pixel-sprite-maker: spritec - .sprite text format -> png/jpg/svg + sprite sheets

- .sprite format: single-char palette keys (hex/rgb()/CSS names/none), grid
  rows, '.' transparent by default, max 256x256 per sprite
- render/sheet/info/preview subcommands; sheets name themselves
  <base>_<cellW>x<cellH>_<cols>x<rows>.<ext> (row-major)
- SVG output RLE-merges pixel runs; JPG composites over --bg
- go tests for parser, colors, scaling, svg, sheet layout

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MmdG9GqfSWCzts7AkDwRDh

pixel-sprite-maker/sprite/render.go

@@ -0,0 +1,75 @@
+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,
+	}
+}