[MM-28638] Improve image thumbnail generation logic (#15534)

* Improve image thumbnail generation logic

* Improve naming

app/file.go

@@ -838,31 +838,16 @@ func (t *UploadFileTask) postprocessImage() {
 		}
 	}
 
-	w := decoded.Bounds().Dx()
-	h := decoded.Bounds().Dy()
-
 	var wg sync.WaitGroup
 	wg.Add(3)
 	go func() {
 		defer wg.Done()
-		thumb := decoded
-		if h > ImageThumbnailHeight || w > ImageThumbnailWidth {
-			if float64(h)/float64(w) < ImageThumbnailRatio {
-				thumb = imaging.Resize(decoded, 0, ImageThumbnailHeight, imaging.Lanczos)
-			} else {
-				thumb = imaging.Resize(decoded, ImageThumbnailWidth, 0, imaging.Lanczos)
-			}
-		}
-		writeJPEG(thumb, t.fileinfo.ThumbnailPath)
+		writeJPEG(genThumbnail(decoded), t.fileinfo.ThumbnailPath)
 	}()
 
 	go func() {
 		defer wg.Done()
-		preview := decoded
-		if w > ImagePreviewWidth {
-			preview = imaging.Resize(decoded, ImagePreviewWidth, 0, imaging.Lanczos)
-		}
-		writeJPEG(preview, t.fileinfo.PreviewPath)
+		writeJPEG(genPreview(decoded), t.fileinfo.PreviewPath)
 	}()
 
 	go func() {
@@ -1083,22 +1068,8 @@ func getImageOrientation(input io.Reader) (int, error) {
 }
 
 func (a *App) generateThumbnailImage(img image.Image, thumbnailPath string, width int, height int) {
-	thumbWidth := float64(IMAGE_THUMBNAIL_PIXEL_WIDTH)
-	thumbHeight := float64(IMAGE_THUMBNAIL_PIXEL_HEIGHT)
-	imgWidth := float64(width)
-	imgHeight := float64(height)
-
-	var thumbnail image.Image
-	if imgHeight < IMAGE_THUMBNAIL_PIXEL_HEIGHT && imgWidth < thumbWidth {
-		thumbnail = img
-	} else if imgHeight/imgWidth < thumbHeight/thumbWidth {
-		thumbnail = imaging.Resize(img, 0, IMAGE_THUMBNAIL_PIXEL_HEIGHT, imaging.Lanczos)
-	} else {
-		thumbnail = imaging.Resize(img, IMAGE_THUMBNAIL_PIXEL_WIDTH, 0, imaging.Lanczos)
-	}
-
 	buf := new(bytes.Buffer)
-	if err := jpeg.Encode(buf, thumbnail, &jpeg.Options{Quality: 90}); err != nil {
+	if err := jpeg.Encode(buf, genThumbnail(img), &jpeg.Options{Quality: 90}); err != nil {
 		mlog.Error("Unable to encode image as jpeg", mlog.String("path", thumbnailPath), mlog.Err(err))
 		return
 	}
@@ -1110,13 +1081,7 @@ func (a *App) generateThumbnailImage(img image.Image, thumbnailPath string, widt
 }
 
 func (a *App) generatePreviewImage(img image.Image, previewPath string, width int) {
-	var preview image.Image
-
-	if width > IMAGE_PREVIEW_PIXEL_WIDTH {
-		preview = imaging.Resize(img, IMAGE_PREVIEW_PIXEL_WIDTH, 0, imaging.Lanczos)
-	} else {
-		preview = img
-	}
+	preview := genPreview(img)
 
 	buf := new(bytes.Buffer)
 

app/image.go

@@ -0,0 +1,50 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package app
+
+import (
+	"image"
+
+	"github.com/disintegration/imaging"
+)
+
+func genThumbnail(img image.Image) image.Image {
+	thumb := img
+	w := img.Bounds().Dx()
+	h := img.Bounds().Dy()
+
+	if h > ImageThumbnailHeight || w > ImageThumbnailWidth {
+		ratio := float64(h) / float64(w)
+		if ratio < ImageThumbnailRatio {
+			// we pre-calculate the thumbnail's width to make sure we are not upscaling.
+			targetWidth := int(float64(ImageThumbnailHeight) * float64(w) / float64(h))
+			if targetWidth <= w {
+				thumb = imaging.Resize(img, 0, ImageThumbnailHeight, imaging.Lanczos)
+			} else {
+				thumb = imaging.Resize(img, ImageThumbnailWidth, 0, imaging.Lanczos)
+			}
+		} else {
+			// we pre-calculate the thumbnail's height to make sure we are not upscaling.
+			targetHeight := int(float64(ImageThumbnailWidth) * float64(h) / float64(w))
+			if targetHeight <= h {
+				thumb = imaging.Resize(img, ImageThumbnailWidth, 0, imaging.Lanczos)
+			} else {
+				thumb = imaging.Resize(img, 0, ImageThumbnailHeight, imaging.Lanczos)
+			}
+		}
+	}
+
+	return thumb
+}
+
+func genPreview(img image.Image) image.Image {
+	preview := img
+	w := img.Bounds().Dx()
+
+	if w > ImagePreviewWidth {
+		preview = imaging.Resize(img, ImagePreviewWidth, 0, imaging.Lanczos)
+	}
+
+	return preview
+}