ebiten: Region size changing

main.go

package main

import (
	"image/color"
	"log"
	"math/rand"

	"github.com/hajimehoshi/ebiten/v2"
)

var (
	w, h      = 512, 512
	s         *ebiten.Shader
	brush     = ebiten.NewImage(w, h)
	tex       = ebiten.NewImage(w, h)
	offscreen = ebiten.NewImage(w, h)
)

func init() {
	var err error

	s, err = ebiten.NewShader([]byte(`
	package main

	func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
		v := imageSrc0UnsafeAt(texCoord)
		_, size := imageSrcRegionOnTexture()
		// size = normalize(size) // Note: this shows the same behaviour
		if v.g == 0. {
			return vec4(size.x, size.y, size.y, 1)
		}

		return v
	}`))
	if err != nil {
		log.Fatal(err)
	}

	brush.Fill(color.White)
	tex.Fill(color.White)
}

type Game struct {
	drawticks uint64
}

func (g *Game) Update() error {
	return nil
}

func (g *Game) Draw(screen *ebiten.Image) {
	g.drawticks++
	if g.drawticks%120 == 0 {
		opts := &ebiten.DrawImageOptions{}
		opts.GeoM.Scale(1./16, 1./16)
		opts.GeoM.Translate(
			rand.Float64()*float64(w),
			rand.Float64()*float64(h),
		)
		opts.ColorScale.ScaleWithColor(color.RGBA{255, 0, 255, 255})
		tex.DrawImage(brush, opts)
	}
	// Fill offscreen
	screen.DrawRectShader(w, h, s, &ebiten.DrawRectShaderOptions{
		Images: [4]*ebiten.Image{
			tex,
		},
	})
}

func (g *Game) Layout(outsideWidth, outsideHeight int) (int, int) {
	return w, h
}

func main() {
	ebiten.SetFullscreen(false)
	ebiten.SetWindowSize(w, h)
	if err := ebiten.RunGame(&Game{}); err != nil {
		log.Fatal(err)
	}
}