Simplify thumbnail resizing logic to ensure target constraints are re…

…spected (mattermost#28012)

Automatic Merge

server/channels/app/file_test.go

@@ -396,7 +396,7 @@ func TestGenerateThumbnailImage(t *testing.T) {
 		// then
 		outputImage, err := os.Stat(thumbnailPath)
 		assert.NoError(t, err)
-		assert.Equal(t, int64(957), outputImage.Size())
+		assert.Equal(t, int64(721), outputImage.Size())
 	})
 }
 

server/channels/app/imaging/preview.go

@@ -26,32 +26,15 @@ func GeneratePreview(img image.Image, width int) image.Image {
 
 // GenerateThumbnail generates the thumbnail for the given image.
 func GenerateThumbnail(img image.Image, targetWidth, targetHeight int) image.Image {
-	thumb := img
 	width := img.Bounds().Dx()
 	height := img.Bounds().Dy()
-	expectedRatio := float64(targetHeight) / float64(targetWidth)
 
-	// If both dimensions are over the target size, we scale down until one is not.
-	// If one dimension is over the target size, we scale down until both are not.
-	// If neither dimension is over the target size, we return the image as is.
-	if height > targetHeight || width > targetWidth {
-		ratio := float64(height) / float64(width)
-		if ratio < expectedRatio {
-			if height >= targetHeight {
-				thumb = imaging.Resize(img, 0, targetHeight, imaging.Lanczos)
-			} else {
-				thumb = imaging.Resize(img, targetWidth, 0, imaging.Lanczos)
-			}
-		} else {
-			if width >= targetWidth {
-				thumb = imaging.Resize(img, targetWidth, 0, imaging.Lanczos)
-			} else {
-				thumb = imaging.Resize(img, 0, targetHeight, imaging.Lanczos)
-			}
-		}
+	// We keep aspect ratio and ensure the output dimensions are never higher than the provided targets.
+	if width > height {
+		return imaging.Resize(img, targetWidth, 0, imaging.Lanczos)
 	}
 
-	return thumb
+	return imaging.Resize(img, 0, targetHeight, imaging.Lanczos)
 }
 
 // GenerateMiniPreviewImage generates the mini preview for the given image.

server/channels/app/imaging/preview_test.go

@@ -0,0 +1,77 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package imaging
+
+import (
+	"image"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestGenerateThumbnail(t *testing.T) {
+	tcs := []struct {
+		name           string
+		inputImg       image.Image
+		targetWidth    int
+		targetHeight   int
+		expectedWidth  int
+		expectedHeight int
+	}{
+		{
+			name:         "empty image",
+			inputImg:     image.NewRGBA(image.Rect(0, 0, 0, 0)),
+			targetWidth:  120,
+			targetHeight: 100,
+		},
+		{
+			name:           "both dimensions lower than targets",
+			inputImg:       image.NewRGBA(image.Rect(0, 0, 100, 50)),
+			targetWidth:    120,
+			targetHeight:   100,
+			expectedWidth:  120,
+			expectedHeight: 60,
+		},
+		{
+			name:           "both dimensions equal to targets",
+			inputImg:       image.NewRGBA(image.Rect(0, 0, 120, 100)),
+			targetWidth:    120,
+			targetHeight:   100,
+			expectedWidth:  120,
+			expectedHeight: 100,
+		},
+		{
+			name:           "both dimensions higher than targets",
+			inputImg:       image.NewRGBA(image.Rect(0, 0, 1000, 500)),
+			targetWidth:    120,
+			targetHeight:   100,
+			expectedWidth:  120,
+			expectedHeight: 60,
+		},
+		{
+			name:           "width higher than target",
+			inputImg:       image.NewRGBA(image.Rect(0, 0, 200, 100)),
+			targetWidth:    120,
+			targetHeight:   100,
+			expectedWidth:  120,
+			expectedHeight: 60,
+		},
+		{
+			name:           "height higher than target",
+			inputImg:       image.NewRGBA(image.Rect(0, 0, 100, 200)),
+			targetWidth:    120,
+			targetHeight:   100,
+			expectedWidth:  50,
+			expectedHeight: 100,
+		},
+	}
+
+	for _, tc := range tcs {
+		t.Run(tc.name, func(t *testing.T) {
+			thumb := GenerateThumbnail(tc.inputImg, tc.targetWidth, tc.targetHeight)
+			require.Equal(t, tc.expectedWidth, thumb.Bounds().Dx(), "expectedWidth")
+			require.Equal(t, tc.expectedHeight, thumb.Bounds().Dy(), "expectedHeight")
+		})
+	}
+}