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resource: Preserve color palette for PNG images

This commit will force a reprocessing of PNG images with new names, so it is adviced to run a `hugo --gc` to remove stale files.

Fixes #4416

--- a/resource/image.go

+++ b/resource/image.go

@@ -30,6 +30,7 @@

 	// Importing image codecs for image.DecodeConfig

 	"image"

+	"image/draw"

 	_ "image/gif"

 	"image/jpeg"

 	_ "image/png"

@@ -65,16 +66,28 @@

 	defaultResampleFilter = "box"

 )

-var imageFormats = map[string]imaging.Format{

-	".jpg":  imaging.JPEG,

-	".jpeg": imaging.JPEG,

-	".png":  imaging.PNG,

-	".tif":  imaging.TIFF,

-	".tiff": imaging.TIFF,

-	".bmp":  imaging.BMP,

-	".gif":  imaging.GIF,

-}

+var (

+	imageFormats = map[string]imaging.Format{

+		".jpg":  imaging.JPEG,

+		".jpeg": imaging.JPEG,

+		".png":  imaging.PNG,

+		".tif":  imaging.TIFF,

+		".tiff": imaging.TIFF,

+		".bmp":  imaging.BMP,

+		".gif":  imaging.GIF,

+	}

+	// Add or increment if changes to an image format's processing requires

+	// re-generation.

+	imageFormatsVersions = map[imaging.Format]int{

+		imaging.PNG: 1, // 1: Add proper palette handling

+	}

+

+	// Increment to mark all processed images as stale. Only use when absolutely needed.

+	// See the finer grained smartCropVersionNumber and imageFormatsVersions.

+	mainImageVersionNumber = 0

+)

+

 var anchorPositions = map[string]imaging.Anchor{

 	strings.ToLower("Center"):      imaging.Center,

 	strings.ToLower("TopLeft"):     imaging.TopLeft,

@@ -117,6 +130,8 @@

 	imaging *Imaging

+	format imaging.Format

+

 	hash string

 	*genericResource

@@ -137,6 +152,7 @@

 	return &Image{

 		imaging:         i.imaging,

 		hash:            i.hash,

+		format:          i.format,

 		genericResource: i.genericResource.WithNewBase(base).(*genericResource)}

 }

@@ -246,6 +262,15 @@

 			return ci, &os.PathError{Op: errOp, Path: errPath, Err: err}

 		}

+		if i.format == imaging.PNG {

+			// Apply the colour palette from the source

+			if paletted, ok := src.(*image.Paletted); ok {

+				tmp := image.NewPaletted(converted.Bounds(), paletted.Palette)

+				draw.Src.Draw(tmp, tmp.Bounds(), converted, converted.Bounds().Min)

+				converted = tmp

+			}

+		}

+

 		b := converted.Bounds()

 		ci.config = image.Config{Width: b.Max.X, Height: b.Max.Y}

 		ci.configLoaded = true

@@ -255,7 +280,7 @@

 }

-func (i imageConfig) key() string {

+func (i imageConfig) key(format imaging.Format) string {

 	k := strconv.Itoa(i.Width) + "x" + strconv.Itoa(i.Height)

 	if i.Action != "" {

 		k += "_" + i.Action

@@ -277,6 +302,14 @@

 		k += "_" + anchor

 	}

+	if v, ok := imageFormatsVersions[format]; ok {

+		k += "_" + strconv.Itoa(v)

+	}

+

+	if mainImageVersionNumber > 0 {

+		k += "_" + strconv.Itoa(mainImageVersionNumber)

+	}

+

 	return k

 }

@@ -410,7 +443,8 @@

 		return nil, err

 	}

 	defer file.Close()

-	return imaging.Decode(file)

+	img, _, err := image.Decode(file)

+	return img, err

 }

 func (i *Image) copyToDestination(src string) error {

@@ -464,13 +498,7 @@

 }

 func (i *Image) encodeToDestinations(img image.Image, conf imageConfig, resourceCacheFilename, filename string) error {

-	ext := strings.ToLower(helpers.Ext(filename))

-	imgFormat, ok := imageFormats[ext]

-	if !ok {

-		return imaging.ErrUnsupportedFormat

-	}

-

 	target := filepath.Join(i.absPublishDir, filename)

 	file1, err := i.spec.Fs.Destination.Create(target)

@@ -509,7 +537,7 @@

 		w = file1

 	}

-	switch imgFormat {

+	switch i.format {

 	case imaging.JPEG:

 		var rgba *image.RGBA

@@ -530,7 +558,7 @@

 			return jpeg.Encode(w, img, &jpeg.Options{Quality: quality})

 		}

 	default:

-		return imaging.Encode(w, img, imgFormat)

+		return imaging.Encode(w, img, i.format)

 	}

 }

@@ -541,6 +569,7 @@

 	return &Image{

 		imaging:         i.imaging,

 		hash:            i.hash,

+		format:          i.format,

 		genericResource: &g}

 }

@@ -555,7 +584,7 @@

 	// Do not change for no good reason.

 	const md5Threshold = 100

-	key := conf.key()

+	key := conf.key(i.format)

 	// It is useful to have the key in clear text, but when nesting transforms, it

 	// can easily be too long to read, and maybe even too long

--- a/resource/image_test.go

+++ b/resource/image_test.go

@@ -19,6 +19,8 @@

 	"strconv"

 	"testing"

+	"github.com/disintegration/imaging"

+

 	"sync"

 	"github.com/stretchr/testify/require"

@@ -255,6 +257,24 @@

 	resized, err := image.Resize("200x")

 	assert.NoError(err)

 	assert.Equal("Sunset #1", resized.Name())

+

+}

+

+func TestImageResize8BitPNG(t *testing.T) {

+

+	assert := require.New(t)

+

+	image := fetchImage(assert, "gohugoio.png")

+

+	assert.Equal(imaging.PNG, image.format)

+	assert.Equal("/a/gohugoio.png", image.RelPermalink())

+	assert.Equal("image", image.ResourceType())

+

+	resized, err := image.Resize("800x")

+	assert.NoError(err)

+	assert.Equal(imaging.PNG, resized.format)

+	assert.Equal("/a/gohugoio_hu0e1b9e4a4be4d6f86c7b37b9ccce3fbc_73886_800x0_resize_linear_1.png", resized.RelPermalink())

+	assert.Equal(800, resized.Width())

 }

--- a/resource/resource.go

+++ b/resource/resource.go

@@ -23,6 +23,8 @@

 	"strings"

 	"sync"

+	"github.com/disintegration/imaging"

+

 	"github.com/spf13/cast"

 	"github.com/gobwas/glob"

@@ -297,8 +299,16 @@

 			return nil, err

 		}

+		ext := strings.ToLower(helpers.Ext(absSourceFilename))

+

+		imgFormat, ok := imageFormats[ext]

+		if !ok {

+			return nil, imaging.ErrUnsupportedFormat

+		}

+

 		return &Image{

 			hash:            hash,

+			format:          imgFormat,

 			imaging:         r.imaging,

 			genericResource: gr}, nil

 	}

binary files /dev/null b/resource/testdata/gohugoio.png differ