use gift filters to create aligned images

internal/epub/filters/position.go

@@ -0,0 +1,55 @@
+package filters
+
+import (
+	"image"
+	"image/draw"
+
+	"github.com/disintegration/gift"
+)
+
+const (
+	PositionCenter = iota
+	PositionLeft
+	PositionRight
+)
+
+func Position(viewWidth, viewHeight int, align int) gift.Filter {
+	return &positionFilter{
+		viewWidth, viewHeight, align,
+	}
+}
+
+type positionFilter struct {
+	viewWidth, viewHeight, align int
+}
+
+func (p *positionFilter) Bounds(srcBounds image.Rectangle) image.Rectangle {
+	return image.Rect(0, 0, p.viewWidth, p.viewHeight)
+}
+
+func (p *positionFilter) Draw(dst draw.Image, src image.Image, options *gift.Options) {
+	draw.Draw(dst, dst.Bounds(), image.White, dst.Bounds().Min, draw.Over)
+
+	srcBounds := src.Bounds()
+	left, top := (p.viewWidth-srcBounds.Dx())/2, (p.viewHeight-srcBounds.Dy())/2
+	if p.align == PositionLeft {
+		left = 0
+	}
+
+	if p.align == PositionRight {
+		left = p.viewWidth - srcBounds.Dx()
+	}
+
+	draw.Draw(
+		dst,
+		image.Rect(
+			left,
+			top,
+			p.viewWidth,
+			p.viewHeight,
+		),
+		src,
+		srcBounds.Min,
+		draw.Over,
+	)
+}

internal/epub/image_filters.go

@@ -21,6 +21,7 @@ func NewGift(options *ImageOptions) *gift.GIFT {
 	g.Add(
 		filters.Resize(options.ViewWidth, options.ViewHeight, gift.LanczosResampling),
 		filters.Pixel(),
+		filters.Position(options.ViewWidth, options.ViewHeight, filters.PositionCenter),
 	)
 	return g
 }
@@ -28,25 +29,30 @@ func NewGift(options *ImageOptions) *gift.GIFT {
 func NewGiftSplitDoublePage(options *ImageOptions) []*gift.GIFT {
 	gifts := make([]*gift.GIFT, 2)
 
-	rightFirst := options.Manga
-
 	gifts[0] = gift.New(
-		filters.CropSplitDoublePage(rightFirst),
+		filters.CropSplitDoublePage(options.Manga),
 	)
 
 	gifts[1] = gift.New(
-		filters.CropSplitDoublePage(!rightFirst),
+		filters.CropSplitDoublePage(!options.Manga),
 	)
 
-	for _, g := range gifts {
+	for i, g := range gifts {
 		if options.Contrast != 0 {
 			g.Add(gift.Contrast(float32(options.Contrast)))
 		}
 		if options.Brightness != 0 {
 			g.Add(gift.Brightness(float32(options.Brightness)))
 		}
+
+		position := filters.PositionLeft
+		if (i == 1) == options.Manga {
+			position = filters.PositionRight
+		}
+
 		g.Add(
 			filters.Resize(options.ViewWidth, options.ViewHeight, gift.LanczosResampling),
+			filters.Position(options.ViewWidth, options.ViewHeight, position),
 		)
 	}
 

internal/epub/image_processing.go

@@ -226,6 +226,7 @@ func (e *ePub) LoadImages() ([]*Image, error) {
 						part := i + 1
 						dst := image.NewGray(g.Bounds(src.Bounds()))
 						g.Draw(dst, src)
+
 						imageOutput <- &Image{
 							Id:         img.Id,
 							Part:       part,