本例根據圖像大小，畫文字自動換行顯示，原理是將文字生成背景透明的陰影圖像，再畫到圖像上去，生成1幅圖像。

如圖像Dpi不為96，調整文字大小或自行修改代碼適應。

/// <summary>
        /// 組件生成圖片
        /// </summary>
        /// <param name="control">組件名稱</param>
        /// <param name="filename">保存路徑</param>
        private void SaveControlImage(FrameworkElement control, string filename)
        {
            Rect rect = VisualTreeHelper.GetDescendantBounds(control);
            DrawingVisual dv = new DrawingVisual();
            using (DrawingContext ctx = dv.RenderOpen())
            {
                VisualBrush brush = new VisualBrush(control);
                ctx.DrawRectangle(brush, null, new Rect(rect.Size));
            }
            int width = (int)control.ActualWidth;
            int height = (int)control.ActualHeight;
            RenderTargetBitmap rtb = new RenderTargetBitmap(width, height, 96, 96, PixelFormats.Pbgra32);
            rtb.Render(dv);
            PngBitmapEncoder encoder = new PngBitmapEncoder();
            encoder.Frames.Add(BitmapFrame.Create(rtb));
            using (FileStream fs = new FileStream(filename,FileMode.Create, FileAccess.Write, FileShare.None))
            {
                encoder.Save(fs);
            }
        }

/// <summary>
        /// 生成背景透明陰影文字圖像
        /// </summary>
        /// <param name="content">文字內容</param>
        /// <param name="fontFace">字體</param>
        /// <param name="fontSize">字體大小</param>
        /// <param name="fontColor">文字顏色</param>
        /// <param name="textWidth">文字范圍寬度</param>
        /// <param name="textHeight">文字范圍高度</param>
        /// <param name="_dpiX">點數</param>
        /// <param name="_dpiY">點數</param>
        /// <param name="shodowOffset">陰影距離</param>
        /// <param name="_radius">模糊值</param>
        /// <param name="_opacity">陰影透明度</param>
        /// <param name="beginColor">文字漸變色起始顏色</param>
        /// <param name="endColor">文字漸變色結束顏色</param>
        /// <returns></returns>
        public static RenderTargetBitmap DrawShadowText(
            string content,
            string fontFace, double fontSize, Brush fontColor,
            int textWidth, int textHeight,
            int _dpiX, int _dpiY,
            Point shodowOffset, int _radius, double _opacity,
            Color beginColor, Color endColor
            )
        {
            var visualShadow = new DrawingVisual();
            using (DrawingContext drawingShadowContext = visualShadow.RenderOpen())
            {
                var pixels = new byte[textWidth * textHeight * 4];
                var maxWH = textWidth > textHeight ? textWidth : textHeight;
                BitmapSource bitmapSource = BitmapSource.Create(textWidth, textHeight, _dpiX, _dpiY, PixelFormats.Pbgra32, null, pixels, maxWH * 4);
                drawingShadowContext.DrawImage(bitmapSource, new Rect(0, 0, textWidth, textHeight));
                FormattedText formatshadow = new FormattedText(content, CultureInfo.InvariantCulture, FlowDirection.LeftToRight, new Typeface(fontFace), fontSize, Brushes.Black);
                formatshadow.MaxTextWidth = textWidth;
                formatshadow.MaxTextHeight = textHeight;
                formatshadow.SetFontWeight(FontWeights.Normal, 0, content.Length);
                drawingShadowContext.DrawText(formatshadow, shodowOffset);
            }
            var image = new DrawingImage(visualShadow.Drawing);
            Rectangle rct = new Rectangle()
            {
                Fill = new ImageBrush(image),
                Effect = new BlurEffect() { Radius = _radius },
                Opacity = _opacity,
            };
            Size sz = new Size(image.Width, image.Height);
            rct.Measure(sz);
            rct.Arrange(new Rect(sz));
            RenderTargetBitmap rtbmp = new RenderTargetBitmap(textWidth, textHeight, _dpiX, _dpiY, PixelFormats.Default);
            rtbmp.Render(rct);
            BitmapSource bgImage = rtbmp;
            RenderTargetBitmap composeImage = new RenderTargetBitmap(bgImage.PixelWidth, bgImage.PixelHeight, _dpiX, _dpiY, PixelFormats.Default);
            DrawingVisual drawingVisual = new DrawingVisual();
            FormattedText formattext = new FormattedText(content, CultureInfo.InvariantCulture, FlowDirection.LeftToRight, new Typeface(fontFace), fontSize, fontColor);
            formattext.MaxTextWidth = textWidth;
            formattext.MaxTextHeight = textHeight;
            formattext.SetFontWeight(FontWeights.Normal, 0, content.Length);
            formattext.SetForegroundBrush( new LinearGradientBrush( beginColor, endColor, fontSize), 0, content.Length);
            using (DrawingContext drawingContext = drawingVisual.RenderOpen())
            {
                drawingContext.DrawImage(bgImage, new Rect(0, 0, bgImage.Width, bgImage.Height));
                drawingContext.DrawText(formattext, new Point(0, 0));
            }
            composeImage.Render(drawingVisual);
            return composeImage;
        }