fix i422ToI420 calculation, add mock FastBoxSampling

pkg/io/video/convert_nocgo.go

@@ -57,8 +57,8 @@ func i422ToI420(img image.YCbCr, dst []uint8) image.YCbCr {
 		for j := 0; j < img.CStride; j++ {
 			cb := uint16(img.Cb[addrSrc]) + uint16(img.Cb[addrSrc+img.CStride])
 			cr := uint16(img.Cr[addrSrc]) + uint16(img.Cr[addrSrc+img.CStride])
-			cbDst[addrDst] = uint8(cb / 4)
-			crDst[addrDst] = uint8(cr / 4)
+			cbDst[addrDst] = uint8(cb / 2)
+			crDst[addrDst] = uint8(cr / 2)
 			addrSrc++
 			addrDst++
 		}

pkg/io/video/scale_test.go

@@ -526,6 +526,9 @@ func TestScale(t *testing.T) {
 }
 
 func TestScaleFastBoxSampling(t *testing.T) {
+	if !hasCGOConvert {
+		t.Skip("Skip: nocgo implementation is not supported for FastBoxSampling")
+	}
 	cases := map[string]struct {
 		src           image.Image
 		width, height int

pkg/io/video/scaler_nocgo.go

@@ -0,0 +1,22 @@
+//go:build !cgo
+// +build !cgo
+
+package video
+
+import (
+	"golang.org/x/image/draw"
+	"image"
+)
+
+// ScalerFastBoxSampling mock scaler for nocgo implementation to pass tests for CGO_ENABLED=0
+var (
+	ScalerFastBoxSampling = Scaler(&FastBoxSampling{})
+)
+
+// FastBoxSampling mock implementation for nocgo implementation
+// TODO implement nocgo FastBoxSampling scaling algorithm
+type FastBoxSampling struct {
+}
+
+func (f *FastBoxSampling) Scale(_ draw.Image, _ image.Rectangle, _ image.Image, _ image.Rectangle, _ draw.Op, _ *draw.Options) {
+}