feat: Logo 嵌入 — QR 码中央叠加自定义图片

- PNG 渲染：RgbaImage 上使用 imageops::overlay 叠加 logo
- SVG 渲染：base64 编码 logo 嵌入 <image> 标签
- QrCode::to_png_bytes / to_svg 新增 Option<logo_bytes> 参数
- Logo 默认占 QR 区域 25%，建议配合 H 级纠错使用

core/src/render/png.rs

@@ -1,9 +1,8 @@
 use crate::qr::QrCode;
-use image::{ImageBuffer, Rgba};
+use image::{imageops, ImageBuffer, Rgba, RgbaImage};
 
-/// 将单个模块填充到图像缓冲区（module_size × module_size 像素块）
 fn fill_module(
-    img: &mut ImageBuffer<Rgba<u8>, Vec<u8>>,
+    img: &mut RgbaImage,
     x: u32,
     y: u32,
     module_size: u32,
@@ -25,7 +24,40 @@ fn fill_module(
     }
 }
 
-pub fn render_png(qr: &QrCode, module_size: u8) -> Result<Vec<u8>, image::ImageError> {
+/// 在 QR 码 PNG 缓冲区中央叠加 logo
+fn overlay_logo(img: &mut RgbaImage, logo_bytes: &[u8], logo_size_pct: f32) -> Result<(), String> {
+    let logo =
+        image::load_from_memory(logo_bytes).map_err(|e| format!("Logo 加载失败: {e}"))?;
+    let logo = logo.to_rgba8();
+
+    let img_w = img.width();
+    let img_h = img.height();
+
+    // Logo 边长 = min(图像边长 * pct, 实际 QR 区域 * pct)
+    let logo_size = (img_w.min(img_h) as f32 * logo_size_pct) as u32;
+    if logo_size < 4 {
+        return Ok(()); // 太小，跳过
+    }
+
+    let resized = imageops::resize(
+        &logo,
+        logo_size,
+        logo_size,
+        imageops::FilterType::Lanczos3,
+    );
+
+    let x = (img_w - logo_size) / 2;
+    let y = (img_h - logo_size) / 2;
+    imageops::overlay(img, &resized, x as i64, y as i64);
+
+    Ok(())
+}
+
+pub fn render_png(
+    qr: &QrCode,
+    module_size: u8,
+    logo: Option<&[u8]>,
+) -> Result<Vec<u8>, image::ImageError> {
     let matrix_size = qr.size() as u32;
     let margin = qr.margin as u32;
     let total_size = matrix_size + 2 * margin;
@@ -47,18 +79,16 @@ pub fn render_png(qr: &QrCode, module_size: u8) -> Result<Vec<u8>, image::ImageE
                 false
             };
 
-            fill_module(
-                &mut img,
-                x,
-                y,
-                module_size as u32,
-                is_dark,
-                &qr.fg_color,
-                &qr.bg_color,
-            );
+            fill_module(&mut img, x, y, module_size as u32, is_dark, &qr.fg_color, &qr.bg_color);
         }
     }
 
+    // Logo 叠加
+    if let Some(logo_data) = logo {
+        // 忽略 logo 叠加错误（logo 有损不影响 QR 主体）
+        let _ = overlay_logo(&mut img, logo_data, 0.25);
+    }
+
     let mut buf = Vec::new();
     img.write_to(&mut std::io::Cursor::new(&mut buf), image::ImageFormat::Png)?;
     Ok(buf)

core/src/render/svg.rs

@@ -1,6 +1,6 @@
 use crate::qr::QrCode;
 
-pub fn render_svg(qr: &QrCode) -> String {
+pub fn render_svg(qr: &QrCode, logo: Option<&[u8]>) -> String {
     let matrix_size = qr.size() as u32;
     let margin = qr.margin as u32;
     let total = matrix_size + 2 * margin;
@@ -20,9 +20,9 @@ pub fn render_svg(qr: &QrCode) -> String {
         .flat_map(|row| row.iter())
         .filter(|&&m| m)
         .count();
-    let mut svg = String::with_capacity(200 + dark_count * 50);
+    let mut svg = String::with_capacity(300 + dark_count * 50);
     svg.push_str(&format!(
-        r#"<svg xmlns="http://www.w3.org/2000/svg" width="{total}" height="{total}" viewBox="0 0 {total} {total}">"#
+        r#"<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="{total}" height="{total}" viewBox="0 0 {total} {total}">"#
     ));
     svg.push_str(&format!(
         r#"<rect width="{total}" height="{total}" fill="{bg}"/>"#
@@ -40,6 +40,42 @@ pub fn render_svg(qr: &QrCode) -> String {
         }
     }
 
+    // Logo 嵌入：base64 PNG data URL
+    if let Some(logo_bytes) = logo {
+        let b64 = base64_encode(logo_bytes);
+        let logo_size = total as f32 * 0.25;
+        let logo_x = (total as f32 - logo_size) / 2.0;
+        let logo_y = (total as f32 - logo_size) / 2.0;
+        svg.push_str(&format!(
+            r#"<image x="{logo_x}" y="{logo_y}" width="{logo_size}" height="{logo_size}" xlink:href="data:image/png;base64,{b64}"/>"#
+        ));
+    }
+
     svg.push_str("</svg>");
     svg
 }
+
+/// 简易 base64 编码（无外部依赖）
+fn base64_encode(data: &[u8]) -> String {
+    const CHARS: &[u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+    let mut result = String::with_capacity((data.len() + 2) / 3 * 4);
+    for chunk in data.chunks(3) {
+        let b0 = chunk[0] as u32;
+        let b1 = if chunk.len() > 1 { chunk[1] as u32 } else { 0 };
+        let b2 = if chunk.len() > 2 { chunk[2] as u32 } else { 0 };
+        let triple = (b0 << 16) | (b1 << 8) | b2;
+        result.push(CHARS[((triple >> 18) & 0x3F) as usize] as char);
+        result.push(CHARS[((triple >> 12) & 0x3F) as usize] as char);
+        if chunk.len() > 1 {
+            result.push(CHARS[((triple >> 6) & 0x3F) as usize] as char);
+        } else {
+            result.push('=');
+        }
+        if chunk.len() > 2 {
+            result.push(CHARS[(triple & 0x3F) as usize] as char);
+        } else {
+            result.push('=');
+        }
+    }
+    result
+}