| Cap. 6. Getting Images out of GIMP | ||
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Parte II. How do I Become a GIMP Wizard? |  |
| Cap. 6. Getting Images out of GIMP | ||
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Parte II. How do I Become a GIMP Wizard? | |
Cuprins
GIMP is capable of reading and writing a large variety of graphics file formats. With the exception of GIMP's native XCF file format, file handling is done by plug-ins. This makes it relatively easy to extend GIMP to support new file types when the need arises.
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When you opened an image in older GIMP releases, let's say a JPG or PNG, the image kept its format and was saved in the same format by using Save. Since GIMP 2.8, all images are imported into GIMP's own XCF format, as a new project. For example, a „sunflower.png” image will be loaded as „*[sunflower] (imported)-1.0 (indexed color, 1 layer)”. The leading asterisk indicates that this file has been changed. This image can be saved as „sunflower.xcf” by using the Save command. If you need your image in another format, you should use the Export command. |
When you are finished working with an image, you will save the results. In fact, it is often a good idea to save at intermediate stages too. GIMP is a pretty robust program, but on rare occasions crashes have happened.
GIMP's native format XCF is special. It is the only format that can store everything about an image (with the exception of „undo” information). This is the reason that saving can only be done in this format. It makes the XCF format especially suited for storing 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. Once you have finished editing your image, you can export it to the format of your choice. GIMP supports a wide range of formats. Most file formats that can be imported, can also be used for exporting.
There are several commands for saving and exporting images. They are listed in the section covering the File Menu. More information on how to use them can be found there.
GIMP allows you to export 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. It is up to you to understand the capabilities of the format you choose.
Exporting an image does not modify the image itself, so you do not lose anything by exporting. See Export file.
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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 (an asterisk is in front of the image name). Saving an image in GIMP's native format XCF will cause the image to be considered "not dirty". On the contrary, exporting an image will not change it to "not dirty", because GIMP can't know for sure that no image information will be lost. |
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The GIF file format does not support some basic image properties such as print resolution. If you care for these properties, use a different file format like PNG. |
Checking interlace allows an image on a web page to be progressively displayed as it is downloaded. Progressive image display is useful with slow connection speeds, because you can stop an image that is of no interest; interlace is of less use today with our faster connection speeds.
GIF comments support only 7-bit ASCII characters. If you use a character outside the 7-bit ASCII set, GIMP will export the image without a comment, and then inform you that the comment was not saved.
When this option is checked, the animation will play repeatedly until you stop it.
You can set the delay, in milliseconds, between frames if it has not been set before. In this case, you can modify every delay in the Layer Dialog.
If this has not been set before, you can set how frames will be superimposed. You can select among three options :
I don't care: you can use this option if all your layers are opaque. Layers will overwrite what is beneath.
Cumulative Layers (combine): previous frames will not be deleted when a new one is displayed.
One frame per layer (replace): previous frames will be deleted before displaying a new frame.
Self-explanatory.
Self-explanatory.
HEIF stands for High Efficiency Image File Format. Also known as HEIC High Efficiency Image Coding.
Twice as much information can be stored in a HEIF image as in a JPEG image of the same size, resulting in a better quality. As of October 2019, no browser supports HEIF natively.
More information in Wikipedia.
These options are self-explanatory.
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.
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.
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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Please note, that the numbers for the JPEG quality level have a different meaning in different applications. Saving with a quality level of 80 in GIMP is not necessarily comparable with saving with a quality level of 80 in a different application. |
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.
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: the image reverts back to its original state when the JPEG dialog is closed.)
If the image you loaded has Exif, XMP, IPTC metadata, they are preserved, and you can keep them or not when exporting to JPEG.
Many applications use the small thumbnail image as a quickly available small preview image.
You can edit the comment attached to the image you loaded or write a new one.
Some information about the advanced settings:
If you enable this option, the optimization of entropy encoding parameters will be used. The result is typically a smaller file, but it takes more time to generate.
JPG compression creates artifacts. By using this option, you can smooth the image when saving, reducing them. But your image becomes somewhat blurred.
Arithmetic encoding is a form of entropy encoding (a lossless data compression scheme) that, since GIMP-2.10 can be used in exporting as JPEG. Images using arithmetic encoding can be 5 - 10 % smaller. But older software may have trouble opening these images.
The image file can include markers which allow the image to be loaded as segments. If a connection is broken while loading the image in a web page, loading can resume from the next marker.
JPEG images are stored as a series of compressed square tiles named MCU (Minimum Coding Unit). You can set the size of these tiles (in pixels).
With this option enabled, the image chunks are stored in the file in an order that allows progressive image refinement during a slow connection web download. The progressive option for JPG has the same purpose as the interlace option for GIF. Unfortunately, the progressive option produces slightly larger JPG files (than without the progressive option).
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Beware that certain older TVs and photo frames (and maybe other devices) may not be able to show jpeg images that have been exported with the progressive setting enabled (which is the default). |
The human eye is not sensitive in the same way over the entire color spectrum. The compression can use this to treat slightly different colors that the eye perceives as very close, as identical colors. Three methods are available :
1x1,1x1,1x1 (best quality): Commonly referred to as (4:4:4), this produces the best quality, preserving borders and contrasting colors, but compression is less.
2x1,1x1,1x1 (4:2:2): This is the standard subsampling, which usually provides a good ratio between image quality and file size. There are situations, however, in which using no subsampling (4:4:4) provides a noticeable increase in the image quality; for example, when the image contains fine details such as text over a uniform background, or images with almost-flat colors.
1x2,1x1,1x1 This is similar to (2x1,1x1,1x1), but the chroma sampling is in the horizontal direction rather than the vertical direction; as if someone rotated an image.
2x2,1x1,1x1 (smallest file): Commonly referred to as (4:1:1), this produces the smallest files. This suits images with weak borders but tends to denature colors.
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”.
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.
integer (the default): This method is faster than „float”, but not as accurate.
fast integer: The fast integer method is much less accurate than the other two.
Checking interlace allows an image on a web page to be progressively displayed as it is downloaded. Progressive image display is useful with slow connection speeds, because you can stop an image that is of no interest; interlace is of less use today with our faster connection speeds.
If your image has many transparency levels, the Internet browsers that recognize only two levels, will use the background color of your Toolbox instead.
Gamma correction is the ability to correct for differences in how computers interpret color values. This saves gamma information in the PNG that reflects the current Gamma factor for your display. Viewers on other computers can then compensate to ensure that the image is not too dark or too bright.
PNG supports an offset value called the „oFFs chunk”, which provides position data. Unfortunately, PNG offset support in GIMP is broken, or at least is not compatible with other applications, and has been for a long time. Do not enable offsets, let GIMP flatten the layers before saving, and you will have no problems.
Save the image resolution, in ppi (pixels per inch).
Date the file was saved.
You can read this comment in the Image Properties.
When this option is checked, the color values are saved even if the pixels are completely transparent. But this is possible only with a single layer, not with a merged composition. When a multi-layer image gets exported to a single-layer file format, there is no way GIMP could preserve the color values in the transparent pixels.
By default your PNG image is exported in a pixelformat that resembles the settings of your image in GIMP. This is usually what you want, but when you need a specific output format you can change this here to 8 or 16 bits per channel RGB or Grayscale, with or without alpha channel.
Since compression is not lossy, the only reason to use a compression level less than 9, is if it takes too long to compress a file on a slow computer. Nothing to fear from decompression: it is as quick whatever the compression level.
If the image you loaded has Exif, XMP, IPTC metadata, they are preserved, and you can keep them or not when exporting to PNG.
Click to save the current settings. Latter, you can use to load the saved settings.
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The PNG format supports indexed images. Using fewer colors, therefore, results in a smaller file; this is especially useful for creating web images; see Secțiune 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. |
This option allows you to specify the algorithm used to compress the image.
None: is fast, and lossless, but the resulting file can be very large.
LZW: The image is compressed using the „Lempel-Ziv-Welch” algorithm, a lossless compression technique. This is efficient and fast. More information at [WKPD-LZW].
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])
Deflate: is a lossless data compression algorithm that uses a combination of the LZ77 algorithm and Huffman coding. It is also used in Zip and Gzip files and PNG images. Source: [WKPD-DEFLATE].
JPEG: is a very good compression algorithm but lossy. This is the same compression as used in JPEG images. Since it is lossy, you should not use this when image quality is important. This compression can not be used when your image is in indexed mode.
CCITT Group 3 fax and CCITT Group 4 fax are black and white formats developed to transfer images by FAX.
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These two compression modes can only be selected, if the image is in indexed mode and reduced to two colors. Use → → to convert the image to indexed mode. Make sure that „Use black and white (1-bit) palette” is checked. |
Since GIMP-2.10.12, you can save layers when exporting to TIFF. Each layer will be a separate page in the TIFF image.
When Save layers is checked, this option, which is enabled by default, will resize all layers to the size of the image. TIFF images can not have negative offsets. This option enables you to import the TIFF again without having to change the position of layers that had a different size as the image in the original.
When this is enabled the color values are saved even if the pixels are completely transparent.
In this text box, you can enter a comment that is associated with the image.
When this option is enabled existing EXIF metadata will be saved in the exported TIFF image.
When this option is enabled existing XMP metadata will be saved in the exported TIFF image.
When this option is enabled existing IPTC metadata will be saved in the exported TIFF image.
When this option is enabled a thumbnail will be saved as the second page in the exported TIFF image. This will also cause certain EXIF tags to be saved even if you have disabled saving EXIF metadata.
When this option is enabled the color profile will be saved in the exported TIFF image.
When this option is enabled GeoTIFF metadata that was present in the original image will be saved in the exported TIFF image.
When checked, the image will be saved in a way that keeps the original quality. This disables the Image quality and Alpha quality settings. When unchecked, the image will be saved based on the quality settings below, which may cause some loss of quality, but at a better compression ratio. This is the default.
These settings determine at what quality the image will be saved. A higher image quality usually means a larger filesize. When filesize is important you can check if a lower quality is still good enough for your purposes.
This selects the type of image you are trying to export. Presumably this will help in deciding the best way to compress the image.
Choices are: Default, Picture for digital pictures, like portraits or inside shots, Photo for outdoor photography with natural lighting, Drawing for hand or line drawing, with high-contrast details, Icon for small-sized colorful images, and Text for images containing mostly text.
When this option is enabled existing EXIF metadata will be saved in the exported image.
When this option is enabled existing IPTC metadata will be saved in the exported image.
When this option is enabled existing XMP metadata will be saved in the exported image.
When this option is enabled the color profile will be saved in the exported image.
When this option is enabled a thumbnail will be saved in the exported image.
MNG is acronym for „Multiple-Image Network Graphics”.
The main problem is that Konqueror is the only Web navigator that recognizes the MNG animation format. Please see http://en.wikipedia.org/wiki/Multiple-image_Network_Graphics.
