Chap60

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Chap60

  1. 1.  Outline the steps to convert images to various file types.
  2. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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.

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