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Chapter 6. Getting Images out of GIMP
The GIMP is capable of reading and writing a large
variety of graphics file formats. With the exception of
GIMP's native XCF file type, file handling is done by
Plugins. Thus, it is relatively easy to extend GIMP
to new file types when the need arises.
Not all file types are equally good for all purposes. This part of the
documentation should help you understand the advantages and disadvantages
of each type.
When you are finished working with an image, you will want to save the
results. (In fact, it is often a good idea to save at intermediate
stages too: GIMP
is a pretty robust program, but we have heard rumors, possibly
apocryphal, that it may have been known on rare and mysterious occasions
to crash.) Most of the file formats that GIMP
can open, can also be used for saving. There is one file format that is
special, though: XCF is GIMP's native format, and is
useful because it stores everything about an image
(well, almost everything; it does not store “undo”
information). Thus, the XCF format is especially suitable for saving
intermediate results, and for saving images to be re-opened later in
GIMP.
XCF files are not readable by most other programs that display images,
so once you have finished, you will probably also want to save the image
in a more widely used format, such as JPEG, PNG, TIFF, etc.
There are several commands for saving images. A list, and information on
how to use them, can be found in the section covering the
File Menu.
GIMP allows you to save the images you create in a
wide variety of formats. It is important to realize that the only
format capable of saving all of the information in
an image, including layers, transparency, etc., is GIMP's native XCF
format. Every other format preserves some image properties and loses
others. When you save an image, GIMP tries to let
you know about this, but basically it is up to you to understand the
capabilities of the format you choose.
As stated above, there is no file format, with the exception of GIMP's
native XCF format, that is capable
of storing all the data in a GIMP
image. When you ask to save an image in a format that will not
completely represent it, GIMP
notifies you of this, tells you what kind of information will be lost,
and asks you whether you would like to “export” the image
in a form that the file type can handle. Exporting an image does not
modify the image itself, so you do not lose anything by doing this. See
Export file.
![[Note]](images/note.png)
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Note |
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When you close an image (possibly by quitting GIMP), you are warned if
the image is "dirty"; that is, if it has been changed without
subsequently being saved. Saving an image in any file format will
cause the image to be considered "not dirty", even if the file format
does not represent all of the information from the image.
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![[Warning]](images/warning.png)
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Warning |
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Please note, that the GIF file format is incapable of storing some
rather basic image informations like
print resolution.
If you care for these properties, you should consider a different
file format like PNG.
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GIF Options
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Interlace
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When this option is checked, the image will be displayed
progressively on the Web page. It was interesting when
computers and modems were slow, as it allowed to stop
loading an image of no interest.
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GIF comment
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Beware of do not insert characters outside the ASCII
range, because of the GIF format supports 7-bits ASCII
texts only, that GIMP can't provide. If you insert
inadvertently a non-ASCII char, the option will be
automatically disabled.
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Animated GIF Options
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Loop forever
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When this option is checked the animation will start
playing again repeatedly until you stop it.
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Delay between frames if unspecified
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You can set the delay, in millisecondes, between frames
if it has not been set before. In this case, you can
modify every delay in the Layer Dialog.
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Frame disposal when unspecified
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If this has not been set before, you can set how frames
will be superimposed. You can select among three options :
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I don't care
: you can use this option if all your layers are
opaque. Layers will overwrite what is beneath.
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Cumulative Layers (combine)
: previous frames will not be deleted when a new one
is displayed.
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One frame per layer (replace)
: previous frames will be deleted before displaying a
new frame.
JPEG files usually have an extension .jpg, .JPG, or .jpeg. It is a
very widely used format, because it compresses images very
efficiently, while minimizing the loss of image quality. No other
format comes close to achieving the same level of compression. It does
not, however, support transparency or multiple layers. For this
reason, saving images as JPEG often requires them to be exported from
GIMP.
The JPEG algorithm is quite complex, and involves a bewildering number
of options, whose meaning is beyond the scope of this documentation.
Unless you are a JPEG expert, the Quality parameter is probably the
only one you will need to adjust.
![[Caution]](images/caution.png)
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Caution |
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After you save an image as a JPEG file, the image is no longer
considered “dirty” by GIMP,
so unless you make further changes to it, you will not receive
any warning if you close it. Because JPEG is lossy and does not
support transparency or multiple layers, some of the information
in the image might then be lost. If you want to save all of the
information in an image, use GIMP's native
XCF format.
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Quality
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When you save a file in JPEG format, a dialog is displayed
that allows you to set the Quality level, which ranges from 0
to 100. Values above 95 are generally not useful, though. The
default quality of 85 usually produces excellent results, but
in many cases it is possible to set the quality substantially
lower without noticeably degrading the image. You can test the
effect of different quality settings by checking
Show Preview in image window
in the JPEG dialog.
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Note |
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Please note, that the numbers for the JPEG quality level have
a different meaning. Saving with a level of 80 in GIMP is not
necessarily comparable with saving with a level of 80 in an
different application.
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Preview in image window
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Checking this option causes each change in quality (or any other
JPEG parameter) to be shown in the image display. (This does not
alter the image, though: it reverts back to its original state
when the JPEG dialog is closed.)
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Advanced settings
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Some information about the advanced settings:
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Optimize
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If you enable this option, the optimization of entropy
encoding parameters will be used.
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Progressive
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With this option enabled the chunks of the image will be
inserted in a certain order in the file. This is done with
the intent to give a progressive refinement of the image
appearance during a slow connection web download, similar
and with the same purpose of the corresponding option
present in the GIF format too. The downside of enabling
this option is, that you get slightly larger files.
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Save EXIF data
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JPEG files from many digital cameras contain extra
information, called EXIF data, which specifies camera
settings and other information concerning the
circumstances under which the image was created. GIMP's
ability to handle EXIF data depends on whether the
“libexif”
library is available on your system; it is not
automatically packaged with GIMP. If GIMP was built with
libexif support, then EXIF data is preserved if you open a
JPEG file, work with the resulting image, and then save it
as JPEG. The EXIF data is not altered in any way when you
do this (which means that some of its fields are no longer
valid). If GIMP was not built with EXIF support, this does
not prevent files with EXIF data from being opened, but it
does mean that the EXIF data will not be present when the
resulting image is later saved.
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Save Preview
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This option lets you save a thumbnail with the image.
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Save XMP data
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If you enable this option, the meta data of the image will
be saved as XMP-structure within the
file.
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Use quality settings from original image
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If a particular quality setting (or
“quantization table”
) was attached to the image when it was loaded, then this
option allows you to use them instead of the standard
ones.
If you have only made a few changes to the image, then
re-using the same quality setting will give you almost the
same quality and file size as the original image. This
will minimize the losses caused by the quantization step,
compared to what would happen if you used different
quality setting.
If the quality setting found in the original file are not
better than your default quality settings, then the option
“Use quality settings from original image”
will be available but not enabled. This ensures that you
always get at least the minimum quality specified in your
defaults. If you did not make major changes to the image
and you want to save it using the same quality as the
original, then you can do it by enabling this option.
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Smoothing
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Compression can create artefacts. By using this option,
you can smooth the image when saving, reducing them. But
your image becomes somewhat blurred.
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Restart markers
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The image file can include some markers which allow to
segment it. If loading this image in a Web page is broken
off, loading can resume from the following marker.
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Subsampling
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Human eye is not sensitive in the same way all over color
spectrum. Compression can use this to consider as
identical slightly different colors. Three methods are
available :
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1x1,1x1,1x1 (best quality)
: preserves borders and contrasting colors, but
compression is less.
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2x1,1x1,1x1 (4:2:2)
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1x2,1x1,1x1
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2x2,1x1,1x1 (smallest file)
: important compression; suits images with weak
borders but tends to denature colors.
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DCT Method
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DCT is “discrete cosine transform”,
and it is the first step in the JPEG algorithm going
from the spatial to the frequency domain. The choices are
“float”, “integer”
(the default), and “fast integer”.
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float
: The float method is very slightly more accurate than
the integer method, but is much slower unless your
machine has very fast floating-point hardware. Also
note that the results of the floating-point method may
vary slightly across machines, while the integer
methods should give the same results everywhere.
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integer
(the default): This method is faster than
“float”, but not as accurate.
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fast integer
: The fast integer method is much less accurate than
the other two.
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Image comments
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In this text box, you can enter a comment which will be
saved with the image.
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Interlacing
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When this option is checked, the image is progressively
displayed on a Web page. So, slow computer users can stop
downloading if they are not interested.
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Save background color
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If your image has many transparency levels, the Internet
browsers which recognize only two levels, will use the
background color of your Toolbox instead. But Internet Explorer
up to version 6 did not use these informations.
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Save gamma
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Informations about your monitor will be saved, so that the
image will be displayed in the same way on other computers,
provided that the display program supports these
informations, what is rarely the case.
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Save layer offset
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No interest. Images with layers are flattened before saving
to PNG and layer offset is taken in account.
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Save Resolution
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Save the image resolution, in dpi (dot per inch).
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Save creation time
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That will be the date of last saving.
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Save comment
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You can read this comment in the
Image Properties.
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Save color values from transparent pixels
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With this option is checked, the color values are saved even if the
pixels are completely transparent.
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Compression level
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Since compression is not lossy, the only reason to use a
compression level less than 9 would be a too long time to
compress file on a slow computer. Nothing to fear from
decompression: it is as quick whatever the compression level.
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Save defaults
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If you click on this button, your settings will be saved and
can be used by other savings by clicking on the
Load defaults.
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Since PNG format supports indexed images, you have better reduce the
number of colors before saving if you want to have the lightest file
for the Web. See Section 6.6, “Indexed mode”.
Computers work on 8 bits blocks named “Byte”.
A byte allows 256 colors. Reducing the number of colors below 256
is not useful: a byte will be used anyway and the file size will not
be less. More, this “PNG8”
format, like GIF, uses only one bit for transparency; only two
transparency levels are possible, transparent or opaque.
If you want PNG transparency to be fully displayed by Internet
Explorer, you can use the AlphaImageLoader DirectX filter in the
code of your Web page. See Microsoft Knowledge Base
[MSKB-294714].
Please note, that this should not be necessary for
InternetExplorer 7 and above.
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Compression
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This option give you the opportunity to choose the compression
method that is appropriate for your image:
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None : fast method, but resulting in
a big file.
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LZW : The image will be compressed
using the “Lempel-Ziv-Welch” algorithm, a
lossless compression technique. This is an old method,
still efficient and fast. More informations at
[WKPD-LZW].
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Pack Bits : is a fast, simple
compression scheme for run-length encoding of data. Apple
introduced the PackBits format with the release of
MacPaint on the Macintosh computer. A PackBits data
stream consists of packets of one byte of header followed
by data. (Source:
[WKPD-PACKBITS])
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Deflate: It is a lossless data
compression algorithm that uses a combination of the LZ77
algorithm and Huffman coding. It is also used in Zip,
Gzip and PNG file formats. Source:
Wikipedia.
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JPEG: this is a very good
compression algorithm but lossy.
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CCITT Group 3 fax;
CCITT Group 4 fax
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Note |
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These options can only be selected, if the image is in
indexed mode and reduced to two colors.
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Save color values from transparent pixels
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With this option the color values are saved even if the pixels
are completely transparent.
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Comment
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In this text box, you can enter a comment which will be
associated with the image.
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