refactor(重构提升): 统一取页与图片导出接口，精简WebUI重复逻辑

ofdgo_renderer_output.go

@@ -19,6 +19,8 @@ import (
 	"bytes"
 	"fmt"
 	"image"
+	"image/jpeg"
+	"image/png"
 	"io"
 
 	"github.com/tdewolff/canvas"
@@ -82,6 +84,53 @@ func (r *Renderer) RenderToImage(page *PageContent) (image.Image, error) {
 	return raster.Image, nil
 }
 
+// RenderToPNG 渲染为PNG，像素尺寸由DPI决定
+// 入参: page 页面内容, writer 输出流
+// 返回: error 错误信息
+func (r *Renderer) RenderToPNG(page *PageContent, writer io.Writer) error {
+	img, err := r.RenderToImage(page)
+	if err != nil {
+		return err
+	}
+	return png.Encode(writer, img)
+}
+
+// RenderToJPEG 渲染为白底JPEG，像素尺寸由DPI决定
+// 入参: page 页面内容, writer 输出流, options 编码选项，nil使用标准库默认值
+// 返回: error 错误信息
+func (r *Renderer) RenderToJPEG(page *PageContent, writer io.Writer, options *jpeg.Options) error {
+	img, err := r.RenderToImage(page)
+	if err != nil {
+		return err
+	}
+	return jpeg.Encode(writer, fillWhiteBackground(img), options)
+}
+
+// fillWhiteBackground 填充白色背景，复用当前导出图片的RGBA像素
+// 入参: img 图片对象
+// 返回: image.Image 图片对象
+func fillWhiteBackground(img image.Image) image.Image {
+	bounds := img.Bounds()
+	if rgba, ok := img.(*image.RGBA); ok {
+		for y := 0; y < bounds.Dy(); y++ {
+			row := rgba.Pix[y*rgba.Stride : y*rgba.Stride+bounds.Dx()*4]
+			for i := 0; i < len(row); i += 4 {
+				if alpha := 255 - row[i+3]; alpha != 0 {
+					row[i] += alpha
+					row[i+1] += alpha
+					row[i+2] += alpha
+					row[i+3] = 255
+				}
+			}
+		}
+		return rgba
+	}
+	dst := image.NewRGBA(bounds)
+	draw.Draw(dst, bounds, image.White, image.Point{}, draw.Src)
+	draw.Draw(dst, bounds, img, bounds.Min, draw.Over)
+	return dst
+}
+
 // RenderToSVG 渲染为SVG
 // 入参: page 页面内容, writer 输出流
 // 返回: error 错误信息