Storm Technology is providing you with a complete application to enable you to decompress still images that have been JPEG compressed. PictureDecomPress is an application that you are free to redistribute with your pictures for non-commercial purposes. This is especially useful when transmitting compressed files over a modem to an individual so that you can be assured that they are able to decompress the image.
The PictureDecomPress application is a subset of our Eddy Award winning PicturePress software. For information on PicturePress or other color imaging software products, please contact us at the above address.
For a free copy of the next version of PictureDecomPress, please provide us with your address on the form at the end of this document. You may fax or email this info to us on the form at the end of this Readme file.
You can use PictureDecomPress in two ways. If you are running System 7, the easiest method is to drag JPEG-compressed image file onto the PictureDecomPress icon. This will automatically launch PictureDecomPress and decompress this file.
A second method is to follow this procedure:
1. Launch the PictureDecomPress application by double-clicking its icon.
2. Select Open from the FILE menu.
3. Navigate to a JPEG-compressed file, then click Open. Two such files appear in the "PictureDecomPress" folder located in the "More Information" folder on the CD.
After choosing either one of these methods for opening a compressed file, you may save the image as a PICT or a TIFF file, which can then be imported into other applications. When you choose “Save As…”, you will be presented with the following dialog box:
Click the appropriate radio button to select PICT or TIFF format. If you choose TIFF, it will be saved with no compression in Macintosh format (as opposed to MS-DOS format). Enter the file name (or keep the default), click on Save.
The folks at Storm Technology hope that this free application gets you going on your way to use color images in your communication. We look forward to creating many applications that will make using color images easy and exciting. Please give us your feedback and insights of how we can make this experience positive for you.
The remainder of this file covers some of the commonly-asked questions about JPEG, QuickTime, and how Storm Technology fits into the picture.
JPEG is an international standard for still image compression, established by the Joint Photographic Experts Group. JPEG specifies a method of compressing and decompressing continuous-tone image files while preserving maximum image quality. JPEG also provides a data stream format so you can send and receive compressed files on various platforms and exchange pictures worldwide. JPEG has been adopted by major corporations in the imaging industry, including Apple® who has incorporated JPEG compression into its QuickTime™ software.
The use of such an international standard has the advantage that users on very different computers can exchange images with no more trouble than is involved in transferring any file between systems. Thus, an MS-DOS user could give a JPEG file to a Mac user, who could decompress the file and make changes, compress it to JPEG again and transfer the file to a user on a Unix system. Also, this means that developers of programs that use images only have to support one standard, rather than many.
The JPEG committee unanimously voted in favor of the standard laid down in the committee draft document CD-10918-1 in June 1991. In November 1991 its status moved to draft international standard (DIS), and by June,1992 twenty-two countries approved the document. Now accepted, the current document has been promoted to international standard (IS-10918-1).
Baseline JPEG, which is what is implemented in Storm’s products currently and in Apple’s QuickTime, is a lossy algorithm, meaning that there is data lost in the compression. Lossless compression keeps data exactly the same after the compression/decompression cycle, but is limited to ratios usually in the range of 2:1 for natural images. Standard data compression methods which are lossless, used in programs such as StuffIt and DiskDoubler, can also be used for natural images to yield ratios around 1.5:1.
A lossy algorithm is the only way possible to achieve high compression ratios, in the range of 5:1 up to 100:1. Lossy algorithms are usually specific to a particular kind of data, such as photographic images in the case of JPEG. One of the principles behind the JPEG algorithm is to remove data whose omission the human eye won’t notice; much research went into determining what visual information the human eye is sensitive and insensitive to, and JPEG uses this. Image quality is a subjective measure and is difficult to quantify.
The complete JPEG process consists of three stages: first an analysis of data to find redundancy, second the selective removal of data less relevant to the human eye to achieve the required compression level, and finally further compaction of the processed data by exploiting its statistical redundancy.
Apple’s use of JPEG in QuickTime is encapsulated within PICT files, thus any program that reads PICT files correctly can decompress them. Applications will have to be modified to save PICT files in this format, however.
Whereas this approach allows many programs to open these files without updating the programs themselves, it has the disadvantage that these files are Macintosh-specific, defeating the interchangeability of files that JPEG otherwise enables. Storm Technology’s PicturePress package allows creation of pure JPEG files which are interchangeable with other platforms.
The JPEG standard specifies what to do in order to produce JPEG data, not how. Many parts of the JPEG algorithm are complex mathematical computations, including the “Discrete Cosine Transform” (DCT) that forms the basis of the algorithm. Different implementations of JPEG differ in the accuracy of these computations, which has a direct effect upon the quality of the image after the compression/decompression cycle. More accuracy yields better quality, but is often sacrificed in order to achieve speed.
Storm Technology’s PhotoFlash™ and Supermac’s ThunderStorm™ , both hardware accelerators for JPEG compression/decompression, use two AT&T Digital Signal Processors to do up to 60 million multiply-accumulates per second at full 36-bit accuracy. This yields not only significant acceleration of the JPEG process, but also a higher level of accuracy that translates into smaller files and better quality at a given level of compression when compared to other implementations.
Another effect that accuracy in JPEG compression has is noticed with repeated compression-decompression cycles, such as opening/saving a compressed image many times to edit it. Storm’s implementation is accurate enough that after the initial compression, there is no further degradation upon repeated compression/decompression steps.
Apple’s QuickTime architecture allows developers to create compressor modules, sometimes referred to as “codecs”, to supplement or replace the standard modules built into QuickTime. Storm Technology is shipping QuickPress™, a JPEG compression module for QuickTime using Storm’s StormCard accelerator. With this module and the PhotoFlash installed, JPEG processes in QuickTime are significantly accelerated and more accurate, thus yielding better image quality.
Accuracy in computation is one method of achieving higher image quality in JPEG; another is algorithmic improvement, including processing the data before and after compression. Storm Technology has already developed several of these improvements and is dedicated to providing the best compression and image quality in the market using these and other techniques.
There are a great many parameters to JPEG, all of which affect how large the JPEG data will be and what quality the decompressed image will have. In general there is a balance between image quality and file size: the smaller the JPEG file, the less quality the decompressed image will have. Storm’s PicturePress gives the user four preset quality settings, and allows them to make a custom setting for more specific applications as well. For example, a scanner manufacturer could develop a custom setting for a particular scanner and exploit the device’s characteristics to yield better quality.
JFIF (JPEG File Interchange Format) is an industry-proposed subset of baseline JPEG, which makes use of one of the “application-specific markers” in JPEG to convey additional information about an image, such as its resolution, that is not included in the JPEG specification. Storm’s products create JPEG files that include this JFIF information.
Technically, JPEG specifies a data stream format, not a file format. A data stream is more general than a file, as it might reside within a file, or be transmitted via modem or over a network.
JPEG++ is an extension to JPEG proprietary to Storm Technology. It only affects compression, and the resulting file is a JPEG file readable by any other JPEG decoder. The JPEG standard specifies one level of compression for the entire image (see Compression Settings above); JPEG++ gives the user the ability to specify two levels—to select a “foreground” and a “background”, and the background will be compressed at a user-specified lower quality level than the foreground, thus yielding a higher compression ratio. Often an image has a clearly-defined foreground in focus and a background out of focus; the increased compression on this background that JPEG++ allows is usually not noticeable visually.
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