Reducing the file size of a very large images, without changing the image dimensions

Consider an application handling uploading of potentially very large PNG files.

All uploaded files must be stored to disk for later retrieval. However, the PNG files can be up to 30 MB in size, but disk storage limitations gives a maximum per file size of 1 MB.

The problem is to take an input PNG of file size up to 30 MB and produce an output PNG of file size below 1 MB.

This operation will obviously be lossy - and reduction in image quality, colors, etc is not a problem. However, one thing that must not be changed is the image dimension. Hence, an input file of dimension 800x600 must produce an output file of dimension 800x600.

The above requirements outlined above are strict and cannot be changed.

Using ImageMagick (or some other 开发者_运维问答open source tool) how would you go about reducing the file size of input PNG-files of size ~30 MB to a maximum of 1 MB per file, without changing image dimensions?

PNG is not a lossy image format, so you would likely need to convert the image into another format-- most likely JPEG. JPEG has a settable "quality" factor-- you could simply keep reducing the quality factor until you got an image that was small enough. All of this can be done without changing the image resolution.

Obviously, depending on the image, the loss of visual quality may be substantial. JPEG does best for "true life" images, such as pictures from cameras. It does not do as well for logos, screen shots, or other images with "sharp" transitions from light to dark. (PNG, on the other hand, has the opposite behavior-- it's best for logos, etc.)

However, at 800x600, it likely will be very easy to get a JPEG down under 1MB. (I would be very surprised to see a 30MB file at those smallish dimensions.) In fact, even uncompressed, the image would only be around 1.4MB:

800 pixels * 600 pixels * 3 Bytes / color = 1,440,000 Bytes = 1.4MB

Therefore, you only need a 1.4:1 compression ratio to get the image down to 1MB. Depending on the type of image, the PNG compression may very well provide that level of compression. If not, JPEG almost certainly could-- JPEG compression ratios on the order of 10:1 are not uncommon. Again, the quality / size of the output will depend on the type of image.

Finally, while I have not used ImageMagick in a little while, I'm almost certain there are options to re-compress an image using a specific quality factor. Read through the docs, and start experimenting!

EDIT: Looks like it should, indeed, be pretty easy with ImageMagick. From the docs:

$magick> convert input.png -quality 75 output.jpg

Just keep playing with the quality value until you get a suitable output.

Your example is troublesome because a 30MB image at 800x600 resolution is storing 500 bits per pixel. Clearly wildly unrealistic. Please give us real numbers.

Meanwhile, the "cheap and cheerful" approach I would try would be as follows: scale the image down by a factor of 6, then scale it back up by a factor of 6, then run it through PNG compression. If you get lucky, you'll reduce image size by a factor of 36. If you get unlucky the savings will be more like 6.

pngtopng big.png | pnmscale -reduce 6 | pnmscale 6 | pnmtopng > big.png

If that's not enough you can toss a ppmquant in the middle (on the small image) to reduce the number of colors. (The examples are netpbm/pbmplus, which I have always found easier to understand than ImageMagick.)

To know whether such a solution is reasonable, we have to know the true numbers of your problem.

Also, if you are really going to throw away the information permanently, you are almost certainly better off using JPEG compression, which is designed to lose information reasonably gracefully. Is there some reason JPEG is not appropriate for your application?

Since the size of an image file is directly related to the image dimensions and the number of colours, you seem to have only one choice: reduce the number of colours.

And ~30MB down to 1MB is a very large reduction.

It would be difficult to achieve this ratio with a conversion to monochrome.

It depends a lot on what you want at the end, I often like to reduce the number of colors while perserving the size. In many many cases the reduced colors does not matter. Here is an example of reducing the colors to 254.

convert -colors 254 in.png out.png

You can try the pngquant utility. It is very simple to install and to use. And it can compress your PNGs a lot without visible quality loss.

Once you install it try something like this:

pngquant yourfile.png
pngquant --quality=0-70 yourfile.png

For my demo image (generated by imagemagick) the first command reduces 350KB to 110KB, and the second one reduces it to 65KB.

• Step 1: Decrease the image to 1/16 of its original size.
• Step 2: Decrease the amount of colors.
• Step 3: Increase the size of the image back to its original size.

I know you want to preserve the pixel size, but can you reduce the pixel size and adjust the DPI stored with the image so that the display size is preserved? It depends on what client you'll be using to view the images, but most should observe it. If you are using the images on the web, then you can just set the pixel size of the <img> tag.

It depends on they type of image, is it a real life picture or computer generated image, for real life images png will do very little it might even not compress at all, use jpg for those images, it the image has a limited number of different colors (it can have a 24 bit image depth but the number of unique images will be low) png can compress quite nicely. png is basicly an implementation of zip for images so if a lot of pixels are the same you can have a rather nice compression ratio, if you need lossless compression don't do resizing.

use optipng it reduce size without loss

http://optipng.sourceforge.net/

Try ImageOptim https://imageoptim.com/mac it is free and open source

If you want to modify the image size in ubuntu, you can try "gimp". I have tried couple of image editing apps in ubuntu and this seemed to be the best among them.

Installation:

• Open terminal
• Type: sudo apt install gimp-plugin-registry
• Give admin password. You'll need net connection for this.
• Once installed, open the image with GIMP image editor. Then go to: File > Export as > Click on 'Export' button
• You will get a small window, where check box on "Show preview in image window". Once you check this option, you will get to see the current size of the file along with Quality level.
• Adjust the quality level to increase/decrease the file size.
• Once adjusting is done, click on 'Export' button finally to save the file.

Right click on the image. Select open with paint. Click on resize. Click on pixel and change the horizontal to 250 or 200.

That's the only thing. It is the fastest way for those who are using Windows XP or Windows 7.