161 lines
5.8 KiB
Python
161 lines
5.8 KiB
Python
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"""图形验证码渲染。
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**为什么自绘而不用 captcha 库的默认渲染**：
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- 该库默认输出随机彩色字符 + 重噪点 + 弧线，与界面视觉体系严重冲突
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- 它最后更新于 2025-03，内部还引用了 Pillow 的旧别名（`PIL.ImageDraw.ImageDraw`）
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自绘后配色与界面统一，而识别难度由**逐字旋转 + 垂直抖动 + 干扰曲线 + 噪点**
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共同保证，强度与原实现相当且可调（见 `_draw_characters` 的注释）。
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仅沿用该库内置的 DroidSansMono 字体文件：自带字体可避开
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在 Linux 容器里找不到系统字体的经典问题。
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"""
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import base64
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import io
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import math
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import random
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import secrets
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from pathlib import Path
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from PIL import Image, ImageDraw, ImageFont
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from app.core.constants import CAPTCHA_CHARSET, CAPTCHA_LENGTH
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# 画布尺寸与前端展示槽位对齐（登录页为 116×40）
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_WIDTH, _HEIGHT = 120, 40
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_FONT_SIZE = 25
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_PADDING = 8
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# 与前端配色呼应：墨色字符、青灰干扰、素纸底
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_INK = (15, 54, 70)
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_INTERFERENCE = (126, 156, 170)
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_SPECKLE = (168, 183, 192)
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_PAPER = (247, 246, 242)
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_SPECKLE_COUNT = 46
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_INTERFERENCE_LINES = 2
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# 备选字体路径：优先用 captcha 包内置字体（跨平台一致），其余为回退
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_FONT_CANDIDATES = (
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"C:/Windows/Fonts/consola.ttf",
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"/usr/share/fonts/truetype/dejavu/DejaVuSansMono.ttf",
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)
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def _font_path() -> Path | None:
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"""定位可用的等宽字体文件。"""
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import captcha
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package_file = getattr(captcha, "__file__", None)
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if package_file:
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bundled = Path(package_file).parent / "data" / "DroidSansMono.ttf"
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if bundled.is_file():
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return bundled
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for candidate in _FONT_CANDIDATES:
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path = Path(candidate)
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if path.is_file():
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return path
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return None
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def _load_font(size: int) -> ImageFont.FreeTypeFont | ImageFont.ImageFont:
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"""加载字体。找不到任何字体文件时退回 PIL 内置位图字体（仍可用，只是较粗糙）。"""
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path = _font_path()
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if path is None:
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return ImageFont.load_default(size)
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return ImageFont.truetype(str(path), size)
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def generate_text() -> str:
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"""生成验证码文本。
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用 `secrets` 而非 `random`：验证码是安全凭据，
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`random` 的 Mersenne Twister 状态可被观测样本反推。
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"""
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return "".join(secrets.choice(CAPTCHA_CHARSET) for _ in range(CAPTCHA_LENGTH))
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def _draw_speckles(draw: ImageDraw.ImageDraw, count: int) -> None:
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"""撒噪点。
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用单像素点与 2×2 方块，**不用 `ellipse`** —— 3×3 包围盒画出的椭圆
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恰好填充中心与四个边中点，形状就是「＋」，会形成一种规则的十字纹理，
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既显眼又容易被整体滤波抹掉，起不到干扰作用。
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"""
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for _ in range(count):
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x = random.randint(0, _WIDTH - 2)
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y = random.randint(0, _HEIGHT - 2)
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if random.random() < 0.75:
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draw.point((x, y), fill=_SPECKLE)
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else:
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draw.rectangle((x, y, x + 1, y + 1), fill=_SPECKLE)
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def _draw_interference(draw: ImageDraw.ImageDraw) -> None:
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"""绘制干扰曲线。
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用低对比度的正弦折线而非粗直线：粗直线会把字符主干割断、
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显著提高正常用户的识错率，而低对比细线对分割算法的干扰并不更差。
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"""
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for _ in range(_INTERFERENCE_LINES):
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amplitude = random.uniform(5.0, 11.0)
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phase = random.uniform(0, math.tau)
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base_y = random.uniform(_HEIGHT * 0.25, _HEIGHT * 0.75)
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points = [
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(x, base_y + amplitude * math.sin(phase + x / 34.0)) for x in range(0, _WIDTH + 1, 4)
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]
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draw.line(points, fill=_INTERFERENCE, width=1)
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def _draw_characters(image: Image.Image, text: str) -> None:
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"""逐字绘制。
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**每个字符单独渲染后旋转再贴回**，而不是整行一起旋转：
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整行旋转只是把同一个倾斜做一次仿射变换，对齐后即可还原；
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逐字随机旋转会破坏字符间的版式一致性，模板匹配难以复用。
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再叠加 ±2px 的垂直抖动，进一步打散基线。
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"""
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font = _load_font(_FONT_SIZE)
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cell = (_WIDTH - 2 * _PADDING) / len(text)
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for index, char in enumerate(text):
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# 每个字形画在独立画布上以便单独旋转；留出余量避免旋转后切角
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glyph = Image.new("RGBA", (int(cell) + 16, _HEIGHT), (0, 0, 0, 0))
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ImageDraw.Draw(glyph).text(
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(8, 3),
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char,
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font=font,
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fill=(*_INK, 255),
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# 加 1 像素描边：等宽细体在小字号下笔画过细，
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# 而本系统的用户是营销人员、屏幕与年龄跨度都大，
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# 验证码过难会直接变成支持负担。加粗的代价是连通像素略多，
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# 但相对于"用户反复输错"的代价是划算的。
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stroke_width=1,
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stroke_fill=(*_INK, 255),
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)
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glyph = glyph.rotate(random.uniform(-14, 14), resample=Image.BICUBIC, expand=False)
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image.paste(glyph, (int(_PADDING + index * cell) - 8, random.randint(-2, 2)), glyph)
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def render_png_base64(text: str) -> str:
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"""把验证码文本渲染为 PNG 并返回 base64（**不含** `data:` 前缀）。
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不含前缀是为了让前端自行拼接；服务端返回纯数据，
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浏览器 / 小程序 / 桌面端可各自决定呈现方式。
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"""
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image = Image.new("RGB", (_WIDTH, _HEIGHT), _PAPER)
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draw = ImageDraw.Draw(image)
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_draw_speckles(draw, _SPECKLE_COUNT // 2)
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_draw_interference(draw)
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_draw_characters(image, text)
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# 字符之上再撒一层，干扰基于连通域的分割
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_draw_speckles(draw, _SPECKLE_COUNT // 2)
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buffer = io.BytesIO()
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image.save(buffer, format="PNG")
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return base64.b64encode(buffer.getvalue()).decode("ascii")
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