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

```go
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:

```go
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:

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

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

```go
// 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.

```go
// 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:

```go
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)

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

### 3.2 Layout (for readers)

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

## 4. Iterating Through Glyphs

After layout, iterate through glyphs using `NextGlyph()`:

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

### 4.1 Glyph Fields

```go
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:

```go
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:

```go
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

```go
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:

```go
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**:
```go
// 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):

```go
// 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):

```go
// 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:

```go
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:

```go
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`:

```go
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`:

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

**Mistake**: Subtracting `first.Y`:

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

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

```go
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)

```go
// 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:

```go
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