text/v2: Mongolian scripts are not rendered correctly

[hajimehoshi]

Ebitengine Version

b9b365a

Operating System

• Windows
• macOS
• Linux
• FreeBSD
• OpenBSD
• Android
• iOS
• Nintendo Switch
• PlayStation 5
• Xbox
• Web Browsers

Go Version (go version)

go version go1.21.1 darwin/amd64

What steps will reproduce the problem?

Add NotoSansMongolian.ttf to examples/texti18n and apply this patch:

diff --git a/examples/texti18n/main.go b/examples/texti18n/main.go
index fe604db8a..56b47ca06 100644
--- a/examples/texti18n/main.go
+++ b/examples/texti18n/main.go
@@ -70,6 +70,19 @@ func init() {
        thaiFaceSource = s
 }
 
+//go:embed NotoSansMongolian-Regular.ttf
+var mongolianTTF []byte
+
+var mongolianFaceSource *text.GoTextFaceSource
+
+func init() {
+       s, err := text.NewGoTextFaceSource(bytes.NewReader(mongolianTTF))
+       if err != nil {
+               log.Fatal(err)
+       }
+       mongolianFaceSource = s
+}
+
 var japaneseFaceSource *text.GoTextFaceSource
 
 func init() {
@@ -139,6 +152,25 @@ func (g *Game) Draw(screen *ebiten.Image) {
                op.GeoM.Translate(float64(x), float64(y))
                text.Draw(screen, thaiText, f, op)
        }
+       {
+               const mongolianText = "ᠬᠦᠮᠦᠨ ᠪᠦᠷ ᠲᠥᠷᠥᠵᠦ ᠮᠡᠨᠳᠡᠯᠡᠬᠦ\nᠡᠷᠬᠡ ᠴᠢᠯᠥᠭᠡ ᠲᠡᠢ᠂ ᠠᠳᠠᠯᠢᠬᠠᠨ"
+               f := &text.GoTextFace{
+                       Source:    mongolianFaceSource,
+                       Direction: text.DirectionTopToBottomAndLeftToRight,
+                       Size:      24,
+                       Language:  language.Mongolian,
+                       // language.Mongolian.Script() returns "Cyrl" (Cyrillic), but we want Mongolian script here.
+                       Script: language.MustParseScript("Mong"),
+               }
+               const lineSpacing = 48
+               x, y := 20, 210
+               w, h := text.Measure(mongolianText, f, lineSpacing)
+               vector.DrawFilledRect(screen, float32(x), float32(y), float32(w), float32(h), gray, false)
+               op := &text.DrawOptions{}
+               op.GeoM.Translate(float64(x), float64(y))
+               op.LineSpacingInPixels = lineSpacing
+               text.Draw(screen, mongolianText, f, op)
+       }
        {
                const japaneseText = "あのイーハトーヴォの\nすきとおった風、\n夏でも底に冷たさを\nもつ青いそら…"
                f := &text.GoTextFace{

What is the expected result?

A vertical Mongolian script is rendered correctly

What happens instead?

The glyphs are not rotated:

Anything else you feel useful to add?

Apparently rotating glyphs is Ebitengine's responsibility.

See a rune's Vertical_Orientation property and rotate it correctly.

[hajimehoshi]

go-text/typesetting#111