The document outlines various file formats for images, including their purposes and characteristics. It discusses formats such as JPEG, GIF, PNG, TIFF, EPS, PDF, PSD and others. It explains that file conversion involves changing the encoding of data from one format to another, such as when upgrading software versions or changing programs or operating systems. Proper file conversion preserves the original file and logs any processing steps.

1.  Outline the steps to convert images to various file types.

2. In This Chapter, you’ll learn on:
 The appropriate file type for conversion and
backward compatibility.
 How to convert files to specific version for media
delivery.
 How to save and export files to appropriate format
for specific applications.

3.  What is a file format?

 A file format is a particular way that information is
encoded for storage in a computer file.
 Since a disk drive, or indeed any computer storage,
can store only bits, the computer must have some
way of converting information to 0s and 1s and vice-
versa. There are different kinds of formats for different
kinds of information. Within any format type, e.g.,
word processor documents, there will typically be
several different formats. Sometimes these formats
compete with each other.


4.  What is the meaning of file conversion?
 Data conversion is the conversion of computer data from one
format to another.
 Throughout a computer environment, data is encoded in a
variety of ways.
 For example, computer hardware is built on the basis of certain
standards, which requires that data contains, for example,
parity bit checks.
 Similarly, the operating system is predicated on certain
standards for data and file handling.
 Furthermore, each computer program handles data in a
different manner.

5.  What is the meaning of file conversion?
 Whenever any one of these variable is changed, data must be
converted in some way before it can be used by a different
computer, operating system or program.
 Even different versions of these elements usually involve
different data structures.
 For example, the changing of bits from one format to another,
usually for the purpose of application interoperability or of
capability of using new features, is merely a data conversion.
 Data conversions may as simple as the conversion of a text file
from one character encoding system to another; or more
complex, such as the conversion of office file formats, or the
conversion of image and audio file formats.

6.  There are many ways in which data is converted within the
computer environment.
 This may be seamless, as in the case of upgrading to a newer
version of a computer program.
 Alternatively, the conversion may require processing by the use
of a special conversion program, or it may involve a complex
process of going through intermediary stages, or involving
complex "exporting" and "importing" procedures, which may
converting to and from a tab-delimited or comma-separated
text file.


7.  In some cases, a program may recognise several data file
formats at the data input stage and then is also capable of
storing the output data in a number of different formats.
 Such a program may be used to convert a file format. If the
source format or target format is not recognised, then at times
third program may be available which permits the conversion
to an intermediate format, which can then be reformatted
using the first program. There are many possible scenarios.

8.  What is the Digital Image Processing?
 Digital image processing is an accepted practice in forensic
science. It is the position of the Scientific Working Group on
Imaging Technologies (SWGIT) that any changes to an image
made through digital image processing are acceptable in
forensic applications provided the following criteria are met:

 The original image is preserved
 The processing steps are logged when they include techniques
other than those used in a traditional photographic darkroom
 The end result is presented as an enhanced image, which may
be reproduced by applying the logged steps to the original
image

9.  The Image Anomalies which may occur during Image
capturing and processing
 In digital image processing, there are various techniques such
as:
 Image Enhancement
 Image Compression
 Image Restoration
 When using digital image processing techniques, use caution
to avoid the introduction of unexplainable artifacts that add
misleading information to the image and the loss of image
detail that could lead to an erroneous interpretation. Any
processing techniques should be applied only to the working
image.

10.  The successful introduction of forensic imagery as evidence in a
court of law is dependant upon the following four legal tests:
 Reliability
 Reproducibility
 Security
 Discovery

11.  Various Image Conversion Techniques (refer to notes in
Chapter 3)
 Various File Formats and Applications
 There are many various compression applications on the
market and in the public domain. For digital imaging Photo
Editing software such as Photoshop, Microsoft Paint and Paint
Shop Pro are compression applications combined with Image
manipulation software.
 Other types of compression applications not native to photo
manipulation are Winzip & WinRar (Windows Platform) or StuffIt
Expander (Macintosh Platform).
 These types of compression applications are Operating System
specific, which means you can compress a number of files into
one archive file.

12. GIF Indexed color
file, for raster
(pixmap) data
only. Primarily for
synthetic,
somewhat flat
images such as
logos, diagrams,
navigation
buttons, etc.
Graphic Image File format. Uses a CLUT (color
lookup table) to define the colors as though they
were individual color chips, and only supports up
to 256 colors per image. Although it can simulate
continuous-tone colors by dithering, that’s
generally best left to the JPEG or PNG formats.
GIF87 was the original Web graphic file format,
way back in 1993. The current version, GIF89a,
supports 1-bit (jagged-edge) transparency,
comments, and simple animation.
GIF is your best Web option for images with flat,
solid colors — the sort of images you normally
create in vector drawing programs such as
Illustrator. Of course we’d love to put our
Illustrator images on the Web in their original
vector form, but for the most part that’s not
possible (but see SVG lower down in this table).
So we export our vectors to raster formats like GIF
or PNG for the Web.

13. GIF Indexed color
file, for raster
(pixmap) data
only. Primarily for
synthetic,
somewhat flat
images such as
logos, diagrams,
navigation
buttons, etc.
GIF is rarely a good choice for non-Web use.
Technically GIF and its LZW compression algorithm
are “lossless,” but since it supports indexed color
only (8-bit or less), you often have to
permanently throw away image data prior to (or
in the process of) exporting your master file as a
GIF. Don’t be confused by Photoshop’s Save for
Web options for GIF, where you’ll see a “lossy”
checkbox: that just rearranges the pixel patterns
slightly, prior to exporting the GIF, to enhance the
compression. GIF being a “lossless” format means
that, unlike with JPEGs (a lossy format), you could
possibly open a GIF repeatedly, edit it, and re-
save it back out again without necessarily
degrading it. That’s not ideal, unless you really
know what you’re doing. It’s generally best to go
back to the master file — often a PSD — for
editing, and then re-export the GIF.

14. JPEG Compressed, lossy
file format, for raster
(pixmap) data only.
Mostly for photo-
type images on the
Web. Can hold RGB
data; many
compression levels
and other options
available.
Joint Photographic Experts Group. Actually a
family of file formats; usually refers to JFIF JPEG.
For Web, digital camera storage, and stock photo
dissemination. (By the way, this latter use is
counter-intuitive, since it’s a lossy format and
stock photos are often intended for high-end
prepress work; but in this kind of use the files are so
huge, and the compression applied so minimal,
that there’s no significant data loss.) But for
everyday use — which generally means in Web
development — the lossiness is a major issue. The
tradeoff is between file size and quantization
artifacts.

15. JPEG Compressed, lossy
file format, for raster
(pixmap) data only.
Mostly for photo-
type images on the
Web. Can hold RGB
data; many
compression levels
and other options
available.
Typically JPEGs are 24-bit RGB files. There are
many variants and choices to be made in
exporting JPEGs. Many people save JPEGs from
Photoshop’s built-in “Save for Web” dialog, but
there are lots of other programs — including Web-
specific graphics applications like Fireworks —
that can save JPEGs. Each has its own vagaries. Be
aware that the JPEG “quality” scales (0–10, 0–100,
or whatever) in these programs have no universal
meaning and are not standard across programs;
they’re only meaningful for comparisons within
the same program.
Also be careful not to edit JPEG files if at all
possible; go back to the original master file (PSD,
TIFF, or whatever) for editing and then re-export a
new JPEG with your changes. JPEG compression
always involves data loss and degradation;
editing JPEGs is just corrupting the corruption — it’s
degrading!

16. PNG The newest of
the three
major Web
graphics file
formats, with
more features
than GIF or
JPEG.
Portable Network Graphics. Relatively recent substitute for
GIFs (and some JPEGs) online. Many technical
advantages, such as…
o Lossless compression (which means you could use it as
an editable format, although you probably shouldn’t
in most cases).
o Multi-bit transparency map (alpha channel), even for
photo-type images.
o Metadata for color management (gamma and ICC
color profile), although this is something of a tease
since most browsers don’t support those things.
o Can hold either RGB data (like a JPEG) or indexed-color
data (like a GIF) — but not CMYK, since that’s designed
for the Web, not for print.

17. PNG The newest of
the three
major Web
graphics file
formats, with
more features
than GIF or
JPEG.
Unfortunately, there are still a fair number of older
browsers in use that don’t support PNG’s alpha-channel
transparency; and most browsers don’t support color
management at all, or have it turned off by default. This
makes the use of these features by Web creators
problematical.
By the way, the Web-graphics creation program Fireworks
uses a kind of PNG as a native file format; but it may
contain vector and animation data, which browsers can’t
read from a PNG. Web creators should be careful to
distinguish between that master-file format and the
program’s Web-export “flat PNG” format.
PNG’s use is growing slowly over time, especially as newer
browsers come into play. It’s already a reasonable
replacement for many GIFs (but not for animated GIFs);
but for photo-type images, JPEG will usually be more
efficient.

18. SVG Attempt to
introduce
a standard
vector
format for
the Web.
Scalable Vector Graphics. Wouldn't you
love to put flat-color or other simple
graphics (like logos and diagrams) online in
compact, scalable vector form? The W3C
has approved this Adobe-sponsored XML
derivative. Not really usable yet in most real-
world projects, because browser support is
still spotty — but improving. The only Web
vector format widely used at present is to
embed images in Flash, but that’s not really
a substitute for SVG (you wouldn’t create a
Flash file just to hold a non-animated
scalable logo, for example).
PostScript File Formats intended primarily for high-end prepress work

19. PS PostScript print-
to-disk file; the
most basic
PostScript file
type.
PostScript is a page-description language that
some programs can generate and some printers
(the expensive kind) can print from. A .ps is a simple
text file that results when you tell a program to send
its PostScript instructions to a file on your hard drive
instead of to a printer; it’s therefore called a “print
to disk file.” (It’s also sometimes called a “pure
PostScript file” or a “PostScript dump.”)
There are basically three things you can do with a
.ps file: send it to a printer (which should then print
the original page, not the PostScript text); convert it
to PDF via Acrobat Distiller; or — if you’re a
PostScript programmer — edit it directly in a text
editor. (I’m not a PostScript programmer but, being
rather geeky for a creative type, I’ve been able to
make some simple, useful changes within .ps files on
occasion. As with any programming code, you
have to be careful — one tiny mistake and the
whole thing may not work.)

20. EPS A useful
but flaky
extension
of the
basic
PostScrip
t file.
Encapsulated PostScript. EPS is essentially a PostScript file in an
“envelope.” It usually — but not always — includes a rasterized
preview in TIFF or PICT, plus some metadata. EPS was originally the
native format of Illustrator, back in the primordial days of PostScript.
Like virtually all vector file formats, EPS can hold raster data too; and
it’s the standard way to import photos with clipping paths (which is
to say, raster data surrounded by a vector) into QuarkXPress and
some other programs. EPS files are mostly exported from a
“creation” program like Illustrator or Photoshop, and then placed in
a layout program like Quark or InDesign... that is to say, EPS files
usually aren’t edited directly. But if you need to, modern apps can
sometimes open them... provided (1) the EPS isn’t too old (the
format has evolved) and (2) you’re feeling lucky. Sometimes the
data is editable; sometimes, even if you can crack open the file, it
turns out to be in a form where editing is impractical. Moral: always
save the master files used to create your export files!
Pros: can contain clipping path, true font data, various kinds of
metadata; widely accepted.
Cons: previews optional & nonstandard; sometimes contains
insufficient data but you wouldn’t know because the preview can
be misleading; file format has evolved so is nonstandard; may only
print properly to PostScript printers; can be flaky at times.

21. AI Adobe
Illustrator's
proprietary file
format —
closely related
to, but not the
same as, EPS.
The Adobe Illustrator program’s
proprietary format originally was EPS.
Illustrator’s current .ai format is
essentially an extension of that original
EPS format, broadened to
accomodate Illustrator’s newer
capabilities. A few other programs
may support .ai files, but it’s a moving
target as Illustrator evolves. Most other
programs stick to the original, more
generic form (EPS).

22. PDF Adobe’s
attempt at a
universal file
format.
Portable Document Format, also known as “Adobe
Acrobat format.” Not really a “graphic file format,” since
it’s designed to contain entire pages including graphics,
type, vector shapes, and overall layout; but I include it
here because it can, in fact, be used purely as a
graphic file format (to contain one or more images). For
example, the Macintosh uses this format to store
screenshot images.
The PDF format attempts to capture or “freeze” the
appearance that a document will have when printed to
a PostScript printer, and make the document scalable
(capable of being enlarged cleanly), even without the
components of the document (images, fonts, etc.)
being available as separate files. (Traditionally, page
layout programs require that the components other
than text be available and linked into the main file.) PDF
files are often slightly editable with Acrobat Pro in case
you don’t have access to the original master file and/or
authoring application.

23. PDF Adobe’s
attempt at a
universal file
format.
PDF tends to be more stable and universal than other
forms of PostScript, but be careful: just because a PDF
looks good (and is cleanly zoomable) on your computer
screen doesn’t necessarily mean it can print out cleanly
at high resolution, especially on a printing press. It
depends on how the file was created.
Adobe has been slowly moving towards PDF as a
universal file format (especially within its own product
line), but it’s not clear whether it will ever replace most
proprietary and generic graphic file formats.
You might like to visit Adobe’s PDF Tech Center online.
Still to come: discussion of prepress variants of PDF with
more specific specs:
PDF/X-1a
PDF/X-3

24. PSD Photoshop’
s native file
format.
PSD stands for “Photoshop document.” It’s an
application-specific proprietary format, but
because of Photoshop’s dominant position in the
pixel-editing world, PSD has become something of a
quasi-standard. A number of other programs, even
some that don’t come from Adobe, support PSD as
an additional file format — but usually as read-only
or for import/export purposes, not as their true
native file format.

25. PSD Photos
hop’s
native
file
format.
In addition to pixel data, the PSD format can contain a
broad array of other data concerning such things as
layers, color modes, color-management profiles, and
even real type and other kinds of vector data.
However, not all programs (other than Photoshop itself)
support all these data types within the PSD format; they
may either ignore such data or (in some cases) actually
throw it away. So you have to be careful if using PSD as
an inter-program transfer format, which is not really its
intended purpose.
The current versions of the two major print-oriented
layout programs, QuarkXPress and Adobe InDesign, can
both import PSD files natively. In some cases you could
now actually go to press without exporting your master
PSDs to TIFF or other more traditional prepress formats.
Web browsers cannot read PSDs, however.

26. TIFF Prepress
file
format
for raster
data like
photos,
etc.
Tagged Image File Format. Raster only. Most widely used
format for photos in prepress world (although EPS is also used
for this purpose). Cross-platform.
Pros: most reliable, widespread format for raster data. Can
hold almost any resolution, color scheme, etc. I like this format
because it’s uncomplicated and (unlike EPS) rarely causes
trouble.
Cons: can’t hold vector data; large, and sometimes slow to
print; Mac and PC have slightly different flavors (although
most Mac apps support both); many nonstandard
“improved” versions floating around.
There are many variations of TIFF — in fact, it’s really a whole
family of file formats — and you can’t assume that a program
that “supports TIFF” will understand all of them. Variations
include compressed (lossless LZW, or other methods) vs.
uncompressed; RGB vs. CMYK; 24 bit vs. 48 bit; and a variety
of TIFF-based alternative file formats, such as TIFF IT (widely
used by ad agencies). TIFF is really designed for use by
professional graphic designers. Although the format itself is
relatively trouble-free, the numerous variations are likely to
confuse non-professionals trying to match individual TIFFs to
the programs and processes that support them.