Choosing the Correct Image formats

Whether you want to print your photos, show documents, publish on the
Internet, etc.., choose the correct format that corresponds to the use
you intend to do with your images.

JPEG, TIFF, PNG … each format has its own characteristics. To choose
one that will match what you want to make your images, it is essential
to know.

First feature: color depth, expressed in bits, it is the number of
color values that can take each pixel in the image. JPEG, for example,
uses a color depth of 24 bits (or 8 bits per primary color: red, green,
blue), corresponding to more than 16.7 million tones for a pixel. Our
eyes are unable to discern all the nuances, and most printers can not
return them, this may seem sufficient.

Question of details

However, cameras and high-end scanners capture images with a color
depth of 12, 14, 16 bits or more per primary color, or billions of
colors. This saves additional information, useful when editing the
picture, to keep even the small details and to apply correction filters
with more precision and it offers greater flexibility than before the
‘it would lead to a visible degradation of the image.

Another important criterion is compression. The lossless compression
possible with Tiff and PNG, for example, reduces a little the weight of
images without affecting their quality. On the other side, the JPEG
compression standard is called destructive: it eliminates image
information. Effective, but more compression, the higher the quality is
degraded.

What is Pixel?

It is the smallest meaningful element of a digital image. Each pixel is
a color, split into red, green and blue. The contrast of pixels, like a
mosaic, create an image (which can represent text). The pixel is used
as a measurement of the size of display screens. A pixel is not
necessarily square. Its shape depends on the support of display used,
its settings and technology.

Jpeg: the most universal

Joint Photographic Expert Group (JPEG) is a widely used format. JPEG
became a standard for photography as the publication of images on the
Internet (all browsers support it). Set in 1992, its a form of lossy
compression: it reduces the image by eliminating – irreversibly –
redundant information.

Each image software defines its own level of compression, for example
from 1 to 99 or 0 to 12, the lowest figure corresponding to the higher
compression. In this case, the file size can be divided by 50, but the
image is very degraded. Beware, each recording of an image causes a new
JPEG compression and thus further degradation. It is therefore
advisable to save an image in JPEG only after making any edits, and not
to manipulate it later.

  • Compression: lossy
  • Number of Colors: 16.7 million
  • Number of grayscale: 256
  • Management of transparency: no
  • Conservation layers: no
  • Uses: photos and illustrations, web publishing, printing

Gif: for animated images

Graphics Interchange Format (GIF), developed by CompuServe in 1987,
this format provides lossless compression and is suitable primarily for
Web images. It is capable of displaying 16.7 million tones, but can
save only 256 at the same time in an image. The GIF is not suitable for
photography. It is, however, very effective for images with few shades
of color (logos, graphics). The GIF also supports transparency effects
and allows for animated images. Therefore, despite the emergence of
competing formats (PNG), it still has some success on the Web for
emoticons (smileys) and banners.

  • Compression: lossless data
  • Number of colors: 256
  • Management transparency: yes
  • Conservation layers: no
  • Uses: web publishing (not shown), moving pictures

JPEG 2000: uncommon

This variant of the JPEG format, developed in 2000 by the Joint
Photographic Experts Group, is little used outside professional
circles. More and more software accept it, but without proposing all
compression options. Few Internet browsers are able to manage it,
unless they add a specific extension. As for cameras, they ignore. The
JPEG 2000 allows you to choose between lossy or lossless data. Its
lossy compression is more efficient than the JPEG standard, to equal
weight files, JPEG 2000 produces images less degraded. However,
visually, the difference is obvious that for very heavy cuts.

  • Compression: with or without loss of data
  • Number of Colors: 16.7 or 32 million
  • Management of transparency: no
  • Conservation layers: no
  • Uses: photos and illustrations, print

Tiff: for printing

Tagged Image File Format (TIFF), developed in 1987 by Aldus since
acquired by Adobe, this format has not changed since 1992. It is used,
the choice of a non-compressed with lossless or lossy, mainly in the
publishing environment. The Tiff keeps images in a very high quality
and supports layers. In its compressed version, it can choose among
many compression systems (ZIP, LZW), even that of JPEG. The production
of the very large files intended primarily for professional use and
preparing images for printing and excludes any use for Web publishing.

  • Compression: with or without loss of data
  • Number of Colors: 16.7 million to several billion
  • Management of transparency: no
  • Conservation Layers: yes
  • Uses: photos and illustrations, print

PNG : the standard of Web

Portable Network Graphics (PNG) was developed by the W3C (World Wide
Web Consortium), the proponent of Web standards, which led to it being
accepted by all browsers. It is intended to replace the GIF and JPEG on
the web. It provides a lossless compression format and can render 256
to billions of colors. Very effective for compressing graphic images
with lots of flat areas, it is much less efficient than JPEG for
photos. As Gif, it handles transparencies allowing, for example, to
display graphical objects clipped sites in maintaining the pattern
overlay. PNG, however, does not accept animated images.

A derivative format, the MNG (Multiple-image Network Graphics) was
developed for this, but most browsers do not accept it, which makes its
use quite confidential.

  • Compression: lossless data
  • Number of colors: 256 to several billion
  • Management transparency: yes
  • Conservation layers: no
  • Uses: web publication (without photos)

RAW: the friend of photographers

The RAW is a rather unusual format. It contains information recorded by
the sensor before any processing software image. After the shooting,
during the editing session, it can change many settings, such as
exposure and white balance, much more finely than a JPEG file, which by
definition removes information.

The Raw presents however some drawbacks. The files are very large
posing problems for storage and handling. Some high-end devices have a
compressed RAW mode that uses a lossless algorithm similar to that of
Tiff, but reducing the size of files is limited. Another drawback of
RAW format: it is specific to each camera. It often requires special
software, provided by the manufacturer of the device for its opening.
Adobe has tried for several years to adopt a more universal RAW format,
the DNG (Digital Negative). For now, it is adopted by some camera
manufacturers (Samsung, Leica, Sigma).

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