ProJPEG¬ Text Docs. Copyright 1996 BoxTop Software, Inc. All rights Reserved.
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Overview
ProJPEG¬ is the most powerful tool available for creating optimized JPEG files for the web. It has many advanced features that make producing JPEG files easy and efficient with no surprises in store.
ProJPEG¬ is an Adobe Photoshop 3.0 File-Format plug-in that integrates seamlessly into Adobe Photoshop. It combines open and save simplicity with the most powerful features available for creating JPEG files.
What is JPEG?
JPEG (pronounced ╘jaypeg╒) is the common usage name of one of two graphic file formats commonly supported by web browsers. Note, that JPEG is a compression method and not the actual name of the file format. JFIF is the correct name of the file format that web browsers support.
JPEG and the JFIF format were developed by the Joint Photographic Experts Group to provide for efficient compression and storage for full-color and gray-scale, continuous tone images of natural, real world scenes, i.e. photographs. JPEG, referring to the common name of the format, supports 24-bit color (16.7 million colors) and uses JPEG compression, which is a ╘lossy╒ method, meaning that image data is lost in the compression process, and the uncompressed image will not match the original image perfectly, as it would if it were compressed with a ╘lossless╒ method. JPEG is highly optimized for compressing continuous tone, photographic images and does not perform as well, both in terms of resultant visual image quality and in terms of achievable compression, for other types of images. Generally, you should not use JPEG for images that are not photographic in nature.
Installing ProJPEG¬
To install ProJPEG¬ simply place the ╘ ProJPEG¬ 2.1╒ plug-in inside your Adobe Photoshop ╘Plug-ins╒ folder and optionally inside the ╘File Formats╒ folder within the ╘Plug-ins╒ folder. ProJPEG¬ will then be available to use as a method to open and save files the next time Photoshop is launched. ╘ ProJPEG 2.0╒ will then appear as a new item in the file format menu in Photoshop╒s open file and save file dialogs.
ProJPEG¬ is a FAT plug-in and will run in native mode on both 68k and PPC based Macintosh computers for best performance.
Key differences between ProJPEG¬ and Photoshop╒s JPEG
There are key differences between ProJPEG¬ and Adobe JPEG. Adobe JPEG implements non JFIF compliant extensions to the format that ProJPEG¬ does not support, specifically CMYK JPEG files as these are not suitable for web use and not supported by web browsers. ProJPEG¬ also supports JPEG variations that Adobe JPEG does not, specifically progressive JPEG files.
This can cause some confusion when attempting to open existing CMYK JPEG files that you created with Adobe JPEG using ProJPEG¬, and also when attempting to open progressive JPEG files that you created with ProJPEG¬ using Adobe JPEG.
ProJPEG¬ will report correctly that a CMYK JPEG file is an unsupported file type, however Adobe JPEG will incorrectly report a progressive JPEG file as being corrupt and damaged.
If you encounter a JPEG file that creates and error when attempting to open it using either ProJPEG¬ or Adobe JPEG first try opening the same file using the other method before assuming that the file may actually be corrupt.
The ProJPEG¬ Interface
ProJPEG¬ has one main options dialog for controlling compression parameters when saving images. The options dialog contains an interactive quality preview, a check box to enable or disable the preview feature, information on image and file size, quality slider control, progressive check box, Huffman optimization checkbox, better subsampling checkbox , a pop-up menu for loading saved presets, and a button for saving presets.
Controls and dialog items
ÑThe compression preview
ProJPEG¬ has a live compression preview that shows you exactly what your image will look like as a JPEG file as you adjust compression settings. The preview is fixed at 100% size and can not be scaled because any scaling would defeat the purpose of the quality preview by altering the way the image appeared. The preview area defaults to the top, left corner of the image being saved but can be scrolled to any portion of the image desired. The preview can also be disabled and enabled as desired.
ÑThe enable/disable preview check box.
This check box enables or disables the preview in the ProJPEG¬ options dialog. Working with the preview on provides interactive feedback as you adjust compression options, but it can be slower than using ProJPEG¬ without the preview enabled.
ÑThe image size readout
To the right of the preview, the top text in the ProJPEG¬ options dialog is the image size. This is the size of the image being saved in pixels wide by pixels tall and does not change as you change compression settings.
ÑThe approximate data size readout
When the quality preview is enabled the approximate data size is displayed below the image size. This is an approximation of the final compressed size of your JPEG file╒s data fork and the compression ratio achieved that is extrapolated from the exact compressed size of the current preview area of the image. If the area of the image being saved that is visible in the preview is representative of the entire image this estimate will be extremely accurate. However, the approximate data size and compression ratio is subject to some inaccuracy because it is based only on the portion of the image within the preview area.
ÑThe quality slider
Below the approximate data size is the most important control in the ProJPEG¬ options dialog, the quality slider. The quality setting is what is most determinant of image quality and compression. Its range is from zero to one hundred, and zero really does mean zero.
It is not advisable to save images at quality settings bellow 10 or above 90 since they are both extremes above or below which no usable advantage in compression or quality is gained.
The quality setting can also be entered numerically in the text edit field to the right of the quality slider. The valid range for the quality setting is zero to one hundred with greater numbers representing greater image quality.
ÑThe progressive check box
This controls whether your image will be a progressive JPEG or a standard baseline compliant JPEG. Progressive JPEGs allow for incremental display similar to interlaced GIFs but more aesthetically pleasing to view. Progressive encoding is also slightly more efficient than non-progressive encoding and progressive JPEG files are generally slightly smaller than the equivalent non-progressive JPEG files.
Progressive JPEGs provide the fastest loading images possible for the web. However, there are still some remaining compatibility issues when using progressive JPEG files.
At the time of writing 12 web browsers support progressive JPEG in their current versions. The JPEG FAQ maintained by Tom Lane of the Independent JPEG Group maintains an up-to-date list of all known applications currently supporting progressive JPEG.
Ñ The Huffman optimization checkbox
Huffman compression is the final step in the JPEG compression process. Normally a pre-calculated, general purpose Huffman table is used in this process as it results in faster compression. However, more efficient compression can be achieved by using a Huffman table that is optimized for the specific image being compressed. The ╘Optimize Huffman Codes╒ option creates an optimized Huffman table for the image being compressed and results in slightly better compression efficiency with no additional quality loss in the image.
Ñ The better subsampling checkbox
Checking this box causes ProJEPG¬ to use a more accurate method of subsampling and improves image quality but generally results in slightly larger JPEG files than if this option were off.
ÑThe presets menu
Presets allow you to save and rapidly set all control options when using ProJPEG¬ to save a JPEG file. The presets menu affects all settings, including controls that are not visible from the basic options dialog. To load a preset simply select the preset in the menu and all controls will be changed automatically to the stored values in the preset. The option to create a new preset is only available from the advanced options.
ÑThe save preset button
To create a new preset first make all the control settings you wish to store in the new preset then click on the ╘Save Presets╒ button. You will be prompted to name the new preset then it will appear in the presets menu until it is deleted. The ╘Save Presets╒ button is also used to delete presets. If you press the option key the ╘Save Presets╒ button title will change to ╘Delete Presets╒ and the currently loaded preset will be deleted from the presets menu when you click on ╘Delete Presets╒.
Opening Files with ProJPEG¬
Opening files with ProJPEG¬ takes no special action from the finder. Simply double-click on a JPEG file or drag a JPEG file onto Photoshop, and ProJPEG¬ will be used as the default method for opening all JPEG files and there is no user control over this behavior.
However, ProJPEG¬ does not support all possible variations of JPEG files. For instance it does not support Adobe CMYK JPEG files, since they are not applicable to web use. There will be times when you may need to open a JPEG file with a method other than ProJPEG¬.
When in Adobe Photoshop you can specify what method to use opening a given file by opening the file from the ╘Open...╒ menu item in the ╘File╒ menu. To do this first select the ╘Open..╒ item then in the dialog that appears check ╘Show all types╒, which is towards the bottom right of the dialog. A pop-up menu titled ╘Format:╒ will appear below the file list in the dialog after ╘show all types╒ has been checked.
Next select the file you wish to open from the file list by single clicking on it. Note that double clicking on the file will begin the opening process before you have selected the method to open the file with.
Once the correct file is highlighted, select the method you would like the file to be opened with from the ╘Format:╒ pop-up menu. Photoshop╒s internal JPEG support appears in this menu simply as ╘JPEG╒. Once you have selected the method to use in the ╘Format:╒ pop-up menu, open the file by hitting the ╘Open╒ button.
Saving Files with ProJPEG¬
There are two means of saving files using ProJPEG¬. The first is using the ╘Save as...╒ command in Photoshop and the second is ╘Save╒ or command - S. You should be very familiar with save and save as from within Photoshop and using ProJPEG¬ works in much the same way as saving in other formats from Photoshop.
If you perform a ╘Save as...╒ you first see Photoshop╒s put file dialog where you select the format to use and supply the name for the new file. Once you hit the ╘Save╒ button in that dialog the options dialog for ProJPEG¬ will appear.
In the options dialog for ProJPEG¬ you will need to make the desired settings for compression. Minimally you should adjust the quality slider to the desired level and check or uncheck the progressive option as desired before hitting the ╘OK╒ button to complete the saving process. You may also cancel at any time from the options dialog without saving the file.
Once you hit the ╘OK╒ button from the options dialog ProJPEG¬ begins to save the file to disk. You may also cancel saving from the progress dialog at any time before ProJPEG¬ completes saving the file to disk.
Command - S saving requires no special user action and no options need be set when saving an image in the normal course of working. With ProJPEG¬ it is not recommended that you ╘save╒ an existing JPEG file immediately after opening it without first performing a ╘save as╒ to ensure the compression setting are what you expect them to be.
Unlike, Adobe╒s JPEG, ProJPEG¬ does not store compression settings in the file. This means that if you ╘Save╒ immediately after opening an existing JPEG file before you ╘Save as...╒ and manually reset the compression options the file will be saved with the default settings of ProJPEG¬ and not necessarily the settings previously used to save the file.
It is not good practice to use JPEG as a working format because of it╒s lossy nature, but it is safe to edit JPEG files and resave them when necessary if done with caution. When saving multiple times in the same editing session cumulative quality loss does not occur, since the original RGB data is recompressed each time you save from the open file.
Optimizing JPEG images for the web with ProJPEG¬
You should limit your use of JPEG for web graphics to continuous tone, full color or greyscale images of photographic nature. These are the types of images where JPEG excels. Also, you should never save images that have been color reduced to 256 or less colors as JPEG files. Doing so will be very detrimental to image quality and also compression efficiency.
Besides those general rules of thumb, there are no practical manual optimizations that can be performed on your image prior to saving as JPEG to improve compression or quality. The final outcome will be largely dependent on the settings used to create the JPEG file.
One thing to remember when you create graphics for the web is that fast download times are often more critical than image quality. The goal is to find a mid point between both extremes that allows for the smallest possible file while maintaining acceptable image quality, with what is acceptable image quality varying by the specific use of the image. For example, a photographers portfolio would require much higher quality images than thumbnail images for an on-line catalog to achieve the desired effectiveness.
The live image compression preview in ProJPEG¬ allows you to have immediate feedback on how changing setting will affect quality and compression and will allow you to safely push compression to the limit to quickly achieve the smallest, best looking files possible.
ProJPEG¬ 2.1 is Copyright 1996 BoxTop Software, Inc. All rights reserved. ProJPEG¬ is a Trade Mark of BoxTop Software, Inc.
