[polari] thumbnailer: Handle different image sizes

[Florian Müllner <fmuellner src gnome org>]

commit 0870f0a5906ecf4f2bb6225b5ee0794e8505c37f
Author: Florian Müllner <fmuellner gnome org>
Date:   Mon Dec 16 03:27:15 2019 +0100

    thumbnailer: Handle different image sizes
    
    We currently assume that snapshots use the same ratio as the final
    previews. While that is correct at the moment, this is about to
    change for image URLs, where we want the snapshot to match the
    actual image.
    
    Prepare for this by taking the preview size as maximum sizes, and
    fit in the source surface as best as possible.
    
    https://gitlab.gnome.org/GNOME/polari/merge_requests/140

 src/thumbnailer.js | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)
---
diff --git a/src/thumbnailer.js b/src/thumbnailer.js
index 4a54548..6cce789 100644
--- a/src/thumbnailer.js
+++ b/src/thumbnailer.js
@@ -111,15 +111,25 @@ class App {
 
         let sourceWidth = surface.getWidth();
         let sourceHeight = surface.getHeight();
+        let ratio = sourceWidth / sourceHeight;
+
+        let targetWidth, targetHeight;
+        if (ratio >= PREVIEW_WIDTH / PREVIEW_HEIGHT) {
+            targetWidth = Math.min(sourceWidth, PREVIEW_WIDTH);
+            targetHeight = targetWidth / ratio;
+        } else {
+            targetHeight = Math.min(sourceHeight, PREVIEW_HEIGHT);
+            targetWidth = targetHeight * ratio;
+        }
 
         let target = new Cairo.ImageSurface(Cairo.Format.ARGB32,
-            PREVIEW_WIDTH,
-            PREVIEW_HEIGHT);
+            targetWidth,
+            targetHeight);
 
         let cr = new Cairo.Context(target);
         cr.scale(
-            PREVIEW_WIDTH / sourceWidth,
-            PREVIEW_HEIGHT / sourceHeight);
+            targetWidth / sourceWidth,
+            targetHeight / sourceHeight);
         cr.setSourceSurface(surface, 0, 0);
         cr.paint();