缩小图像尺寸，同时保持质量

Question

How to reduce the size of images to 40 KB while maintaining quality, and the dimensions of images are greater than 400 px , in C# ?

Note, was used:

• image.GetThumbnailImage : (The size is appropriate but the dimensions are small, and the larger the dimensions the bigger the size)

• Drawing : (Large size, larger than 700 KB)

Code:

System.Drawing.Bitmap newImage = new System.Drawing.Bitmap(newWidth, newHeight); 
using (System.Drawing.Graphics gr = System.Drawing.Graphics.FromImage(newImage))
{
    gr.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
    gr.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
    gr.PixelOffsetMode = System.Drawing.Drawing2D.PixelOffsetMode.HighQuality;
    gr.DrawImage(im3age, new System.Drawing.Rectangle(new System.Drawing.Point(0, 0), new System.Drawing.Size(newWidth, newHeight)));
    newImage.Save(FullPhat, System.Drawing.Imaging.ImageFormat.Jpeg);
}

Answer 1

Your solution is going to be based purely on what image format you choose, what compression options you select, and most importantly, the nature of the image itself.

The popular image formats for this task would be either JPEG or PNG. Both of those formats use, among other things, clusters of color in the images to achieve their compression. That means if your image is largely the same or similar colors, you will get small file sizes, and if your image has a lot of distributed colors, the file sizes are going to be larger.

Here are some examples:

(Images saved with Photoshop. PNGs saved using highest compression levels, JPEGs saved using default level of quality "8", highest level of quality "12", and lowest level of quality "1".)

• PNG: 4 KB
• JPEG (8): 9 KB
• JPEG (12): 15 KB
• JPEG (1): 5 KB

• PNG: 141 KB
• JPEG (8): 378 KB
• JPEG (12): 590 KB
• JPEG (1): 146 KB

• PNG: 341 KB
• JPEG (8): 84 KB
• JPEG (12): 274 KB
• JPEG (1): 22 KB

As you can see, different kinds of images at the same resolution produce wildly different file sizes, and JPEG is not always better than PNG or vice versa. Some of the JPEG images dip below your criteria, but keep in mind that they are saved with essentially the poorest quality settings, so the image is going to come out looking like garbage.

The general rule, then, is that a 400x400 px image, saved with perfect lossless quality, in a file under 40 KB... unless the image is mostly a single flat color, that isn't going to happen.