agent-tools/pixel-sprite-maker/sprite/render.go

package sprite

import (
	"fmt"
	"image"
	"image/color"
	"image/jpeg"
	"image/png"
	"io"
)

// PixelGrid is anything that can be rendered pixel by pixel: a single
// Sprite or a composed Sheet.
type PixelGrid interface {
	Bounds() (w, h int)
	At(x, y int) color.NRGBA
}

// Image renders a PixelGrid to an NRGBA image, scaled up by the integer
// factor scale (nearest neighbour, keeps pixels crisp).
func Image(g PixelGrid, scale int) *image.NRGBA {
	if scale < 1 {
		scale = 1
	}
	w, h := g.Bounds()
	img := image.NewNRGBA(image.Rect(0, 0, w*scale, h*scale))
	for y := 0; y < h; y++ {
		for x := 0; x < w; x++ {
			c := g.At(x, y)
			for dy := 0; dy < scale; dy++ {
				for dx := 0; dx < scale; dx++ {
					img.SetNRGBA(x*scale+dx, y*scale+dy, c)
				}
			}
		}
	}
	return img
}

// WritePNG encodes g as PNG with transparency preserved.
func WritePNG(w io.Writer, g PixelGrid, scale int) error {
	return png.Encode(w, Image(g, scale))
}

// WriteJPG encodes g as JPEG. JPEG has no alpha channel, so transparent
// pixels are composited over bg first.
func WriteJPG(w io.Writer, g PixelGrid, scale int, bg color.NRGBA, quality int) error {
	if quality < 1 || quality > 100 {
		return fmt.Errorf("jpg quality %d out of range 1-100", quality)
	}
	src := Image(g, scale)
	b := src.Bounds()
	flat := image.NewRGBA(b)
	for y := b.Min.Y; y < b.Max.Y; y++ {
		for x := b.Min.X; x < b.Max.X; x++ {
			flat.Set(x, y, blendOver(src.NRGBAAt(x, y), bg))
		}
	}
	return jpeg.Encode(w, flat, &jpeg.Options{Quality: quality})
}

// blendOver composites src over an opaque background color.
func blendOver(src, bg color.NRGBA) color.NRGBA {
	if src.A == 255 {
		return src
	}
	a := uint32(src.A)
	inv := 255 - a
	return color.NRGBA{
		R: uint8((uint32(src.R)*a + uint32(bg.R)*inv) / 255),
		G: uint8((uint32(src.G)*a + uint32(bg.G)*inv) / 255),
		B: uint8((uint32(src.B)*a + uint32(bg.B)*inv) / 255),
		A: 255,
	}
}