Image Compression Experiments


on the image of a GUI for

'morphing' between 2 images


JPEG, PNG, GIF files
- - -
FILE-SIZE (compression)
versus IMAGE-QUALITY preservation


This is a reduced-size version of the image used in the
PNG, JPEG, GIF experiments below --- experiments in
image-COMPRESSION versus image-QUALITY-preservation.

FE Home page > JPEG-vs-PNG-vs-GIF page >

This Image-Compression-versus-Quality-Experiments-
-on-an-Screenshot-of-a-'WheeeMorph'-GUI page

Note:
Text, images, file-size-data, or links may be added
or improved --- if/when I revisit this page.

Go to File Size Results, below.

Go to some Conclusions, below.

Go to Start of Test Images, below.

INTRODUCTION to the IMAGE EXPERIMENTS :

This is a page of experiments in image compression-versus-quality. Images are shown, and text under each image explains how the image was processed.

Before the images, a summary of file sizes is listed, at the bottom of this introduction --- along with some discussion of how one may be able to get relatively small GIF or JPEG or PNG image files while preserving (for the most part) the quality of the original, captured image.

The summary of file sizes is followed by the images, along with some information about each image, underneath each image.

    If you want to examine the images --- for example, by magnifying them --- you can 'right-click' on each image to do a 'Save As ...' of the image file to your local computer. Then you can use image-viewing software on your computer to examine any of these images in detail.

This page is a link on the 'parent' web page JPEG vs. PNG vs. GIF - image-quality vs. file-size. The 'Introduction' on that page describes some of the considerations in compressing GIF/JPEG/PNG files while preserving the quality of the original image (in terms of what the eye can see, without magnifying the image).

See 'The bottom line' section of that page. It contains a summary of GIF/JPEG/PNG compression-versus-quality guidelines that were deduced from the image compression experiments in these web pages.


The making of the images on this page :

The images on this page were made from an 'original' image in a PNG file.

That 'original' image-file was created by doing an 'entire-screen' capture (to a PNG file) when the 'wheeeMorph' GUI was displayed on the screen. The capture was done with the 'gnome-screenshot' utility. I then used the 'mtpaint' image-editor program to read the PNG file and crop the GUI image from the image of the entire screen. I saved the cropped image to a PNG file, from the 'mtpaint' program.

So the 'original' PNG file was made using the 'gnome-screenshot' and 'mtpaint' programs.

Other, compressed image files (PNG, GIF, and JPEG) were made from this 'original' PNG file by using either

  • the ImageMagick 'convert' command, OR
  • the 'pngcrush' command.

In a little more detail, the steps in making these images were as follows.

  • The entire screen was captured as a PNG file using the 'gnome-screenshot' utility --- on Ubuntu 9.10 Linux, where 9.10 means '2009 October'.

  • The 'mtpaint' image editor program was used to crop the GUI image out of the 'entire-screen' image, and the cropped image was saved as a PNG file. That file is the first of the images below.

      (The 'mtpaint' image editor tends to make rather large PNG files --- larger than the PNG file that was read in --- even after some cropping.)

  • Various PNG and JPEG and GIF files are shown below --- in file-size order, largest first. Most of the compressed images were made with the ImageMagick 'convert' command. One image was made with 'pngcrush'.

  • The JPEG files shown below were made, from the 'original' PNG file at the top of these images, by using the ImageMagick 'convert' command, with the '-quality' and '-sampling factor' and '-filter' options.

    The '-strip' option of the 'convert' command was used to assure that the JPEG file would be stripped of text data such as EXIF text data.

    Experiments on some other PNG image captures have indicated that smaller JPEG files are yielded when the '-sampling-factor 2x2' option is used --- rather than '1x1' or '2x1'.

    Some of those JPEG-making experiments also indicated that the choice of '-filter' may not have a great influence on file-size --- at least not as much influence as '-quality' and '-colors' options. So '-filter Mitchell' was used to make these JPEG files.

  • Some compressed PNG files were made from the 'original' PNG file at the top of these images, using either the 'pngcrush' command with the '-brute' option --- OR the 'convert' command with the '-quality' option.

  • GIF files were made from the 'original' PNG file at the top of these images, by using the 'convert' command with the '-colors' option --- and with '+dither' to turn OFF dithering. A range of '-colors' values were used --- --- from 256 to 16.

  • File-size results are summarized just below. See the images further below to examine the image quality.

Here is a list of the sizes (in bytes) and names of the image files shown below --- sorted by size, largest files first. The filenames indicate the kind of conversion/compression that was done.

File size results from various compression tests are listed below --- in file size order --- largest to smallest.

The descriptive file names are quite long and have been 'folded' onto a second line.


The 'original' PNG screenshot file:
('mtpaint' creates large PNG files)

   2,201,589   wheeeMorph_Cheney_drawing-photo_deformed-grids_
               gnome-screenshot_fromMTpaintCrop_1024x715.png

to-compressed-PNG via ImageMagick 'convert' PNG-compression:

     503,095   wheeeMorph_Cheney_drawing-photo_deformed-grids_
               gnome-screenshot_fromMTpaintCrop_1024x715_CompressType00.png

to-compressed-PNG via 'pngcrush' compression:

     376,393   wheeeMorph_Cheney_drawing-photo_deformed-grids_
               gnome-screenshot_fromMTpaintCrop_1024x715_pngcrushBRUTE.png

to-JPEG - quality-92 (via ImageMagick 'convert'):

     277,349   wheeeMorph_Cheney_drawing-photo_deformed-grids_
               gnome-screenshot_fromMTpaintCrop_1024x715_Quality92_
               SampFact2x2_FilterMitchell_Stripped.jpg

to-GIF up-to-256-colors (via ImageMagick 'convert'):
 
     219,802   wheeeMorph_Cheney_drawing-photo_deformed-grids_
               gnome-screenshot_fromMTpaintCrop_1024x715_PALETTE256.gif

to-JPEG - quality-80:

     185,566   wheeeMorph_Cheney_drawing-photo_deformed-grids_
               gnome-screenshot_fromMTpaintCrop_1024x715_Quality80_
               SampFact2x2_FilterMitchell_Stripped.jpg

to-GIF up-to-128-colors:

     176,983   wheeeMorph_Cheney_drawing-photo_deformed-grids_
               gnome-screenshot_fromMTpaintCrop_1024x715_PALETTE128.gif

to-JPEG - quality-70:

     152,433   wheeeMorph_Cheney_drawing-photo_deformed-grids_
               gnome-screenshot_fromMTpaintCrop_1024x715_Quality70_
               SampFact2x2_FilterMitchell_Stripped.jpg

to-GIF up-to-64-colors:

     147,541   wheeeMorph_Cheney_drawing-photo_deformed-grids_
               gnome-screenshot_fromMTpaintCrop_1024x715_PALETTE64.gif

to-GIF up-to-16-colors:

      92,879   wheeeMorph_Cheney_drawing-photo_deformed-grids_
               gnome-screenshot_fromMTpaintCrop_1024x715_PALETTE16.gif

See quality of these images in the 'images section' below.

The file-sizes versus image-quality is discussed there.

PNG vs. GIF vs. JPEG :

Note that we achieved the maximum LOSS-LESS compression of the PNG file (to another PNG file) in the '_pngcrushBRUTE.png' file --- created with the 'pngcrush' command with the '-brute' option.

We also created a PNG file about one-fourth the size of the 'original' by using the ImageMagick 'convert' command with a '-quality' compression parameter of '00'.

For the PNG '-quality' parameter of 'convert':

** 00 is recommended for images with mostly AREAS OF SOLID COLORS.
** 05 is recommended for images like NATURAL LANDSCAPES.
** 00 and 90 seem to give small file sizes with good quality.

    The first digit (tens) is the zlib compression level, 1-9. However if a setting of '0' is used you will get Huffman compression rather than 'zlib' compression, which is often better!

    The second digit is the PNG data encoding filtering type (before the data is compressed): 0 is none, 1 is 'sub', 2 is 'up', 3 is 'average', 4 is 'Paeth', and 5 is 'adaptive'.

However, we can get still smaller files, than these PNG files, by allowing some 'lossiness' --- by going to GIF files (with a max of 256 colors) or by using the 'lossy' compression inherent in creating JPEG files.

On GIF :

Since this GUI image contains 2 canvas areas on which color-shaded images may be displayed --- and those two images may be composed of hundreds of shades of various colors --- we can expect that we are going to lose some image quality if we convert the 'original' PNG file to a GIF file ---- even if we choose to specify the maximum of 256 colors.

In fact, it turned out that for this GUI image, when we go down to about 64 colors, the GIF file differs quite markedly in appearance from the 'original' PNG file from which it came. Some 'color banding' is apparent on the photo-portrait image. And when we go to a 16-color GIF, the color banding gets clearly worse.

(We could try converting the 'original' PNG file to PNG files with a restricted number of colors --- such as 256 or 16 --- but if we are going to do that, I would rather use GIF --- since it has been supported by web browsers for about 10 more years than PNG --- and there may be some cases of software not reading certain types of PNG files.)

On JPEG :

Note that when small text fonts are in an image, the 'lossy' compression of JPEG typically introduces 'mosquito noise' around the small text characters --- especially when we use a 'quality' value lower than the range of 100 to 92.

In this GUI image, there are many 'fine', small text characters --- so we DO have to be concerned about introducing 'mosquito noise' when converting the 'original' PNG file to a JPEG file of 'quality' less than 92.

These less-than-92-quality JPEG files DO show some 'mosquito noise' --- which, on close examination, is seen in the '-quality 80' JPEG file --- and even more so in the '-quality 70' JPEG file.

On choosing a file-type :

For this particular image, IF file size was my main concern (for example, if I were posting the image on a web page along with a lot of other images), I would be tempted to use a JPEG file of 'quality' 70 (or, better yet, 80) rather than use the loss-less, maximally-crushed PNG file.

Those JPEG's would be about half the size of the loss-less, maximally crushed PNG file.

Alternatively, I would use a GIF file with at least 64-colors --- because its size and quality compare favorably with the JPEG's that were created with '-quality' of about 70 to 80.

To me, it would be almost a coin-flip whether to use the JPEG-quality-80 or the GIF-colors-128 image file of this GUI image. If it proved to be the case that a GIF file would load into a web page faster (or with less CPU processing) than the JPEG file, then I would probably choose the GIF file.


Below are the images. Judge for yourself.


The 'original' cropped PNG file from 'gnome-screenshot' and 'mtpaint'.
File size: 2,201,589 bytes
('mtpaint' saves to a rather large PNG file.)



PNG from 'convert -quality 00'
File size: 503,095 bytes
(NOTE: Nice quality and about 4 times smaller than the 'original' PNG file.)



PNG from the 'pngcrush' command with the '-brute' option.
File size: 376,393 bytes
(NOTE: Nice quality and about 6 times smaller than the 'original' PNG file.
This is the smallest PNG we can get without sacrificing some colors.)



JPEG from 'convert -quality 92 -sampling-factor 2x2 -filter Mitchell'
File size: 277,349 bytes
(NOTE: There is NO apparent 'mosquito noise' around the text items. This image
is quite usable, and is about 25 percent smaller than the maximally crushed PNG file.)



GIF from 'convert -colors 256 +dither' (no dithering).
File size: 219,802 bytes
(NOTE: There is no 'mosquito noise' generated in GIF files,
so none in this one. And if there were many colors lost in
generating this 256-color GIF file, it is not readily apparent
in the photo-portrait on the right-side of the GUI. This image looks
quite usable, and it is about 40 percent smaller than the smallest PNG file, above.)



JPEG from 'convert -quality 80 -sampling-factor 2x2 -filter Mitchell'
File size: 185,566 bytes
(NOTE: If you look closely at this 'quality-80' JPEG image,
there is hardly any 'mosquito noise' detectable around the text items.
If you wanted a file about 50 percent smaller than the smallest
PNG file, above --- and you did not want to use a GIF file ---
you might tolerate this miniscule level of 'mosquito noise' and use
a JPEG file compressed to this extent.)



GIF from 'convert -colors 128 +dither' (no dithering).
File size: 176,983 bytes
(NOTE: There is no 'mosquito noise' generated in GIF files,
so none in this one. There may be quite a few colors lost in
generating this 128-color GIF file. There appears to be some degradation
in the photo-portrait on the right-side of the GUI --- for example, some
'color banding' on the cheeks. However, this image looks quite usable,
and it is less than half the size of the smallest PNG file, above.)



JPEG from 'convert -quality 70 -sampling-factor 2x2 -filter Mitchell'
File size: 152,433 bytes
(NOTE: If you look closely at this 'quality-70' JPEG image, a
little bit of 'mosquito noise' is detectable around some of the text items.
However, if you wanted a file less than half the size of the smallest
PNG file, above --- and you did not want to use a GIF file ---
you might tolerate this level of 'mosquito noise' and use
a JPEG file compressed to this extent.)



GIF from 'convert -colors 64 +dither' (no dithering).
File size: 147,541 bytes
(NOTE: The text is quite clear, BUT 'color-banding' on the
cheeks is quite evident --- in this colors-64 GIF image.
I would NOT want to use this image.)



GIF from 'convert -colors 16 +dither' (no dithering).
File size: 92,879 bytes
(NOTE: Although the text is still clear, even more severe
'color-banding' appears in this colors-16 GIF image. Unusable.)

MORE IMAGE EXPERIMENTS :

Some other PNG-JPEG-GIF compression-versus-quality experiments (using different image types) are available from the 'parent' page of this page --- the JPEG vs. PNG vs. GIF - image-quality vs. file-size page.

'External' WEB LINKS to other JPEG/PNG/GIF info :

For more information on image file quality and compression issues, for JPEG and PNG and GIF files, you can try the following Google searches on the indicated keywords.

You may wish to change or add keywords to these queries in order to hone in on answers to your particular questions.

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Page was created 2015 Feb 03.
Page was changed 2018 Aug 18. (Added css and javascript to try to handle text-size for smartphones, esp. in portrait orientation.)