image: Move background color guess to other thread

Also, skip it completely for JPEG and BMP

Issue #111

src/decoder/formats/mod.rs

@@ -31,6 +31,13 @@ impl ImageFormat {
     pub fn is_svg(&self) -> bool {
         matches!(self, Self::Svg)
     }
+
+    pub fn is_potentially_transparent(&self) -> bool {
+        !matches!(
+            self,
+            Self::ImageRs(image_rs::ImageFormat::Bmp) | Self::ImageRs(image_rs::ImageFormat::Jpeg)
+        )
+    }
 }
 
 impl std::fmt::Display for ImageFormat {

src/widgets/image.rs

@@ -810,8 +810,14 @@ impl LpImage {
                     new_tiles.cleanup(imp.zoom_target.get(), self.viewport());
                     new_tiles
                 });
-                if self.imp().background_color.borrow().is_none() {
+                if imp.background_color.borrow().is_none() {
-                    self.set_background_color(self.background_color_guess());
+                    spawn!(glib::clone!(@weak self as obj => async move {
+                        let color = obj.background_color_guess().await;
+                        obj.set_background_color(color);
+                        if obj.is_mapped() {
+                            obj.queue_draw();
+                        }
+                    }));
                 }
             }
             DecoderUpdate::Error(err) => {
@@ -1635,7 +1641,16 @@ impl LpImage {
     /// Returns a background color that should give suitable contrast with transparent images
     ///
     /// For non-transparent images this always returns `BACKGROUND_COLOR_DEFAULT`
-    pub fn background_color_guess(&self) -> Option<gdk::RGBA> {
+    pub async fn background_color_guess(&self) -> Option<gdk::RGBA> {
+        // Shortcut for formats that don't support transparency
+        if !self
+            .format()
+            .map_or(true, |x| x.is_potentially_transparent())
+        {
+            log::debug!("This format does not support transparency");
+            return Some(*BACKGROUND_COLOR_DEFAULT);
+        }
+
         let (width, height) = self.original_dimensions();
         let max_size = i32::max(width, height);
 
@@ -1660,14 +1675,17 @@ impl LpImage {
             return None;
         }
 
+        // Render the small version of the image and download to RAM
         let texture = renderer.render_texture(node, None);
-
         let mut downloader = gdk::TextureDownloader::new(&texture);
         downloader.set_format(gdk::MemoryFormat::R8g8b8a8);
         let (bytes, stride) = downloader.download_bytes();
 
+        gio::spawn_blocking(move || {
             let mut bytes_iter = bytes.iter();
+            // Number of transparent pixels
             let mut transparent = 0;
+            // Number of non-transparent pixels with bad contrast
             let mut bad_contrast = 0;
             'img: loop {
                 for _ in 0..texture.width() {
@@ -1713,5 +1731,8 @@ impl LpImage {
             } else {
                 Some(*BACKGROUND_COLOR_DEFAULT)
             }
+        })
+        .await
+        .ok()?
     }
 }