This is a reduced-size version of the GUI image used in
PNG, JPEG, GIF experiments below --- experiments in
image-COMPRESSION versus image-QUALITY-preservation.
Text, images, file-size-data, or links may be added
or improved --- if/when I revisit this page.
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 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
In a little more detail, the steps in making these images were as follows.
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.
See quality of these images in the 'images section' below.
File-size-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 with the '_pngcrushBRUTE.png' file --- created with the 'pngcrush' command with the '-brute' option.
We also created a PNG file more than 10 times smaller than 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.
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 a canvas area on which a shaded-image of a 'super formula shape' is displayed --- and that shaded image is composed of hundreds of shades of a cyan color --- 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 32 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 'superformula shape'. And when we go to a 16-color GIF, the color banding gets worse. And at 4 colors, the color shading on the 'superformula shape' disappears.
(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 appreciable 'mosquito noise' --- which, on close examination, is seen in the '-quality 70' JPEG file --- and even more so in the '-quality 50' and '-quality 30' JPEG files.
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 92' rather than use the loss-less, maximally-crushed PNG file.
The quality-92 JPEG is about 10 percent smaller than the loss-less, maximally crushed PNG file --- and the quality-92 JPEG shows no apparent difference in quality compared to the maximally crushed PNG file.
Alternatively, if I were not concerned about undetectable color loss, I would use a GIF file with about 64-colors in its color palette --- because it is about half the size of the quality-92 JPEG file. The quality of the 64-color Gif file compares favorably with the quality of the quality-92 JPEG file.
To me, it would be almost a coin-flip whether to use the JPEG-quality-92 or the GIF-colors-64 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 chose the GIF file.
Below are the images. Judge for yourself.
The 'original' cropped PNG file from 'gnome-screenshot' and 'mtpaint'.
File size: 1,384,803 bytes
('mtpaint' saves to a rather large PNG file.)
PNG from 'convert -quality 00'
File size: 96,644 bytes
(NOTE: Nice quality and more than 10 times smaller than the 'original' PNG file.)
PNG from the 'pngcrush' command with the '-brute' option.
File size: 95,356 bytes
(NOTE: More than 10 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: 86,660 bytes
(NOTE: There is NO apparent 'mosquito noise' around the text items. This image
is quite usable, and is about 10 percent smaller than the maximally crushed PNG file.)
GIF from 'convert -colors 256 +dither' (no dithering).
File size: 58,374 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 color-shaded 'superformula shape'. This image looks quite usable,
and it is about 40% smaller than the smallest PNG file, above.)
JPEG from 'convert -quality 70 -sampling-factor 2x2 -filter Mitchell'
File size: 51,152 bytes
(NOTE: If you look closely at this 'quality-70' JPEG image,
some 'mosquito noise' is detectable around the text items.
However, if you wanted a file about 40% smaller than 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.)
JPEG from 'convert -quality 50 -sampling-factor 2x2 -filter Mitchell'
File size: 40,010 bytes
(NOTE: Even if you could not detect the 'mosquito noise' in the quality-70
JPEG image above, you should be able to see considerable 'mosquito noise'
around text in this quality-50 JPEG image. I would not want to use this.)
GIF from 'convert -colors 64 +dither' (no dithering).
File size: 38,690 bytes
(NOTE: 'Color-banding' has NOT appeared yet, in this colors-64 GIF image.
And the text is quite clear.
If I wanted a really small file, I would consider this usable.)
GIF from 'convert -quality 30 -sampling-factor 2x2 -filter Mitchell'.
File size: 31,569 bytes
(NOTE: The 'mosquito noise' in this quality-30 JPEG image is quite pronounced.
I would definitely consider this unacceptable for usage.)
GIF from 'convert -colors 32 +dither' (no dithering).
File size: 30,758 bytes
(NOTE: 'Color-banding' has appeared in this colors-32 GIF image. Unusable.)
GIF from 'convert -colors 16 +dither' (no dithering).
File size: 27,054 bytes
(NOTE: Even more severe 'color-banding' appears in this colors-16 GIF image. Unusable.)
GIF from 'convert -colors 4 +dither' (no dithering).
File size: 15,127 bytes
(NOTE: All color-shading on the 'superformula shape' has disappeared.
This image is definitely unusable.
It looks like about 38,000 bytes is the smallest we can go
and still have a good quality image --- and that is
with a 64-color GIF file.)
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.