add trim option to remove solid color borders

Fixes #441

data.go

@@ -30,6 +30,7 @@ const (
 	optCropWidth       = "cw"
 	optCropHeight      = "ch"
 	optSmartCrop       = "sc"
+	optTrim            = "trim"
 )
 
 // URLError reports a malformed URL error.
@@ -80,6 +81,9 @@ type Options struct {
 
 	// Automatically find good crop points based on image content.
 	SmartCrop bool
+
+	// If true, automatically trim pixels of the same color around the edges
+	Trim bool
 }
 
 func (o Options) String() string {
@@ -123,6 +127,9 @@ func (o Options) String() string {
 	if o.SmartCrop {
 		opts = append(opts, optSmartCrop)
 	}
+	if o.Trim {
+		opts = append(opts, optTrim)
+	}
 	sort.Strings(opts)
 	return strings.Join(opts, ",")
 }
@@ -132,7 +139,7 @@ func (o Options) String() string {
 // the presence of other fields (like Fit).  A non-empty Format value is
 // assumed to involve a transformation.
 func (o Options) transform() bool {
-	return o.Width != 0 || o.Height != 0 || o.Rotate != 0 || o.FlipHorizontal || o.FlipVertical || o.Quality != 0 || o.Format != "" || o.CropX != 0 || o.CropY != 0 || o.CropWidth != 0 || o.CropHeight != 0
+	return o.Width != 0 || o.Height != 0 || o.Rotate != 0 || o.FlipHorizontal || o.FlipVertical || o.Quality != 0 || o.Format != "" || o.CropX != 0 || o.CropY != 0 || o.CropWidth != 0 || o.CropHeight != 0 || o.Trim
 }
 
 // ParseOptions parses str as a list of comma separated transformation options.
@@ -207,7 +214,7 @@ func (o Options) transform() bool {
 //
 // # Format
 //
-// The "jpeg", "png", and "tiff"  options can be used to specify the desired
+// The "jpeg", "png", and "tiff" options can be used to specify the desired
 // image format of the proxied image.
 //
 // # Signature
@@ -219,6 +226,13 @@ func (o Options) transform() bool {
 // See https://github.com/willnorris/imageproxy/blob/master/docs/url-signing.md
 // for examples of generating signatures.
 //
+// # Trim
+//
+// The "trim" option will automatically trim pixels of the same color around
+// the edges of the image.  This is useful for removing borders from images
+// that have been resized or cropped.  The trim option is applied before other
+// options such as cropping or resizing.
+//
 // Examples
 //
 //	0x0         - no resizing
@@ -251,6 +265,8 @@ func ParseOptions(str string) Options {
 			options.Format = opt
 		case opt == optSmartCrop:
 			options.SmartCrop = true
+		case opt == optTrim:
+			options.Trim = true
 		case strings.HasPrefix(opt, optRotatePrefix):
 			value := strings.TrimPrefix(opt, optRotatePrefix)
 			options.Rotate, _ = strconv.Atoi(value)

transform.go

@@ -267,6 +267,11 @@ func transformImage(m image.Image, opt Options) image.Image {
 	timer := prometheus.NewTimer(metricTransformationDuration)
 	defer timer.ObserveDuration()
 
+	// trim
+	if opt.Trim {
+		m = trimEdges(m)
+	}
+
 	// Parse crop and resize parameters before applying any transforms.
 	// This is to ensure that any percentage-based values are based off the
 	// size of the original image.
@@ -311,3 +316,41 @@ func transformImage(m image.Image, opt Options) image.Image {
 
 	return m
 }
+
+// trimEdges returns a new image with solid color borders of the image removed.
+// The pixel at the top left corner is used to match the border color.
+func trimEdges(img image.Image) image.Image {
+	bounds := img.Bounds()
+	minX, minY, maxX, maxY := bounds.Max.X, bounds.Max.Y, bounds.Min.X, bounds.Min.Y
+
+	// Get the color of the first pixel (top-left corner)
+	baseColor := img.At(bounds.Min.X, bounds.Min.Y)
+
+	// Check each pixel and find the bounding box of non-matching pixels
+	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
+		for x := bounds.Min.X; x < bounds.Max.X; x++ {
+			if img.At(x, y) != baseColor { // Non-matching pixel
+				if x < minX {
+					minX = x
+				}
+				if y < minY {
+					minY = y
+				}
+				if x > maxX {
+					maxX = x
+				}
+				if y > maxY {
+					maxY = y
+				}
+			}
+		}
+	}
+
+	// If no non-matching pixels are found, return the original image
+	if minX >= maxX || minY >= maxY {
+		return img
+	}
+
+	// Crop the image to the bounding box of non-matching pixels
+	return imaging.Crop(img, image.Rect(minX, minY, maxX+1, maxY+1))
+}

transform_test.go

@@ -375,3 +375,77 @@ func TestTransformImage(t *testing.T) {
 		}
 	}
 }
+
+func TestTrimEdges(t *testing.T) {
+	x := color.NRGBA{255, 255, 255, 255}
+	o := color.NRGBA{0, 0, 0, 255}
+
+	tests := []struct {
+		name string
+		src  image.Image // source image to transform
+		want image.Image // expected transformed image
+	}{
+		{
+			name: "empty",
+			src:  newImage(0, 0),
+			want: newImage(0, 0), // same as src
+		},
+		{
+			name: "solid",
+			src:  newImage(8, 8, x),
+			want: newImage(8, 8, x), // same as src
+		},
+		{
+			name: "square",
+			src: newImage(4, 4,
+				x, x, x, x,
+				x, o, o, x,
+				x, o, o, x,
+				x, x, x, x,
+			),
+			want: newImage(2, 2,
+				o, o,
+				o, o,
+			),
+		},
+		{
+			name: "diamond",
+			src: newImage(5, 5,
+				x, x, x, x, x,
+				x, x, o, x, x,
+				x, o, o, o, x,
+				x, x, o, x, x,
+				x, x, x, x, x,
+			),
+			want: newImage(3, 3,
+				x, o, x,
+				o, o, o,
+				x, o, x,
+			),
+		},
+		{
+			name: "irregular",
+			src: newImage(5, 5,
+				x, o, x, x, x,
+				x, o, o, x, x,
+				x, o, o, x, x,
+				x, x, x, x, x,
+				x, x, x, x, x,
+			),
+			want: newImage(2, 3,
+				o, x,
+				o, o,
+				o, o,
+			),
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := trimEdges(tt.src)
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("trimEdges() returned image %#v, want %#v", got, tt.want)
+			}
+		})
+	}
+}