x/image/font/opentype: Thai language font does not render properly

[zapkub]

What version of Go are you using (go version)?

$ go version go1.15.6 darwin/amd64

Does this issue reproduce with the latest release?

Yes,

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/rungsikorn.r/Library/Caches/go-build"
GOENV="/Users/rungsikorn.r/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/rungsikorn.r/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/rungsikorn.r/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/rungsikorn.r/go/src/***********/sliputil/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/78/kqk2z8953g9gy7z_lg29zqkr0000gq/T/go-build201421536=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I use opentype to render text to image with Thai language character and it rendered incorrectly

What did you expect to see?

The "tone mark" and the "vowel" should be rendered in the right places

มั้ย เนื้อ อื้อ เป็น เบิ้ด หนึ่งหมื่น ฏู ญู  ฤา 

compare to the render result. The glyph which is rendered above and below the line does not pick a glyph correctly

(image from http://aodr.org/_common/do.php?a=full&b=12&bidx=717&aidx=9769 )

What did you see instead?

opentype must pick the right glyph from the character index table which need to be calculated from the combination rules of the Thai language

C90 Thai character combination rules

#27281

[zapkub]

I have already try to achieve the correct result which needs to be

• support post script info table parsing (which already support in opentype package)
• calculate the combination of Thai glyph rules

some work in process here golang/freetype#81 but I found this package is likely to deprecate so I will focus on change in x/image/font/opentype to apply Thai glyph combination rules

Ligature example

B = Base
U = Upper Vowel
L = Lower Vowel
T = Tone mark

B + U + T
ม + ั + ้ = มั้ย