Why '.png' files produced by ImageMagick are so much bigger than '.jpg' & '.gif' files?

Question

Hello.

I'm using ImageMagick to convert some files from one format to another. I was always under the impression that .png files were supposed to be as big/small as .jpg if not smaller, and definitely smaller than .gif.

However when I run

convert photo.jpg photo.png 

The files I'm getting out is about 6 times bigger than the original jpg.
Original jpg is a regular photo about 300x500 px, 52 kb. Output is a proper png of the same dimensions, but size is about 307 kb?

Does anyoone know what the hack is going on? Am I doing something wrong?

P.S.:

I tried both on Debian and Windows with the same results.

P.P.S.:

Also when I add resize option to this and resize to 10000 x 10000. Converting and resizing to jpg takes a few seconds, but it works, if I do the same of png, I jsut strt running out of memory altogether

Answer 1

JPG is a lossy compression algorithm while PNG is a lossless one.

This fact alone will (in general) make JPG images smaller than PNG ones. You can tweak the compression ratios for each format, so it could also be that your not compressing your PNG files as much as your JPG ones.

For a photographic image saving as JPG will usually produce a smaller file than PNG as there's more noise or randomness in the image for the compression to work with. Images created by graphic art tools will tend to have more hard edges which will compress better in PNG.

GIF is smaller because it's based on an colour palette (of 256 colours) rather than the separate RGB values for each pixel (or group of pixels) in JPG and PNG.

Answer 2

The .jpeg file format is a lossy format, it throws information away. Which makes for good compression ratios. The .gif file format is not lossy but loses other information, it only supports 256 colors. The .png file format is not lossy and preserves the color range.

Answer 3

It depends on the kind of image. JPEG's lossy compression works very well on images with lots of color gradients (i.e. photos).

Try it on an image consisting of areas with the same color separated by sharp edges (screenshots are good, unless they show a photo desktop background, and the fancy window borders and task bar of Vista and Windows 7 also tend to mess this up). JPEG is bad at that kind of thing, and you'll probably find PNG to compress better (probably better than GIF too).

Answer 4

The PNG format can provide images of 24 bits per pixel or 8 bits per pixel. JPG is a 24 bit format, but it uses lossy compression to reduce the file size significantly. PNG and GIF both use lossless compression, but GIF only works with 8 bits so it requires your image to have 256 colors or less - this often results in a grainier picture.

Try the -colors and -dither options in ImageMagick to reduce the number of bits in your PNG output. At that point it should be comparable to the GIF file size. If you need to reduce it further, there are utilities to optimize the PNG.