How to reduce an image's file size?

Frequently Asked Questions regarding XnView (including Answers)

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How to reduce an image's file size?

Postby helmut » Sat Aug 02, 2008 8:53 am

For sending out image via e-mail or when saving an edited image you want to get an image with smaller or reasonable file size. Below please find a list of various approaches to reduce an image's file size.


Any graphic program that can save an image can reduce it's filesize. Some ways to reduce the filesize include:

a. Use a different format
Lossless formats like TIFF or BMP are inherantly large, switching to JPEG will greatly decrease the filesize of photographic images.

b. Choose a higher compression level.
If you're using a lossless format and already at the highest compression, you'll have to use a different method. Compression level can be set by clicking the "options" button when you go to save an image.

c. Reduce the dimensions of the image
By cropping or reducing an image, you will reduce the amount of pixels and graphic data in the file. This in turn will reduce the filesize.

d. Reduce the colors of the image.
If you reduce the image to 256 colors or less, or convert it to greyscale, that will again reduce the data and therefore the filesize.

e. Blur the image
You can reduce the size of a photo in JPEG format by blurring or softening the image.

XnView's Export function
The easiest way to get an image at a low filesize and still keep it at an acceptable quality (nearly anything that will reduce the filesize will have an impact on quality (although often it isn't a noticeable one), it's simply an unavoidable fact) is to use the export function of XnView. This will allow you to see the results of using a different format, higher compression, and reducing colors before you actually save the image (cropping and blurring still have to be done before exporting though).

Note:
All the above info has been originally posted by Drahken in topic Can you shrink a picture file size?. Thank you, Drahken!


Related topics:
- 'Lossy? Lossless? 256 colors? This is confusing!'
- How to prepare an image for the internet
- How to Batch convert several images at once
- Smaller image has same file size
- Resizing image results in larger file
- What format should I save this image in?
- Common graphic formats
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Re: How to reduce an image's file size?

Postby DOS386 » Thu Aug 28, 2008 2:40 am

helmut wrote:a. Use a different format
Lossless formats like TIFF or BMP are inherently large


It's not only because of "lossless", rather that both BMP and TIFF do have compression, but is rarely used, also RLE algo of BMP is not very efficient.

b.


c. Reduce the dimensions of the image


File size is proportional to image area ... in average ... special content can violate this rule viewtopic.php?t=1253

d.
e.


f. Use PNG format instead of GIF

g. Use a PNG optimizer (PNGOUT, OptiPNG, DeflOPT)
Last edited by DOS386 on Sun Jun 13, 2010 12:35 am, edited 1 time in total.
There is indeed no WinZIP under my rock.
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Postby Aokromes » Thu Aug 28, 2008 9:39 am

You forgot the "Use a external compresor" like zip, rar, 7zip etc.
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Postby Drahken » Sun Aug 31, 2008 3:49 pm

A very large portion of the massive filesize of a lossless format IS the fact that it's lossless. Compress a photograph losslessly with any format & settings you want (jpeg2000 is usually the best), it'll still be several times the size of a lossy format of similar visual quality.
TIFF compression is almost always used, uncompressed TIFFs are extremely rare. BMPs almost never use any compression, the RLE compression can only be used on indexed BMPs (and even when it is used, it's not remotely efficient, which is why BMPs are about the worst format you can use. Ever.)

The only time GIF should ever be chosen over PNG is if you want animation. Other than that, PNG is much better on every level.
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Postby DOS386 » Thu Sep 04, 2008 9:41 pm

Aokromes wrote:You forgot the "Use a external compresor" like zip, rar, 7zip etc.


This fails in 99% of cases ... because PNG is better than ZIP ;-)

In 1%, when having many large and similar screenshots for example, putting them as BMP's (! not PNG's) in 1 (! no bunch) solid (!) 7-ZIP (! ZIP is a no-go) can be ways better than a bunch of PNG's :-)

I wrote:

rather that both BMP and TIFF do have compression, but is rarely used


Drahken wrote:

TIFF compression is almost always used, uncompressed TIFFs are extremely rare.


Interesting negation ... but did you ever look / test ?

Image

Code: Select all
*******************************

480x320x24bpp TrueColor

BU.BMP 460'854 
BU.TIF 461'560 Default NC/XN
BU.TIF 149'563 LZW-Pre lossless
BU.PNG 118'630 PNGOUT

*******************************


You can see 2 things:

1. NCONVERT and XNVIEW (like most others) save TIFF by default as uncompressed (even bigger than BMP) :shock:

2. PNG is ways better than any of TIFF's lossless methods

BMPs are about the worst format you can use. Ever.


BMP is very simple and very compatible ... very good for temporary storage ... and very bad for long term storage or uploading.
Last edited by DOS386 on Sat May 08, 2010 6:37 am, edited 1 time in total.
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Postby RGBA » Fri Sep 05, 2008 11:43 am

@DOS386
TIFF can handle Meta Infos (EXIF, IPTC, XMP), as well as JPG.
BMP has no ability for this.

Related to PNG: it supports also Meta Infos,
but doesn't follows neither IPTC- nor Exif-specs.

146.313 TGA (RLE)
135.201 TIFF (ZIP)
119.842 PNG (9, Filter all)
118.832 PNG (optipng -o7)

If you are only interested in low size just resample the pic and take GIF.
Color: Floyd or adapt., because there is no visible difference and your result is already under 100KB.

My fav. PNG recompression tools are currently:
http://www.vbgore.com/PNG_Monster
or
http://psydk.org/PngOptimizer
The second one is pretty good, because its looks automatic in subfolders and is fine for portable usage.
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Re: How to reduce an image's file size?

Postby georgesg » Mon Apr 12, 2010 7:54 pm

In the french version, the window "resize" proposes 3 options for finalizing the resizing: "charger", "enregistrer", "exporter" in french, that is "load", "save", "export" in english.
How to use these buttons, what do they do?
Thanks for explaining in detail..... georges :?
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Re: How to reduce an image's file size?

Postby Drahken » Thu Apr 15, 2010 3:04 pm

The only thing I can find with load/save/convert in the english version is the batch & sequence convert. In both of these cases the load/save/export only applies to conversion scripts (save saves changes to the existing script, load loads a new script, export saves settings to a new script). If this is the dialog you are looking at, then you should see "go" and "cancel" at the bottom of the window, "go" is what you want to click.
The normal resize option (should be located under image->resize, or by hitting shift+s) only has go & cancel.

The batch convert dialog:
Image


The normal resize dialog:
Image
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