height = 0;
   if (quality)
     *quality = -1;