图片与压缩 (学习笔记)

refer : 

http://blog.csdn.net/qq_16542775/article/details/51792092

http://www.cnblogs.com/xinzhao/p/5130410.html⁠⁠⁠⁠

https://zhidao.baidu.com/question/495382546600556724.html

https://zh.m.wikipedia.org/zh/无损数据压缩

先说说常用的图片格式 : 

jpeg, jpg 

有损

大部分网页图片都是 jpg 格式的. 

特点就是小

PNG (无损)

特点是支持透明 

Gif (无损)

特点是支持动画 

质量压缩 (有损要损和无损的区别)

有损指的是图片压缩后质量会流失, 压缩过程中会丢失图片信息 (由于人的肉眼有限, 所以部分信息即使丢失了，我们在网页也是看不出来的)

无损是通过去除冗余信息实现的，过程中不会丢失信息，类似 zip file 一样可以还原. 

有损好处是可以压缩的很小.

尺寸压缩 

尺寸指的是把 1000px * 1000px 的图片压缩成 100px * 100px 

这样也可以让图片体积变少.

asp.net c# 代码 : 

            try
            {
                long quality = 90; //越高越清楚，越低size越小
                int targetWidth = 800; 
                int targetHeight = 0;
                string originalPath = @"C:keatkeatmy projectsThumbnailImageProjectProject	est.jpg";
                string newPath = @"C:keatkeatmy projectsThumbnailImageProjectProject	est123.jpg";

                using (Bitmap original = new Bitmap(originalPath))
                {
                    ImageCodecInfo codecInfo = GetEncoderInfo("image/jpeg");
                    Encoder encoder = Encoder.Quality;
                    EncoderParameters encoderParameters = new EncoderParameters(1);
                    EncoderParameter encoderParameter = new EncoderParameter(encoder, quality);
                    encoderParameters.Param[0] = encoderParameter;

                    int newWidth = 0;
                    int newHeight = 0;

                    if (targetWidth != 0)
                    {
                        decimal ratio = (decimal)targetWidth / original.Width;
                        newWidth = targetWidth;
                        newHeight = (int)(original.Height * ratio);
                    }
                    else
                    {
                        decimal ratio = (decimal)targetHeight / original.Height;
                        newHeight = targetHeight;
                        newWidth = (int)(original.Width * ratio);
                    }      
                  
                    using (Bitmap newImage = new Bitmap(newWidth, newHeight))
                    {
                        newImage.SetResolution(original.HorizontalResolution, original.VerticalResolution);
                        using (Graphics g = Graphics.FromImage(newImage))
                        {
                            g.SmoothingMode = SmoothingMode.HighQuality;
                            g.InterpolationMode = InterpolationMode.HighQualityBicubic;
                            g.PixelOffsetMode = PixelOffsetMode.HighQuality;
                            g.DrawImage(original, 0, 0, newWidth, newHeight);
                        }
                        newImage.Save(newPath, codecInfo, encoderParameters);
                    }
                }
            }
            catch (Exception ex)
            {

            }

private static ImageCodecInfo GetEncoderInfo(string mimeType)
{
int j;
ImageCodecInfo[] encoders;
encoders = ImageCodecInfo.GetImageEncoders();
for (j = 0; j < encoders.Length; ++j)
{
if (encoders[j].MimeType == mimeType)
return encoders[j];
}
return null;
}

 尺寸压缩原理就是重绘, js canvas 也可以完成. 质量压缩只能用于 jpg. js 也是可以做到的，只是兼容比较蛋疼.. /. 

refer : https://github.com/think2011/localResizeIMG