How do I draw characters without corresponding cmap through the system API?

A font file can allow the character to have glyf data, but it does not necessarily have to have a cmap mapping. (no codepoint)

Therefore, I want to know if there is a system API that can render character in this situation.

I know that I can get all the point set data of this contours and the flags (onCurve,...) information from the glyf raw data, so that I can draw the glyph myself.

However, I am seeking a solution using a system API (preferably Windows, but solutions for other platforms are also welcome).

If it has a code point, I can directly use TextOutW to draw the character, but if there is no code point and I only know the glyphIndex, is there a way to achieve that?

ExtTextOut combined with ETO_GLYPH_INDEX seems to be able to do it, but I'm not very sure.

Solution

Indeed, using ExtTextOut can accomplish it. The options should be set to ETO_GLYPH_INDEX (0x10). The content that lpString points to should be filled according to the glyph index.

ExtTextOut(hdc, 0, 0, ETO_GLYPH_INDEX, nil, []uint16{49, 50, 51}) // glyphIndex: 49, 50, 51
ExtTextOut(hdc, 0, 0, 0, nil, utf16.Encode([]rune("Hello world 世界")) // draw string

Example

func main() {
    lf := w32.LogFont{
        Height:         -64,
        Width:          0,
        Escapement:     0,
        Orientation:    0,
        Weight:         400,
        Italic:         0,
        Underline:      0,
        StrikeOut:      0,
        CharSet:        w32.DEFAULT_CHARSET,
        OutPrecision:   w32.OUT_TT_PRECIS,
        ClipPrecision:  w32.CLIP_DEFAULT_PRECIS,
        Quality:        w32.ANTIALIASED_QUALITY,
        PitchAndFamily: w32.FF_DONTCARE,
    }
    hFont := gdiDll.CreateFont(
        lf.Height,
        lf.Width,
        lf.Escapement,
        lf.Orientation,
        lf.Weight,
        uint32(lf.Italic),
        uint32(lf.Underline),
        uint32(lf.StrikeOut),
        uint32(lf.CharSet),
        uint32(lf.OutPrecision),
        uint32(lf.ClipPrecision),
        uint32(lf.Quality),
        uint32(lf.PitchAndFamily),
        "Arial",
    )
    defer gdiDll.DeleteObject(w32.HGDIOBJ(hFont))

    hdc := userDll.GetDC(0)
    defer userDll.ReleaseDC(0, hdc)

    hMemDC := gdiDll.CreateCompatibleDC(hdc)
    defer gdiDll.DeleteObject(w32.HGDIOBJ(hMemDC))

    const width = 1000
    const height = 500
    hBitmap := gdiDll.CreateCompatibleBitmap(hdc, width, height)
    hObjOld := gdiDll.SelectObject(hMemDC, w32.HGDIOBJ(hBitmap))
    defer gdiDll.SelectObject(hMemDC, hObjOld)
    oldFont := gdiDll.SelectObject(hMemDC, w32.HGDIOBJ(hFont))
    defer gdiDll.SelectObject(hMemDC, oldFont)

    gdiDll.SetTextColor(hMemDC, w32.RGB(255, 0, 0))
    gdiDll.ExtTextOut(hMemDC, 20, 20, w32.ETO_GLYPH_INDEX, nil, []uint16{49, 50, 51}) // GlyphIndex 49, 50, 51
    gdiDll.ExtTextOut(hMemDC, 500, 300, 0, nil, utf16.Encode([]rune("Hello world 世界")))

    _ = userDll.OpenClipboard(0)
    _ = userDll.EmptyClipboard()
    _, _ = userDll.SetClipboardData(w32.CF_BITMAP, w32.HANDLE(hBitmap))
    _ = userDll.CloseClipboard()
}