here u go

common/nori/export.go

@@ -6,6 +6,7 @@ import (
 	"image/draw"
 
 	"git.tuxpa.in/a/zlog/log"
+	"github.com/phrozen/blend"
 	"gitlab.com/gfxlabs/gfximg/apng"
 )
 
@@ -30,18 +31,24 @@ func (n *Nori) ExportAnimation() (*apng.APNG, error) {
 	a := &apng.APNG{
 		Frames: make([]apng.Frame, 0, len(g.Images)),
 	}
+	images := make([]*image.NRGBA64, 0, len(n.Animations))
 	for i := 0; i < len(n.Animations); i++ {
-		for _, v := range n.Animations[i].Frames {
-			canvasRect := image.Rect(0, 0, 0, 0)
-			for _, plane := range v.Planes {
+		if len(n.Animations[i].Frames) < 1 {
+			continue
+		}
+		planes := n.Animations[i].Frames[0].Planes
+		var canvasRect image.Rectangle
+		for _, plane := range planes {
 			if int(plane.BitmapId) >= len(n.Gawi.Images) {
 				log.Printf("could not find bitmap %d, only have %d", plane.BitmapId, len(n.Gawi.Images))
 			}
 			bitmap := n.Gawi.Images[plane.BitmapId]
-				canvasRect = canvasRect.Union(bitmap.Img.Bounds())
+			pt := image.Pt(int(plane.PlaneX), int(plane.PlaneY)) // where to put the point
+			rc := bitmap.Img.Bounds()
+			canvasRect = canvasRect.Union(rc.Add(pt))
 		}
-			img := image.NewRGBA64(canvasRect)
-			for _, plane := range v.Planes {
+		img := image.NewNRGBA64(canvasRect)
+		for _, plane := range planes {
 			bitmap := n.Gawi.Images[plane.BitmapId]
 			transparent := false
 			//flipx
@@ -55,29 +62,58 @@ func (n *Nori) ExportAnimation() (*apng.APNG, error) {
 				transparent = true
 			}
 			_ = transparent
+			pt := image.Pt(int(plane.PlaneX), int(plane.PlaneY)) // where to put the point
+			src := bitmap.Img
+			sr := src.Bounds()
+			r := image.Rectangle{pt, pt.Add(sr.Size())}
 			switch plane.Blend {
 			case BlendMode_Alpha:
 			case BlendMode_Mul, BlendMode_Mul7:
+				blend.BlendImage(img, bitmap.Img, blend.Multiply)
 			case BlendMode_Add, BlendMode_Add8:
+				blend.BlendImage(img, bitmap.Img, blend.Add)
 			case BlendMode_InvertMul, BlendMode_InvertMul5:
+				blend.BlendImage(img, bitmap.Img, blend.Multiply)
 			case BlendMode_None:
-					break
 			default:
 				return nil, fmt.Errorf("unknown blend mode: %d", plane.Blend)
 			}
 			draw.Draw(img,
-					img.Rect,
-					bitmap.Img,
-					image.Point{X: int(plane.PlaneX),
-						Y: int(plane.PlaneY)}, draw.Over)
+				r,
+				src,
+				sr.Min,
+				draw.Over,
+			)
 		}
+		if len(planes) > 0 {
+			images = append(images, img)
+
+		}
+	}
+	or := images[0].Bounds()
+	for _, realFrame := range images {
+		or = realFrame.Bounds().Union(or)
+	}
+	// TODO: this is bugged
+	for i, realFrame := range images {
+		big := image.NewNRGBA64(or)
+		draw.Draw(
+			big,
+			big.Rect,
+			realFrame,
+			realFrame.Bounds().Min,
+			draw.Src,
+		)
 		fr := apng.Frame{
-				Image:            g.Images[i].Img,
-				DelayNumerator:   uint16(g.Images[i].Delay),
+			Image:            big,
+			DisposeOp:        apng.DISPOSE_OP_PREVIOUS,
+			BlendOp:          apng.BLEND_OP_OVER,
 			DelayDenominator: 1000,
 		}
+		if g.Images[i].Delay != 0 {
+			fr.DelayNumerator = uint16(g.Images[i].Delay)
+		}
 		a.Frames = append(a.Frames, fr)
 	}
-	}
 	return a, nil
 }

common/nori/image.go

@@ -51,11 +51,13 @@ func (i *Image) Decode(rd *bufio.Reader, palette *PaletteSection) error {
 			red := uint8(((0x7c00 & bgr) >> 10) * 255 / 31)
 			green := uint8(((0x3e0 & bgr) >> 5) * 255 / 31)
 			blue := uint8((0x1f & bgr) * 255 / 31)
-			if red != 0 && blue != 0 && green != 0 {
+			alpha := uint8(0xff)
+			if red == 0xff && blue == 0xff {
+				alpha = 0x00
 			}
 			col := &color.RGBA{
 				red, green, blue,
-				0xff,
+				alpha,
 			}
 			cf.Set(idx%int(i.Width), idx/int(i.Width), col)
 		}
@@ -68,9 +70,13 @@ func (i *Image) Decode(rd *bufio.Reader, palette *PaletteSection) error {
 			if err != nil {
 				return err
 			}
+			alpha := uint8(0xff)
 			col := &color.RGBA{
 				rgb[0], rgb[1], rgb[2],
-				0xff,
+				alpha,
+			}
+			if rgb[0] == 0xff && rgb[2] == 0xff {
+				alpha = 0x00
 			}
 			cf.Set(idx%int(i.Width), idx/int(i.Width), col)
 		}

common/nori/nori_test.go

@@ -5,7 +5,6 @@ import (
 	"os"
 	"testing"
 
-	"git.tuxpa.in/a/zlog/log"
 	"gitlab.com/gfxlabs/gfximg/apng"
 )
 
@@ -59,14 +58,21 @@ func TestParsePalette(t *testing.T) {
 	if err != nil {
 		t.Errorf("export: %s", err)
 	}
-	writeApng(a, "./nori_test/palette.apng")
+	err = writeApng(a, "./nori_test/palette.apng")
+	if err != nil {
+		t.Errorf("export: %s", err)
+	}
 }
 
-func writeApng(a *apng.APNG, fp string) {
+func writeApng(a *apng.APNG, fp string) error {
 	out := new(bytes.Buffer)
 	err := apng.Encode(out, *a)
 	if err != nil {
-		log.Errorf("fail to encode apng %s", err)
+		return err
 	}
-	os.WriteFile(fp, out.Bytes(), 0740)
+	err = os.WriteFile(fp, out.Bytes(), 0740)
+	if err != nil {
+		return err
+	}
+	return nil
 }

go.mod

@@ -10,5 +10,6 @@ require (
 require (
 	github.com/mattn/go-colorable v0.1.12 // indirect
 	github.com/mattn/go-isatty v0.0.14 // indirect
+	github.com/phrozen/blend v0.0.0-20210220204729-f26b6cf7a28e // indirect
 	golang.org/x/sys v0.0.0-20210927094055-39ccf1dd6fa6 // indirect
 )

go.sum

@@ -6,6 +6,8 @@ github.com/mattn/go-colorable v0.1.12 h1:jF+Du6AlPIjs2BiUiQlKOX0rt3SujHxPnksPKZb
 github.com/mattn/go-colorable v0.1.12/go.mod h1:u5H1YNBxpqRaxsYJYSkiCWKzEfiAb1Gb520KVy5xxl4=
 github.com/mattn/go-isatty v0.0.14 h1:yVuAays6BHfxijgZPzw+3Zlu5yQgKGP2/hcQbHb7S9Y=
 github.com/mattn/go-isatty v0.0.14/go.mod h1:7GGIvUiUoEMVVmxf/4nioHXj79iQHKdU27kJ6hsGG94=
+github.com/phrozen/blend v0.0.0-20210220204729-f26b6cf7a28e h1:r8tWFp1HMiodzOwFtEVZ41Q0PuX/G5PWHZS14kAQMoI=
+github.com/phrozen/blend v0.0.0-20210220204729-f26b6cf7a28e/go.mod h1:8LjAsvtcQgvNmMQZ/iSuduOKYgRA37KcsgPg8ZC6Krc=
 github.com/pkg/errors v0.9.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/rs/xid v1.3.0/go.mod h1:trrq9SKmegXys3aeAKXMUTdJsYXVwGY3RLcfgqegfbg=
 gitlab.com/gfxlabs/gfximg v0.0.5 h1:jtHE6In6axz0zGPy7YnLIZV9RFdcFEZzpdqoXb52K9s=