upscale or downscale image automatically

will make panel view works for small image

internal/epub/filters/resize.go

@@ -0,0 +1,47 @@
+package filters
+
+import (
+	"image"
+	"image/draw"
+
+	"github.com/disintegration/gift"
+)
+
+func Resize(viewWidth, viewHeight int, resampling gift.Resampling) gift.Filter {
+	return &resizeFilter{
+		viewWidth, viewHeight, resampling,
+	}
+}
+
+type resizeFilter struct {
+	viewWidth, viewHeight int
+	resampling            gift.Resampling
+}
+
+func (p *resizeFilter) Bounds(srcBounds image.Rectangle) image.Rectangle {
+	w, h := p.viewWidth, p.viewHeight
+	srcw, srch := srcBounds.Dx(), srcBounds.Dy()
+
+	if w <= 0 || h <= 0 || srcw <= 0 || srch <= 0 {
+		return image.Rect(0, 0, 0, 0)
+	}
+
+	wratio := float64(srcw) / float64(w)
+	hratio := float64(srch) / float64(h)
+
+	var dstw, dsth int
+	if wratio > hratio {
+		dstw = w
+		dsth = int(float64(srch)/wratio + 0.5)
+	} else {
+		dsth = h
+		dstw = int(float64(srcw)/hratio + 0.5)
+	}
+
+	return image.Rect(0, 0, dstw, dsth)
+}
+
+func (p *resizeFilter) Draw(dst draw.Image, src image.Image, options *gift.Options) {
+	b := p.Bounds(src.Bounds())
+	gift.Resize(b.Dx(), b.Dy(), p.resampling).Draw(dst, src, options)
+}

internal/epub/image_filters.go

@@ -19,7 +19,7 @@ func NewGift(options *ImageOptions) *gift.GIFT {
 		g.Add(gift.Brightness(float32(options.Brightness)))
 	}
 	g.Add(
-		gift.ResizeToFit(options.ViewWidth, options.ViewHeight, gift.LanczosResampling),
+		filters.Resize(options.ViewWidth, options.ViewHeight, gift.LanczosResampling),
 		filters.Pixel(),
 	)
 	return g