Adobe Photoshop CS5 for Photographers - Martin Evening [38]
Pixel order
The Photoshop TIFF format has traditionally saved the pixel values in an interleaved order. So if you were saving an RGB image, the pixel values would be saved as clusters of RGB values using the following sequence: RGBRGBRGB. All TIFF readers are able to interpret this pixel order. The Per Channel pixel order option saves the pixel values in channel order, where all the red pixel values are saved first, followed by the green, then the blue. So the sequence is more like: RRRGGGBBB. Using the Per Channel order can therefore provide faster read/write speeds and better compression. Most third-party TIFF readers should support Per Channel pixel ordering, but there is a very slim chance that some TIFF readers won't.
Figure 1.93.
This dialog shows the save options that are available when you save an image as a TIFF.
Byte order
The byte order can be made to match the computer system platform the file is being read on, but there is usually no need to worry about this and it shouldn't cause any compatibility problems.
Redundant formats
Note that the Filmstrip file format is no longer supported in Photoshop CS5. The same applies to the PICT format. Photoshop is still able to read raster PICT files (though not QuickDraw PICTs), it just won't allow you to write to the PICT file format.
Save Image Pyramid
The ‘Save Image Pyramid’ option saves a pyramid structure of scaled-down versions of the full resolution image. TIFF pyramid-savvy DTP applications (and there are none I know of yet) will then be able to display a good quality TIFF preview, but without having to load the whole file.
TIFF compression options
An uncompressed TIFF is usually about the same megabyte size as the figure you'll see displayed in the Image Size dialog box, but the TIFF format in Photoshop offers several compression options. LZW is a lossless compression option, where the image data is compacted and the file size is reduced, but without any image detail being lost. Saving and opening takes longer when LZW is utilized so some clients request that you do not use it. ZIP is another lossless compression encoding that like LZW is most effective where you have images that contain large areas of a single color. JPEG image compression offers a lossy method, which can offer even greater levels of file compression, but, again, be warned that this option can cause problems downstream with the printer RIP if it is used when saving output files for print. If there are layers present in the image, separate compression options are available for the layers. RLE stands for Run Length Encoding and provides the same type of lossless compression as LZW. The ZIP compression is another form of lossless compression. Alternatively, you can choose ‘Discard Layers and Save a Copy’, which saves a copy version of the master image as a flattened TIFF.
Flattened TIFFs
If an open image contains alpha channels or layers, the Save dialog shown in Figure 1.92 indicates this and you can keep these items checked when saving as a TIFF. If you have ‘Ask Before Saving Layered TIFF Files’ switched on in the File Saving preferences, a further alert dialog will warn you that ‘including layers will increase the file size’ the first time you save an image as a layered TIFF.
Figure 1.92 If the file format you choose to save in won't support all the components in the image, such as layers, then a warning triangle alerts you to this when you attempt to save the document, reminding you that the layers will not be included. Note that the Mac OS dialog shown here can be collapsed or expanded by clicking on the downward pointing disclosure triangle to toggle the expanded folder view.
Photoshop PDF
The PDF (Portable Document Format) is a cross-platform file format that was initially designed to provide an electronic publishing medium for