types of graphic files (as lossy or lossless .JPEG,
and compressed .TIFF) may be difficut to be efficiently
compressed, as some picture/photo file formats employ specific
algorithms meant to efficiently compress graphic data, and consequently
the already compressed image formats leads to very poor compression
ratio when added to an archive using general purpose compression
algorithms, even if higest compression levels are set.
archiving already compressed graphic files it is recommended to
use lowest/fastest compression settings in order to spare compuational
resources, rather than gaining a small percentual improvement of
compression using maximum compression sets or slower algorithms, as the
tradeoff between performance penaly and compression ratio gain would
not be favourable - if it is needed to keep
output under a maximum size limit (i.e. for email attachment),
it may be recommended, instead, to split
output in multiple volumes.
Alternative ways, detailed below, to reduce images disk occupation are
to conver picture format to a more suitable one (this apply
graphic-specific compression on the data), or to reduce actual image
size, either by cropping (reduce the picture to a smaller selection,
containing relevant images) or resizing it to a smaller size suited to
the intended use.
How to convert picture format
An alternative approach to enhance compression efficiency of pictures,
photo and CGI graphic files is chosing the most appropriate graphic
coding format, and convert original picture to a more suitable image
In example, PNG
(and to a lessen extent compressed TIFF) is extremely
efficient on regular shapes with low noise (geometric shapes,
simple drawings and computer-generated imagery imagery) so
recompressing a raw BMP or
uncompressed TIFF file to PNG format can really improve disk occupation
- moreover, being PNG compression lossless, no actual picture's data is
lost in the transformation.
On the other side, JPEG
is more efficient on photographic images, even
with average or high noise, so it should be chose over BMP,
or uncompressed TIFF, and PNG when saving actual photo or scans
rather than CGI graphic; it is consequently a popular choice for
consumer / prosumer digital photograpy.
JPEG compression, however, degrades the quality of the picture
data at each iteration, so for better results it is recommended
original data, edit it using lossless formats (PNG, TIFF, or specific
photoediting file formats) for intermediate steps, and only apply lossy
JPEG compression when finalizing the work - keeping in account the
platform the graphic is meant to be used for.
Uncompressed graphic files formats (i.e. RAW images, BMP files,
uncompressed TIFF) usually compresses well with traditional general
purpose compression (ZIP, 7Z...), but in this case is often
preferable to evaluate if it is needed to keep the original, large,
files, or rather convert them to a lossless picture format (usually PNG
or TIFF) that does not modify the original file information but stores
it in a smaller file, ready to use/view without requiring extraction.
PeaZip features a simple set
editing tools, with Transform dialog
providing the needed functions to convert any readable
picture format to BMP, PNG, or JPEG.
If JPEG format is chosen, lossy JPG
compression quality can be set in a range from 0 to 100: lower quality
means more efficient picture compression and smaller output pictures,
but the increase of JPG compression artifacts will degrade the quality
the image; higher JPG quality value will result in better pictures, but
saved to larger output files.
How to crop and resize images
Another critical factor for optimizing graphic files disk occupation is
picture size, i.e. reducing
the original picture size to 50% brings the
output to the 25% of the original file size.
Resizing the picture
to a smaller size (featured in PeaZip graphic files' Transform dialog)
is a lossy operation: the removed information is lost
and expanding back the image to a larger size will not add back the
original information, so resizing the graphic should be done in the
final step, accurately chosen for the intended platform.
If the smaller size is acceptable for the intended use, reducing
picture size is a very effective mean to decrease file size.
PeaZip's Transform dropdown menu also features fit to screen pre-sets entries to
scale the image to most common screen sizes.
(provided by PeaZip picture Crop dialog),
removes unwanted or less important area of the raw image: it is too an
obviously lossy operation and cut area cannot be recovered from the
cropped image, but in this way both size of the picture and size of the
file can be reduced without altering the quality of the remaining
portion of the graphic.
External online resources: computer
generated imagery (CGI), digital
image format, lossy and lossless
compression definition on Wikipedia.
Topics and serach suggestions: crop, resize, compress pictures wiki,
screen, convert graphic
files, optimize and re-compress images, reduce disk occupation of
pictures collections, improve JPEG, PNG, TIFF, GIF, BMP,
raw files compression, photo scaling and cropping for the web.