feat: 投影法波谷检测+白底硬切断+帧底部对齐+gifmaker透明GIF

- prompt_agent: 精灵表/瓦片集间隙从2-4px放宽到8-16px纯白
- splitsprite removeWhiteBg: dist<threshold/2直接A=0，消除半透明残留
- splitsprite findCuts: 波峰-波谷检测替代死阈值，归并原阈值回退
- splitsprite alignCenter: 全局画布底部对齐替代独立居中，人物不跳帧
- splitsprite MinFillRatio 调至0.14，MinGapWidth调至2
- 新增 pkg/gifmaker: 统一画布+透明索引0+DisposalBackground
- test_prompt_to_gif 改用gifmaker包

backend/tools/test_prompt_to_gif.go

@@ -7,14 +7,12 @@ import (
 	"context"
 	"fmt"
 	"image"
-	"image/color"
-	"image/draw"
-	"image/gif"
 	"image/png"
 	"os"
 
 	"gen2d/internal/config"
 	"gen2d/internal/service"
+	"gen2d/pkg/gifmaker"
 	"gen2d/pkg/splitsprite"
 )
 
@@ -28,10 +26,10 @@ func main() {
 	// 1. PromptAgent 优化提示词
 	fmt.Println("=== Step 1: Optimize prompt via PromptAgent ===")
 	agentIn := service.PromptAgentInput{
-		Tags:      []string{"像素", "战士", "持剑", "精灵表"},
+		Tags:      []string{"像素", "横版动作", "大剑战士", "精灵表", "4x4网格"},
 		AssetType: "sprite",
-		Prompt:    "生成一个像素风持剑战士的4方向行走精灵表",
-		UserNote:  "需要4方向（上下左右），每方向4帧行走动画",
+		Prompt:    "生成一个2D横版动作游戏角色的连续攻击连招精灵表，侧视角，4行4列网格排列",
+		UserNote:  "角色为持大剑的战士，连招包含4段攻击：横斩→上挑→跳劈→终结重击。每段攻击4帧关键帧，共16帧，按4行×4列网格排列。帧间留8-16px纯白间隙(#FFFFFF)。要求动作流畅有力量感，大剑挥舞轨迹清晰",
 	}
 	out, err := service.RunPromptAgent(ctx, agentIn)
 	if err != nil {
@@ -42,7 +40,7 @@ func main() {
 	// 2. 调用文生图 API
 	fmt.Println("=== Step 2: Generate sprite sheet via image API ===")
 	params := service.AssetParams{
-		Resolution: 1024,
+		Resolution: 1536,
 		Format:     "spritesheet",
 	}
 	images, err := service.GenerateImages(ctx, out.Prompt, params)
@@ -63,17 +61,13 @@ func main() {
 	}
 	fmt.Printf("Saved sprite sheet → %s (%d bytes)\n", sheetPath, len(images[0].Data))
 
-	// 3. splitsprite 拆分精灵表
-	fmt.Println("\n=== Step 3: Split sprite sheet ===")
+	// 3. splitsprite: 洗白底 → 波谷投影拆分 → 有效像素≥40% → 裁切像素边界 → 统一最大分辨率居中
+	fmt.Println("\n=== Step 3: Wash white bg → valley projection split → trim → center align ===")
 	sheetImg, err := decodePNG(images[0].Data)
 	if err != nil {
 		fatalf("decode sheet: %v", err)
 	}
-	opts := splitsprite.DefaultOptions()
-	opts.GridRows = 4
-	opts.GridCols = 4
-	opts.GridPadding = 2
-	opts.CenterAlign = true
+	opts := splitsprite.DefaultOptions() // WhiteBg=true, MinFillRatio=0.4, CenterAlign=true
 	frames, err := splitsprite.Process(sheetImg, opts)
 	if err != nil {
 		fatalf("split failed: %v", err)
@@ -92,7 +86,7 @@ func main() {
 	// 4. 生成 GIF 预览
 	fmt.Println("\n=== Step 4: Generate GIF preview ===")
 	gifPath := "test_output/preview.gif"
-	if err := genGIF(gifPath, frames, 12); err != nil {
+	if err := gifmaker.Save(gifPath, frames, nil); err != nil {
 		fatalf("generate GIF: %v", err)
 	}
 	fmt.Printf("GIF preview → %s (%d frames)\n", gifPath, len(frames))
@@ -104,63 +98,6 @@ func main() {
 	fmt.Println("  test_output/preview.gif       — animated GIF preview")
 }
 
-func genGIF(path string, frames []image.Image, delay int) error {
-	f, err := os.Create(path)
-	if err != nil {
-		return err
-	}
-	defer f.Close()
-
-	pal := buildPalette(frames)
-	anim := &gif.GIF{}
-	for _, frame := range frames {
-		b := frame.Bounds()
-		paletted := image.NewPaletted(b, pal)
-		draw.Draw(paletted, b, frame, b.Min, draw.Src)
-		anim.Image = append(anim.Image, paletted)
-		anim.Delay = append(anim.Delay, delay)
-	}
-	anim.LoopCount = 0 // loop forever
-	return gif.EncodeAll(f, anim)
-}
-
-func buildPalette(frames []image.Image) color.Palette {
-	hist := make(map[color.RGBA]int)
-	sampleStep := max(1, len(frames)/8)
-	for i := 0; i < len(frames); i += sampleStep {
-		b := frames[i].Bounds()
-		step := max(1, (b.Dx()*b.Dy())/4096)
-		n := 0
-		for y := b.Min.Y; y < b.Max.Y; y++ {
-			for x := b.Min.X; x < b.Max.X; x++ {
-				if n%step != 0 {
-					n++
-					continue
-				}
-				n++
-				r, g, bl, a := frames[i].At(x, y).RGBA()
-				if a > 0 {
-					c := color.RGBA{R: uint8(r >> 8), G: uint8(g >> 8), B: uint8(bl >> 8), A: uint8(a >> 8)}
-					hist[c]++
-				}
-			}
-		}
-	}
-	pal := make(color.Palette, 0, 256)
-	for c := range hist {
-		pal = append(pal, c)
-		if len(pal) >= 240 {
-			break
-		}
-	}
-	pal = append(pal,
-		color.RGBA{0, 0, 0, 0},
-		color.RGBA{0, 0, 0, 255},
-		color.RGBA{255, 255, 255, 255},
-	)
-	return pal
-}
-
 func decodePNG(data []byte) (image.Image, error) {
 	img, _, err := image.Decode(bytes.NewReader(data))
 	if err != nil {