Add helper PixelCoordinateForPosition function in Canvas

This is required for certain use-cases to know where in a render output a position is.
Cannot be done with current API due to Gl texture scale in glPainter on macOS

canvas.go

@@ -32,4 +32,8 @@ type Canvas interface {
 	AddShortcut(shortcut Shortcut, handler func(shortcut Shortcut))
 
 	Capture() image.Image
+
+	// PixelCoordinateForPosition returns the x and y pixel coordinate for a given position on this canvas.
+	// This can be used to find absolute pixel positions or pixel offsets relative to an object top left.
+	PixelCoordinateForPosition(Position) (int, int)
 }

internal/driver/glfw/canvas.go

@@ -2,6 +2,7 @@ package glfw
 
 import (
 	"image"
+	"math"
 	"sync"
 
 	"fyne.io/fyne"
@@ -32,8 +33,8 @@ type glCanvas struct {
 	onKeyUp     func(*fyne.KeyEvent)
 	shortcut    fyne.ShortcutHandler
 
-	scale, detectedScale float32
-	painter              gl.Painter
+	scale, detectedScale, texScale float32
+	painter                        gl.Painter
 
 	dirty                              bool
 	dirtyMutex                         *sync.Mutex
@@ -192,6 +193,21 @@ func (c *glCanvas) SetScale(_ float32) {
 	c.context.RescaleContext()
 }
 
+func (c *glCanvas) setTextureScale(scale float32) {
+	c.texScale = scale
+	c.painter.SetFrameBufferScale(scale)
+}
+
+func (c *glCanvas) PixelCoordinateForPosition(pos fyne.Position) (int, int) {
+	texScale := c.texScale
+	multiple := float64(c.Scale() * texScale)
+	scaleInt := func(x int) int {
+		return int(math.Round(float64(x) * multiple))
+	}
+
+	return scaleInt(pos.X), scaleInt(pos.Y)
+}
+
 func (c *glCanvas) OnTypedRune() func(rune) {
 	return c.onTypedRune
 }
@@ -463,7 +479,7 @@ func (c *glCanvas) contentPos() fyne.Position {
 }
 
 func newCanvas() *glCanvas {
-	c := &glCanvas{scale: 1.0}
+	c := &glCanvas{scale: 1.0, texScale: 1.0}
 	c.content = &canvas.Rectangle{FillColor: theme.BackgroundColor()}
 	c.contentTree = &renderCacheTree{root: &renderCacheNode{obj: c.content}}
 	c.padded = true

internal/driver/glfw/canvas_test.go

@@ -42,6 +42,30 @@ func TestGlCanvas_Resize(t *testing.T) {
 	assert.Equal(t, size, over.Size())
 }
 
+func TestGlCanvas_Scale(t *testing.T) {
+	w := d.CreateWindow("Test").(*window)
+	c := w.Canvas().(*glCanvas)
+
+	c.scale = 2.5
+	assert.Equal(t, 5, int(2*c.Scale()))
+}
+
+func TestGlCanvas_PixelCoordinateAtPosition(t *testing.T) {
+	w := d.CreateWindow("Test").(*window)
+	c := w.Canvas().(*glCanvas)
+
+	pos := fyne.NewPos(4, 4)
+	c.scale = 2.5
+	x, y := c.PixelCoordinateForPosition(pos)
+	assert.Equal(t, 10, x)
+	assert.Equal(t, 10, y)
+
+	c.texScale = 2.0
+	x, y = c.PixelCoordinateForPosition(pos)
+	assert.Equal(t, 20, x)
+	assert.Equal(t, 20, y)
+}
+
 func Test_glCanvas_SetContent(t *testing.T) {
 	fyne.CurrentApp().Settings().SetTheme(theme.DarkTheme())
 	var menuHeight int

internal/driver/glfw/window.go

@@ -469,7 +469,7 @@ func (w *window) frameSized(viewport *glfw.Window, width, height int) {
 
 	winWidth, _ := w.viewport.GetSize()
 	texScale := float32(width) / float32(winWidth) // This will be > 1.0 on a HiDPI screen
-	w.canvas.painter.SetFrameBufferScale(texScale)
+	w.canvas.setTextureScale(texScale)
 	w.canvas.painter.SetOutputSize(width, height)
 }
 

internal/driver/gomobile/canvas.go

@@ -132,6 +132,10 @@ func (c *mobileCanvas) SetScale(_ float32) {
 	c.scale = fyne.CurrentDevice().SystemScale()
 }
 
+func (c *mobileCanvas) PixelCoordinateForPosition(pos fyne.Position) (int, int) {
+	return int(float32(pos.X) * c.scale), int(float32(pos.Y) * c.scale)
+}
+
 func (c *mobileCanvas) Overlay() fyne.CanvasObject {
 	return c.overlay
 }

internal/driver/gomobile/canvas_test.go

@@ -12,6 +12,16 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+func TestCanvas_PixelCoordinateAtPosition(t *testing.T) {
+	c := NewCanvas().(*mobileCanvas)
+
+	pos := fyne.NewPos(4, 4)
+	c.scale = 2.5
+	x, y := c.PixelCoordinateForPosition(pos)
+	assert.Equal(t, 10, x)
+	assert.Equal(t, 10, y)
+}
+
 func TestCanvas_Tapped(t *testing.T) {
 	tapped := false
 	altTapped := false

test/testcanvas.go

@@ -118,6 +118,10 @@ func (c *testCanvas) SetScale(scale float32) {
 	c.scale = scale
 }
 
+func (c *testCanvas) PixelCoordinateForPosition(pos fyne.Position) (int, int) {
+	return int(float32(pos.X) * c.scale), int(float32(pos.Y) * c.scale)
+}
+
 func (c *testCanvas) OnTypedRune() func(rune) {
 	return c.onTypedRune
 }

test/testcanvas_test.go

@@ -24,3 +24,13 @@ func TestTestCanvas_Capture(t *testing.T) {
 	assert.Equal(t, b1, b2)
 	assert.Equal(t, a1, a2)
 }
+
+func TestGlCanvas_PixelCoordinateAtPosition(t *testing.T) {
+	c := NewCanvas().(*testCanvas)
+
+	pos := fyne.NewPos(4, 4)
+	c.scale = 2.5
+	x, y := c.PixelCoordinateForPosition(pos)
+	assert.Equal(t, 10, x)
+	assert.Equal(t, 10, y)
+}