Image file format


Painting software (such as PhotoShop) deals with images made up of a collection of pixels. Such an image is called a "bitmap" because bits of data representing the intensity of RGB components are stored in order. Drawing software data that expresses shape data with numerical values ​​such as coordinates of points is called "vector data", whereas bitmap data is sometimes called "raster data". Raster means a collection of points and lines.
The followings are the formats for bitmap data:

JPEG format (extension .jpg)
JPEG (ISO Joint Photographic Experts Group) is a general-purpose image format developed by ISO. It is widely used on the web because browsers support it by default. It is also used to store digital camera data and mobile photos. The feature is that the compression ratio is high and the file size is as small as a several tens percent to a few percent. Instead, an irreversible compression algorithm that cannot be restored to the original data is used, and the image quality decreases more or less. The image quality or compression ratio can be selected when saving (in the case of Photoshop, the image quality is 0 to 12). Degradation is practically negligible if stored at high image quality. This is suitable for storing full color images such as photos. Noise called block noise or mosquito noise occurs in areas where color changes suddenly (particularly when the image quality is low), so it is not suitable for storing an image like illustration. To save such an image, PNG or GIF format (only for 256 colors or less) is recommended.
Normally, a JPEG image is saved in RGB mode, but you can save it even after converting to CMYK mode. When placing a photo in Illustrator or InDesign to be used as a print manuscript, convert it to CMYK mode and save it in JPEG format (for most printing, JPEG format is also acceptable) or save in EPS format described later.

JPEG 2000 format (extension .jp2, .j2k)
An improved version of JPEG. It is characterized by less image degradation than JPEG. In particular, block noise hardly appears. You can also select a storage format (lossless) that does not degrade the image. Mac OS X and Adobe products (Acrobat, Photoshop, etc.) for Mac support this format, but few web browsers do not (as of 2014).

GIF format (extension .gif)
A compressed image format popularized by PC communication company CompuServe. Widely used on the web. A format for saving images in index color (palette color) mode. The maximum number of colors is 256 (8 bits). Images containing more colors should be reduced in color number before saving. Since the number of colors is limited, it is suitable for storing illustrations with few colors. The data is compressed.
With internal versions GIF89a, you can specify one of the index colors as transparent. By combining several GIF images, a GIF animation can be created. Free software for creating GIF animations is available. GIF animations can also be created with Adobe ImageReady or Adobe Photoshop.

PNG format (extension .png)
Abbreviation for Portable Network Graphics, pronounced 'ping'. It was developed as a successor to GIF. It supports lossless image compression and is suitable for storing illustrations. Not only index colors (256 colors) but also full color images (with 24-bit colors) can be saved. It also supports transparent color. It is widely used on the web because the browsers support it by default.

Photoshop PSD format (extension .psd)
Adobe Photoshop's native format. The file size tends to be large because it saves lossless image data. In addition to RGB and CMYK modes, Lab mode and 16-bit mode images can also be saved. It also supports layers.

EPS format (extension .eps)
Encapsulated PostScript is a kind of PostScript file with a bitmap embedded in it. Normally save in CMYK mode. CMYK mode EPS is often used for the purpose of placing photos in Illustrator or InDesign. There are two formats: an ASCII format that embeds data as characters and a binary format that embeds data in hexadecimal. The ASCII format is the most versatile, but has twice the file size of the binary format. When saving in Photoshop EPS format, you can save in uncompressed format (ASCII / binary) or JPEG compressed. JPEG compression can significantly reduce file size. When saving, it is possible to include a low-quality "preview image" separately from the data itself, but this is a feature left for compatibility with older versions of Illustrator, and usually including a preview image is not needed.

Photoshop EPS saving window  Saving as EPS with Photoshop

EPS format includes DCS 1.0 and DCS 2.0 in addition to normal EPS format. DCS is an abbreviation of Desktop Color Separation. When this is selected, the image is saved as a set of files for color plates. DCS 2.0 is a format for using a special color ink.

TIFF format (extension .tif)
Tagged Image File Format, a general-purpose format developed by Aldus (acquired by Adobe). This format is popular among publishing industries and photographers. As well as saving uncompressed data, saving with LZW compression is supported. It is commonly used as a standard format of monochrome images (with high resolution) for a print shop.

Windows Bitmap format (extension .bmp)
Windows' standard image format. The file size tends to be large because image is not compressed.

PICT format (extension .pct, .pic)
The former standard image format for Macintosh and "QuickDraw."

* In a manuscript containing a bitmap image such as a photo for a print shop, a resolution of 350 pixels/inch or more is required in printing size. For monochrome images, a resolution of 1200 pixels/inch or more is required.
* In addition to the standard formats for Illustrator and InDesign, PDF/X-1a: 2001 (Japan) compliant PDF formats have been increasingly accepted.

Reference: Examples of noise in JPEG images
JPEG noise1
Block noises in a low-quality
image (enlarged view)

    Mosquito noises (enlarged view)
Noise appears when the color
changes suddenly. Such images
should be saved as PNG or GIF.

Reference: Example of JPEG 2000 image
high-quality JPEG
low-quality JPEG
High-quality JPEG
(file size 72KB)
Low-quality JPEG
(file size 27 KB)
Low-quality JPEG 2000
(file size 25 KB)
Original JPEG 2000 file

Reference: Image file size

For an uncompressed full-color image, data of 24 bits = 3 bytes are needed to represent the color of each pixel. This means that the required file size = number of pixels x 3 (bytes).
For example, an image of 1600 x 1200 pixels (approximately 2 million pixels or 2 megapixels) requires
1600 x 1200 x 3 = 1920000 x 3 = 5760000 bytes (roughly 6 MB).
This is a net data size of an uncompressed image, but the actual file size might be slightly larger because of a header.

By irreversibly compressing this with JPEG, the file size can be significantly reduced (compression rate depends on image quality). With lossless compression (compression other than JPEG), the compression ratio is not very large, but the file size is somewhat smaller. In particular, when the same color appears continuously like in a monochrome image or in an illustration, the compression ratio will be improved. (On the other hand, in an image where the color changes frequently from place to place, the file size might become larger when compression is applied.)

Vector Graphics

An image file described as a set of coordinate values is called a vector image, while an image file made up of a set of pixels is a bitmap. A vector graphic image is usually made of a combination of curves (strokes) and shapes filled with color. Character fonts are also made up of outlines, so they can be included in vector images. Since the vector image is made up of numerical values, it can be freely scaled up or down (no jaggies appear even when it is scaled up). This is a major feature of vector graphics. Images of drawing software are usually saved as vector graphics data.
Adobe Illustrator format (extension .ai)
This is the standard format for Illustrator. Since the format differs depending on the Illustrator version, the new version may not be readable by the older versions. Since the contents are text, an editor can open the file to check the contents. AI file is written in PostScript, a language for printers developed by Adobe.

PDF (extension .pdf)
Portable Document Format (PDF) developed by Adobe is a general-purpose format for exchanging documents. It can include bitmap images, but is basically made of vector images. If we select the PDF format when saving with Illustrator, we can reduce the file size and optionally save it as a PDF that is editable by Illustrator. Many applications other than graphic applications also support PDF output. A PDF file written with only simple functions may be opened with Illustrator.

PostScript format (extension .ps)
A general-purpose file written in the PostScript language developed by Adobe. It can be opened with any application that understands the PostScript (PS) language. Illustrator also supports PS files. AI format is also a type of PS file.

SVG format (extension .svg)
SVG stands for Scalable Vector Graphics. It is a general-purpose vector image format. Since it is a text file, we can see the contents with an editor. This format was developed by the W3C (World Wide Web Consortium) as an image file for the web. It is widely used for graphs and illustrations on the web because images can be freely scaled without jaggies. Of course, it can be placed on the web screen as it is. Illustrator has the ability to export data as SVG files.