邮件正文添加图片，设置图片大小，分割图片

背景

邮件中需要发送一些图片报表，报表长宽比过大， 长度 1800，宽 8000，此时发送到邮件显示的图片宽度过小。此时在HTML设置长宽等参数无效

解决方案

尝试发送一张 1000 * 1000的图片，设置图片的长宽各为1000，此时在邮件中显示正常。

使用代码将图片进行分割，除了最后一张图片，所以图片的高度都为1000，接图片分割之后插入到HTML模板中，发送到邮箱。

代码

1、先将base64转图片

        public Image ConvertBase64ToImage(string base64String)
        {
            byte[] imageBytes = Convert.FromBase64String(base64String);
            using (MemoryStream ms = new MemoryStream(imageBytes, 0, imageBytes.Length))
            {
                ms.Write(imageBytes, 0, imageBytes.Length);
                Image image = Image.FromStream(ms, true);
                return image;
            }
        }

2、计算长宽比,按照图片宽度1000的比例

var ratio = img.Width / 1000.0;
img = ResizeImage(img, 1000, Convert.ToInt32(img.Height / ratio));

3、重新设置图片大小,按照图片宽度1000生成

        public Bitmap ResizeImage(Image originalImage, int maxWidth, int maxHeight)
        {
            int originalWidth = originalImage.Width;
            int originalHeight = originalImage.Height;
            float aspectRatio = (float)originalWidth / (float)originalHeight;

            int resizedWidth = maxWidth;
            int resizedHeight = (int)(maxWidth / aspectRatio);

            if (resizedHeight > maxHeight)
            {
                resizedHeight = maxHeight;
                resizedWidth = (int)(maxHeight * aspectRatio);
            }

            Bitmap resizedImage = new Bitmap(resizedWidth, resizedHeight);
            using (Graphics graphics = Graphics.FromImage(resizedImage))
            {
                graphics.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
                graphics.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
                graphics.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
                graphics.DrawImage(originalImage, 0, 0, resizedWidth, resizedHeight);
            }
            return resizedImage;
        }

4、计算需要分割几张图片

var count = Math.Ceiling(img.Height / 1000.0);

５、生成分割的图片并转 Base64字符串

            List<Image> imgs = new List<Image>();
            var builder = new StringBuilder();
            for (int i = 0; i < count; i++)
            {
                var height = 1000;
                var width = 1000;
                if ((i + 1) == count)
                {
                    height = img.Height - i * 1000;
                }
                var temp = CropImage(img, new Rectangle(0, i * 1000, width, height));

                //temp.Save(@"C:\myfiles\" + i + ".png");

                var imgBase64Str = ImageToBase64(temp);

                builder.Append($"<img width=\"{width}\" height=\"{height}\"  src=\"data:image/png;base64,{imgBase64Str}\" />");
            }

6、将生成的小图转Base64

        public string ImageToBase64(Image image)
        {
            using (MemoryStream ms = new MemoryStream())
            {
                image.Save(ms, ImageFormat.Jpeg);
                byte[] imageBytes = ms.ToArray();
                return Convert.ToBase64String(imageBytes);
            }
        }