Pad/doc/shaper_usage.md

Text Shaper Usage Guide

This document describes how to use the Gio text shaper correctly for text layout, measurement, and rendering. It serves as a reference to avoid common mistakes and ensure consistency across the codebase.

1. Creating a Shaper

import (
    "gioui.org/text"
    "gioui.org/x/gofont"
)

shp := text.NewShaper(text.WithCollection(gofont.Collection()))

Important: Always provide a font collection. Without it, the shaper may not load fonts correctly and will return zero-width glyphs.

2. Layout Parameters — CRITICAL

The text.Parameters struct controls how text is shaped. Three fields are mandatory for correct single-line text layout:

params := text.Parameters{
    PxPerEm:    fixed.I(gtx.Sp(size)),  // Font size in device pixels (REQUIRED)
    MinWidth:   0,                       // Minimum width (REQUIRED for single-line)
    MaxWidth:   availableWidth,          // Maximum width (REQUIRED for single-line)
    MaxLines:   1,                       // Limit to one line (REQUIRED for single-line)
}

2.1 PxPerEm Calculation

CORRECT: Use gtx.Sp(size) to convert SP to device pixels:

params := text.Parameters{
    PxPerEm: fixed.I(gtx.Sp(size)),  // size is unit.Sp
}

WRONG: Don't use arbitrary multipliers like 1024 * pixelsPerDp:

// WRONG - this produces incorrect widths
params := text.Parameters{
    PxPerEm: fixed.Int26_6(1024 * pixelsPerDp),
}

Why: gtx.Sp(size) correctly converts SP to device pixels accounting for DPI scaling. The fixed.I() function converts the result to fixed.Int26_6 format.

2.2 MinWidth/MaxWidth/MaxLines — REQUIRED

Without MinWidth, MaxWidth, and MaxLines, the shaper has no horizontal space constraint and will wrap every character into its own line. This is the most common mistake when using the shaper.

// WRONG - every character wraps to its own line
shp.LayoutString(text.Parameters{PxPerEm: fixed.I(gtx.Sp(size))}, str)

// CORRECT - text flows horizontally within constraints
shp.LayoutString(text.Parameters{
    PxPerEm:  fixed.I(gtx.Sp(size)),
    MinWidth: 0,
    MaxWidth: availableWidth,
    MaxLines: 1,
}, str)

Why: Gio's widget.Label sets these from gtx.Constraints:

• MinWidth: cs.Min.X
• MaxWidth: cs.Max.X
• MaxLines: l.MaxLines

Without these, the shaper treats each character as a separate line with FlagLineBreak set.

2.3 Word Wrap

Set MaxWidth to enable word wrap:

params := text.Parameters{
    PxPerEm:    fixed.I(gtx.Sp(size)),
    MinWidth:   0,
    MaxWidth:   int(widthInPixels),
    MaxLines:   0,  // 0 = no limit, allows multi-line
    WrapPolicy: text.WrapHeuristically,
}

The shaper automatically breaks text into lines at word boundaries when MaxWidth is set and MaxLines is 0 or unset.

3. Layout Functions

3.1 LayoutString (for single strings)

shp.LayoutString(params, "Hello, World!")

3.2 Layout (for readers)

shp.Layout(params, strings.NewReader("Hello, World!"))

4. Iterating Through Glyphs

After layout, iterate through glyphs using NextGlyph():

for {
    g, ok := shp.NextGlyph()
    if !ok {
        break
    }
    // Process glyph g
}

4.1 Glyph Fields

type Glyph struct {
    ID      GlyphID         // Glyph ID
    X       fixed.Int26_6   // Dot position in document coordinates
    Y       int32           // Baseline position (same for all glyphs on a line)
    Ascent  fixed.Int26_6   // Line ascent
    Descent fixed.Int26_6   // Line descent
    Advance fixed.Int26_6   // Logical width (horizontal advance)
    Runes   uint16          // Number of runes this glyph represents
    Offset  fixed.Point26_6 // Glyph offset from (X, Y)
    Bounds  fixed.Rectangle26_6 // Glyph bounds relative to dot
    Flags   Flags           // FlagLineBreak, FlagRunBreak, FlagClusterBreak, etc.
}

4.2 Accumulating Widths

To get per-character cumulative widths:

func charWidths(gtx layout.Context, shp *text.Shaper, str string, size unit.Sp) []int {
    shp.LayoutString(text.Parameters{
        PxPerEm:  fixed.I(gtx.Sp(size)),
        MinWidth: 0,
        MaxWidth: 1000,
        MaxLines: 1,
    }, str)
    var widths []int
    var cumWidth fixed.Int26_6 = 0
    for {
        g, ok := shp.NextGlyph()
        if !ok {
            break
        }
        cumWidth += g.Advance
        widths = append(widths, int(cumWidth>>6)) // Convert from Int26_6 to int
    }
    return widths
}

Key points:

• g.Advance is in fixed.Int26_6 format (16.6 fixed-point)
• Convert to int by shifting right by 6 bits: int(cumWidth>>6)
• The cumulative width is in device pixels
• Always set MinWidth, MaxWidth, MaxLines or widths will be wrong

4.3 Getting Per-Character Widths

For hit testing (mapping UI coordinates → byte offset), we need per-character cumulative widths:

func charWidths(gtx layout.Context, shp *text.Shaper, str string, size unit.Sp) []int {
    shp.LayoutString(text.Parameters{
        PxPerEm:  fixed.I(gtx.Sp(size)),
        MinWidth: 0,
        MaxWidth: 1000,
        MaxLines: 1,
    }, str)
    var widths []int
    var cumWidth fixed.Int26_6 = 0
    for {
        g, ok := shp.NextGlyph()
        if !ok {
            break
        }
        cumWidth += g.Advance
        widths = append(widths, int(cumWidth>>6))
    }
    return widths
}

5. Rendering Glyphs Directly

Gio's paintGlyph (widget/label.go) draws glyphs using shaper.Shape() (for vector glyphs) and shaper.Bitmaps() (for bitmap glyphs like emoji). This is the correct approach to avoid double-shaping.

5.1 Drawing Text — Match Gio's paintGlyph Exactly

func drawText(gtx layout.Context, shp *text.Shaper, str string, size unit.Sp, x, y unit.Dp, col color.NRGBA) {
    // Layout text with width constraints
    shp.LayoutString(text.Parameters{
        PxPerEm:  fixed.I(gtx.Sp(size)),
        MinWidth: 0,
        MaxWidth: 1000,
        MaxLines: 1,
    }, str)
    drawLineText(gtx, shp, x, y, col)
}

func drawLineText(gtx layout.Context, shp *text.Shaper, x, y unit.Dp, col color.NRGBA) {
    // Match Gio's textView: record into a macro for clipping
    m := op.Record(gtx.Ops)
    var glyphs [32]text.Glyph
    line := glyphs[:0]
    for g, ok := shp.NextGlyph(); ok; g, ok = shp.NextGlyph() {
        line = append(line, g)
        if g.Flags&text.FlagLineBreak != 0 || cap(line)-len(line) == 0 {
            drawLine(gtx, shp, line, x, y, col)
            line = line[:0]
        }
    }
    if len(line) > 0 {
        drawLine(gtx, shp, line, x, y, col)
    }
    call := m.Stop()
    call.Add(gtx.Ops)
}

func drawLine(gtx layout.Context, shp *text.Shaper, line []text.Glyph, x, y unit.Dp, col color.NRGBA) {
    if len(line) == 0 {
        return
    }
    first := line[0]
    // shaper.Shape(line) returns a path where glyph positions are relative to
    // the first glyph. Offset by (x + first.X, y + first.Y) to place the line
    // at the desired document position. Matches Gio's paintGlyph:
    //   lineOff = (glyph.X, glyph.Y) - viewport.Min
    //   op.Affine(f32.Affine2D{}.Offset(lineOff))
    offX := float32(gtx.Dp(x)) + float32(first.X)/64.0
    offY := float32(gtx.Dp(y)) + float32(first.Y)
    t := op.Affine(f32.Affine2D{}.Offset(f32.Pt(offX, offY))).Push(gtx.Ops)

    // Draw vector glyphs
    path := shp.Shape(line)
    outline := clip.Outline{Path: path}.Op().Push(gtx.Ops)
    paint.ColorOp{Color: col}.Add(gtx.Ops)
    paint.PaintOp{}.Add(gtx.Ops)
    outline.Pop()

    // Draw bitmap glyphs (emoji, etc.)
    if call := shp.Bitmaps(line); call != (op.CallOp{}) {
        call.Add(gtx.Ops)
    }

    t.Pop()
}

5.2 Offset Calculation Explained

The offset calculation is critical:

offX := float32(gtx.Dp(x)) + float32(first.X)/64.0
offY := float32(gtx.Dp(y)) + float32(first.Y)

• x, y are the desired document position (in DP)
• first.X is the first glyph's X position in fixed.Int26_6 (divide by 64 to get pixels)
• first.Y is the first glyph's Y position (baseline) in pixels
• shp.Shape(line) returns a path where glyph positions are relative to the first glyph
• So we offset by (x + first.X, y + first.Y) to place the line at (x, y)

This matches Gio's paintGlyph exactly:

// Gio's paintGlyph:
if len(line) == 0 {
    it.lineOff = f32.Point{X: fixedToFloat(glyph.X), Y: float32(glyph.Y)}
        .Sub(layout.FPt(it.viewport.Min))
}
// ...
t := op.Affine(f32.Affine2D{}.Offset(it.lineOff)).Push(gtx.Ops)

Since viewport.Min = (0,0) in our case, lineOff = (glyph.X, glyph.Y).

5.3 Avoiding Double-Shaping

WRONG (shapes text twice):

// First pass: measure
widths := charWidths(gtx, shp, text, size)

// Second pass: render (material.Label calls shaper again)
material.Label(th, size, text).Layout(gtx)

CORRECT (shapes text once for rendering):

// Single pass: layout and draw using shaper.Shape()
drawText(gtx, shp, text, size, x, y, col)

NOTE: For truncation, we may need to layout twice:

1. First layout to measure widths and find truncation point
2. Second layout to draw the truncated text

This is acceptable because truncation is only needed when text doesn't fit, and the string is short (filename in StatusBar).

6. Comparison with Gio's paintGlyph

Gio's paintGlyph (widget/label.go) demonstrates the correct approach:

func (it *textIterator) paintGlyph(gtx layout.Context, shaper *text.Shaper, glyph text.Glyph, line []text.Glyph) ([]text.Glyph, bool) {
    visibleOrBefore := it.processGlyph(glyph, true)
    if it.visible {
        if len(line) == 0 {
            it.lineOff = f32.Point{X: fixedToFloat(glyph.X), Y: float32(glyph.Y)}
                .Sub(layout.FPt(it.viewport.Min))
        }
        line = append(line, glyph)
    }
    if glyph.Flags&text.FlagLineBreak != 0 || cap(line)-len(line) == 0 || !visibleOrBefore {
        t := op.Affine(f32.Affine2D{}.Offset(it.lineOff)).Push(gtx.Ops)
        path := shaper.Shape(line)
        outline := clip.Outline{Path: path}.Op().Push(gtx.Ops)
        it.material.Add(gtx.Ops)  // sets color
        paint.PaintOp{}.Add(gtx.Ops)
        outline.Pop()
        if call := shaper.Bitmaps(line); call != (op.CallOp{}) {
            call.Add(gtx.Ops)
        }
        t.Pop()
        line = line[:0]
    }
    return line, visibleOrBefore
}

Key insights:

1. lineOff is set on the first glyph: (glyph.X, glyph.Y) - viewport.Min
2. shaper.Shape(line) returns a path relative to the first glyph
3. The offset places the first glyph at lineOff, and subsequent glyphs follow
4. it.material is a pre-recorded color call; we use paint.ColorOp + paint.PaintOp instead
5. Everything is wrapped in op.Record/m.Stop() for clipping

7. Common Pitfalls

7.1 Missing MinWidth/MaxWidth/MaxLines

Mistake: Calling LayoutString without width constraints:

shp.LayoutString(text.Parameters{PxPerEm: fixed.I(gtx.Sp(size))}, str)

Result: Every character wraps to its own line with FlagLineBreak set and X=0.

Fix: Always set MinWidth, MaxWidth, and MaxLines:

shp.LayoutString(text.Parameters{
    PxPerEm:  fixed.I(gtx.Sp(size)),
    MinWidth: 0,
    MaxWidth: availableWidth,
    MaxLines: 1,
}, str)

7.2 Wrong PxPerEm Calculation

Mistake: Using arbitrary multipliers like 1024 * pixelsPerDp

Fix: Use fixed.I(gtx.Sp(size)) to get the font size in device pixels

7.3 Double-Shaping

Mistake: Calling charWidths() to measure, then material.Label() to render

Fix: Use shp.Shape() to render the already-laid-out glyphs

7.4 Wrong Offset Calculation

Mistake: Using just (x, y) without adding first.X/first.Y:

// WRONG - glyphs will be at wrong position
offX := float32(gtx.Dp(x))
offY := float32(gtx.Dp(y))

Mistake: Subtracting first.Y:

// WRONG - pushes text off-screen
offY := float32(gtx.Dp(y)) - float32(first.Y)

Fix: Add first.X and first.Y to the offset:

offX := float32(gtx.Dp(x)) + float32(first.X)/64.0
offY := float32(gtx.Dp(y)) + float32(first.Y)

7.5 Not Converting Int26_6 to int

Mistake: Using int(g.Advance) directly without shifting

Fix: Use int(g.Advance >> 6) to convert from fixed.Int26_6 to int

8. Usage in Pad

8.1 Filename Truncation (StatusBar)

// Measure widths
shp.LayoutString(text.Parameters{
    PxPerEm:  fixed.I(gtx.Sp(size)),
    MinWidth: 0,
    MaxWidth: availableWidth,
    MaxLines: 1,
}, filename)

// Truncate if needed
widths := measureWidths(shp)
if widths[len(widths)-1] > availableWidth {
    // Find truncation point
    truncated = filename[:i] + "..."
}

// Re-layout and render
shp.LayoutString(text.Parameters{...}, truncated)
drawLineText(gtx, shp, x, y, col)

8.2 TextField (Editor Content)

For the TextField element, we'll use the same approach:

1. Layout text with MaxWidth for word wrap
2. Iterate through glyphs to get per-character positions
3. Use glyph positions for:
   • Hit testing (UI coordinates → byte offset)
   • Cursor positioning
   • Selection rendering
   • IME bridge sync

8.3 BottomBar

The BottomBar uses op.Offset + material.Label() for simple text that doesn't need precise positioning. This is acceptable because:

• The text is short and fixed
• We don't need per-character widths for hit testing
• The performance impact is negligible

9. Debugging Tips

When text doesn't render correctly, add debug logging to trace:

fmt.Printf("[DEBUG] glyph X=%d Y=%d advance=%d flags=%b\n",
    g.X, g.Y, g.Advance, g.Flags)
fmt.Printf("[DEBUG] offX=%f offY=%f\n", offX, offY)
fmt.Printf("[DEBUG] clip=(%d, %d, %d, %d)\n", clipMin.X, clipMin.Y, clipMax.X, clipMax.Y)

Look for:

• All X=0: Missing MaxWidth/MinWidth/MaxLines
• All FlagLineBreak set: Same as above
• Text off-screen: Wrong offset calculation
• Text clipped: Clip region doesn't include text bounds

10. Summary

• Always use fixed.I(gtx.Sp(size)) for PxPerEm
• Always set MinWidth, MaxWidth, MaxLines in LayoutString parameters
• Always offset by (x + first.X, y + first.Y) in drawLine
• Always wrap glyph drawing in op.Record/m.Stop() for clipping
• Avoid using material.Label() when you need precise glyph positions
• Reuse glyph data for measurement, hit testing, and rendering
• Follow Gio's paintGlyph as the reference implementation