Important File Formats for Practical Work

When saving an image, you should select a file format that corresponds to the requirements of the image as well as your stylistic intentions. This section will introduce you to the most commonly used file formats.

XCF: GIMP's Native File Format

GIMP's XCF format was created for the primary purpose of saving images with layers; however, it can also be used to save images that aren't finished yet. The XCF format saves image layers by employing a lossless compression method. So the file size of an XCF formatted image will be smaller than most other image file formats and about 30 percent smaller than the PSD format, described later.

Since overly large files are cumbersome and can be unmanageable, GIMP's native XCF format is the best choice for storing images with layers. The only drawback of GIMP's current version is that XCF files cannot be opened in another image editing program. If you want to export an XCF file into another program, you must first convert a copy of it into JPEG, PNG, or TIF. If you plan to consistently use other programs in conjunction with GIMP 2.6, you should save images with layers in the PSD format.

XCF Characteristics

• 16.78 million colors, 24-bit color depth

• Alpha transparency (color gradient from transparent to opaque)

• Lossless compression

• Supports layers

PSD: Adobe Photoshop's Native File Format

PSD (PhotoShop Document) is the native file format of Adobe Photoshop, one of the most popular image editing programs. This file format is considered a de-facto standard and can be used by almost all other image editing programs, including GIMP. It is a high-quality format that is frequently used to export images with layers.

The downside of saving images in PSD is that the files are often quite large because the format provides no compression options.

Since GIMP 2.4, Photoshop layer masks are readable, and it can even write in the PSD format. However, GIMP does not support some PSD formats such as smart objects or smart filters.

PSD Characteristics

• 16.78 million colors, 24-bit color depth

• Alpha transparency (color gradient from transparent to opaque)

• Supports layers

PNG: Portable Network Graphics

The PNG format is capable of preserving the transparencies of an image with full 24-bit color depth. Moreover, it uses a lossless high compression method that considerably reduces the image file size.

The PNG format is also suitable for Internet use.

PNG Characteristics

• 256 or 16.78 million colors, 8- or 24-bit color depth

• Alpha transparency (color gradient from transparent to opaque)

• Lossless, settable compression

• Suitable for the Internet

• Interlaced (immediate display, layered refresh rate in web pages)

JPG/JPEG: Joint Photographic Experts Group

Photographs and photo-realistic images with a color depth of 24 bits can be efficiently compressed with the JPEG format, which reduces image files to a fraction of the original size. However, the compression method used by the JPEG format is not lossless. This means that the image quality will suffer in correlation with the degree of the compression as well as the decrease in file size. The JPEG format was developed primarily as a way to quickly load photographs on the Internet. JPEG format should be avoided when archiving digital photographs. You should also refrain from repeatedly saving an image in the JPEG format because the quality of your image will drop with each subsequent save. To preserve the integrity of your images, use the PNG format to save when you are working with them and to archive your images after you've finished editing them.

For exporting images in the JPEG format, GIMP offers a programmable compression option with a preview feature. This option will display the file size of the compressed photo prior to saving it.

JPG Characteristics

• 16.78 million colors, 24-bit color depth

• High but lossy compression; settable by the user

• Suitable for use on the Internet

• Progressive (faster display in Web pages, layered image refresh rate, comparable with the interlaced characteristic



GIF: Graphics Interchange Format

Unlike other file formats, the GIF format requires a color palette. This means that a maximum of 256 colors can be saved in conjunction with an image. GIMP can create these color palettes automatically, but there is a major drawback to doing so. Converting images with an original color depth of 24 bits (or more) to GIF will usually produce an unsatisfactory result.

However, if you save an image with 256 or fewer colors (such as a simple logo) to GIF, the GIF compression is lossless. In addition, GIF files allow you to save one transparent color as well as simple animations (animated GIF). The GIF format is often used to upload images to the Internet

GIF Characteristics

• 2 to 256 colors, 8-bit color depth (at least one, possibly transparent, color)

• Lossless compression for images containing up to 256 colors

• Suitable for use on the Internet

• One color can be transparent

• Interlaced (immediate display, layered refresh rate in web pages)

• Animated GIF available

BMP: Windows Bitmap

Microsoft developed BMP, therefore it is supported by most Windows image editing programs and is a suitable format for image sharing between different programs. BMP has a color depth of 24 bits and the image resolution remains unchanged when exporting. However, because the size of BMP files is normally quite large, the format is not particularly suitable for the Internet.

BMP Characteristics

• 16.78 million colors, 24-bit color depth

• Rather unsuitable for use on the Internet (for Microsoft Internet Explorer only)

TIF/TIFF: Tagged Image File Format

This is one of the oldest image file formats still in use. Almost all image editing programs can read and write a TIF formatted image, even if they're being run on different operating systems. For this reason, it is the best file format to use when sharing high-quality images without layers. Basically, the file format also allows you to save images in the CMYK mode for the four-color printing process.

The TIF format preserves all transparencies of an image with the full color depth of 24 bits. It uses a lossless, but not particularly high, compression method. With an adapted TIF format, you may save images with the 48-bit color depth (i.e., HDRI images). The major drawback of using TIF is that it doesn't support layers (except in Adobe Photoshop).

TIF Characteristics

• 16.78 million colors, 24-bit color depth

• Alpha transparency (color gradient from transparent to opaque)

• Lossless LZW compression

• Different settings for saving when used on IBM/Intel and Macintosh PowerPC computers

DNG: Adobe's Digital Negative

Adobe's DNG format was developed to replace the proprietary RAW files and create an open standard. It offers advantages in the long-term archiving of RAW files and provides photographers with a certain amount of freedom from the camera's own software. GIMP'S plug-in UFRaw can work with DNG and most cameras' RAW formats. Several camera manufacturers have introduced this format to save images in their cameras.

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  • james
    Why image compression is important and how it can be used to store files on the Internet?
    7 years ago
  • petros
    How to know 24 bit or 8 bit jpg in gimp?
    7 years ago
  • amaranto
    What format to save transparancy on Gimp 2.6?
    7 years ago

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