usernode/internal/auth/captcha_img.go

package auth

import (
	"bytes"
	"fmt"
	"image"
	"image/color"
	"image/draw"
	"image/png"
	"math/rand"
	"time"
)

// 验证码图像渲染：内置 5x7 点阵数字 + 噪点 + 干扰线，不依赖第三方字体库
// （PLAN §4：内置生成，不依赖第三方服务）。

// digitGlyphs 为 0-9 的 5x7 点阵，每行低 5 位表示该行像素。
var digitGlyphs = [10][7]byte{
	{0b01110, 0b10001, 0b10011, 0b10101, 0b11001, 0b10001, 0b01110}, // 0
	{0b00100, 0b01100, 0b00100, 0b00100, 0b00100, 0b00100, 0b01110}, // 1
	{0b01110, 0b10001, 0b00001, 0b00010, 0b00100, 0b01000, 0b11111}, // 2
	{0b11111, 0b00010, 0b00100, 0b00010, 0b00001, 0b10001, 0b01110}, // 3
	{0b00010, 0b00110, 0b01010, 0b10010, 0b11111, 0b00010, 0b00010}, // 4
	{0b11111, 0b10000, 0b11110, 0b00001, 0b00001, 0b10001, 0b01110}, // 5
	{0b00110, 0b01000, 0b10000, 0b11110, 0b10001, 0b10001, 0b01110}, // 6
	{0b11111, 0b00001, 0b00010, 0b00100, 0b01000, 0b01000, 0b01000}, // 7
	{0b01110, 0b10001, 0b10001, 0b01110, 0b10001, 0b10001, 0b01110}, // 8
	{0b01110, 0b10001, 0b10001, 0b01111, 0b00001, 0b00010, 0b01100}, // 9
}

const (
	glyphW    = 5
	glyphH    = 7
	scale     = 3 // 点阵放大倍数
	charGap   = 4 // 字符间距（像素）
	edgePad   = 6 // 画布边距
)

// RenderCaptchaPNG 将 4 位数字验证码渲染为 PNG 字节流。
// 仅接受数字字符，其余返回错误。
func RenderCaptchaPNG(text string) ([]byte, error) {
	if len(text) == 0 || len(text) > 8 {
		return nil, fmt.Errorf("auth: captcha text length must be 1~8")
	}
	for _, r := range text {
		if r < '0' || r > '9' {
			return nil, fmt.Errorf("auth: captcha text must be digits")
		}
	}
	canvasW := edgePad*2 + len(text)*glyphW*scale + (len(text)-1)*charGap
	canvasH := edgePad*2 + glyphH*scale
	img := image.NewRGBA(image.Rect(0, 0, canvasW, canvasH))
	draw.Draw(img, img.Bounds(), &image.Uniform{C: color.RGBA{248, 250, 252, 255}}, image.Point{}, draw.Src)

	rng := rand.New(rand.NewSource(time.Now().UnixNano()))

	// 噪点（浅灰，稀疏）
	for i := 0; i < canvasW*canvasH/7; i++ {
		x, y := rng.Intn(canvasW), rng.Intn(canvasH)
		g := uint8(150 + rng.Intn(90))
		img.Set(x, y, color.RGBA{g, g, g, 255})
	}

	// 干扰线（穿过字符区域的浅色斜线）
	for i := 0; i < 3; i++ {
		g := uint8(160 + rng.Intn(80))
		c := color.RGBA{g, g, g, 255}
		x1, y1 := rng.Intn(canvasW/2), rng.Intn(canvasH)
		x2, y2 := canvasW/2+rng.Intn(canvasW/2), rng.Intn(canvasH)
		drawLine(img, x1, y1, x2, y2, c)
	}

	// 逐字符绘制（颜色随机取深色系）
	inkPalette := []color.RGBA{
		{40, 60, 110, 255}, {120, 45, 45, 255}, {30, 90, 60, 255}, {80, 60, 110, 255},
	}
	for i, r := range text {
		glyph := digitGlyphs[r-'0']
		ink := inkPalette[rng.Intn(len(inkPalette))]
		x0 := edgePad + i*(glyphW*scale+charGap)
		y0 := edgePad
		for row := 0; row < glyphH; row++ {
			for col := 0; col < glyphW; col++ {
				if glyph[row]&(1<<(glyphW-1-col)) != 0 {
					fillRect(img, x0+col*scale, y0+row*scale, scale, scale, ink)
				}
			}
		}
	}

	var buf bytes.Buffer
	if err := png.Encode(&buf, img); err != nil {
		return nil, fmt.Errorf("auth: encode captcha png: %w", err)
	}
	return buf.Bytes(), nil
}

// fillRect 填充实心矩形。
func fillRect(img *image.RGBA, x, y, w, h int, c color.RGBA) {
	for dy := 0; dy < h; dy++ {
		for dx := 0; dx < w; dx++ {
			img.Set(x+dx, y+dy, c)
		}
	}
}

// drawLine 使用 DDA 算法画线。
func drawLine(img *image.RGBA, x1, y1, x2, y2 int, c color.RGBA) {
	steps := abs(x2-x1)
	if d := abs(y2 - y1); d > steps {
		steps = d
	}
	if steps == 0 {
		img.Set(x1, y1, c)
		return
	}
	for i := 0; i <= steps; i++ {
		x := x1 + (x2-x1)*i/steps
		y := y1 + (y2-y1)*i/steps
		if x >= 0 && x < img.Bounds().Dx() && y >= 0 && y < img.Bounds().Dy() {
			img.Set(x, y, c)
		}
	}
}

func abs(n int) int {
	if n < 0 {
		return -n
	}
	return n
}