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Compressing Digital Images

Key Benefits for Optimizing File Size for your Digital Asset Management System

One of the most effective ways to increase the efficiency of your advertising workflows is to optimize the file size consumed by your digital images.  The size of these files determines how many CD’s you will have to burn for a project, which images you can send by email and which images you can maintain “online” on your file or web server.  The size of these files also impacts the time it takes to print your design files, transfer from disk or over the network and the time to actually process the images in your digital imaging program.  This article is an overview for several techniques to reduce the file storage size of your digital images as a key to improving the efficiency of your advertising workflows.

Resolution: Any method that reduces the pixels in a digital image effectively reduces the resolution.  Usually this is associated with creating a “low res” image which is 72 pixels per inch compared with a “high res” image which is typically 300 pixels per inch.  The resolution however can also be reduced by changing the height and width of an image.  By reducing the height and width of an image you are effectively changing the resolution if you reduce the total number of pixels.  Even though your digital imaging process may capture a very high resolution file, if that amount of resolution will never be utilized, it needlessly increases file size and that associated cost.

Our suggestion is to reduce high resolution files in Photoshop to be the height, width and resolution that is required for the intended end use.  Note that digital asset file size increases exponentially with increased dimensions.  A 10x10 image is 4 times larger than a 5x5 image.  The opposite of that is also true, a 5x5 image is ¼ the size of a 10x10 image.  Maintaining images at the size of their intended use can dramatically reduce your digital asset file size cost.

Color Mode: There are 3 common color mode variations that our customers typically use.  These color modes are 8 bit/channel RGB images (standard RGB image in Photoshop), 16 bit/channel RGB Images (captured by many newer digital cameras) and 8 bit/channel CMYK images (standard CMYK image in Photoshop).
 
8 Bit vs 16 Bit RGB Images: The standard RGB image in Photoshop is an 8 bit/channel RGB image.  Essentially, this means that a standard RGB image has a range of 256 color values for each channel (red, green & blue).  When combined together this creates 16.7 million color possibilities.  Up until recent advances in digital cameras and Photoshop, virtually all RGB images were 8 bit/channel.  Newer cameras and Photoshop also support 16 bit/channel RGB images.  So what does this mean?  Well in theory a 16 bit image will have greater color fidelity and capture greater nuances of the color spectrum.  That sounds like a great idea but there are some practical considerations and limitations.  First of all, that additional data doubles the file size of your image.  The second implication is how much of that theoretical color nuance is usable for your application.  The vast majority of our clients are using their high resolution color images for printed advertising.  Printed advertising of digital photos almost always uses process printing of CMYK inks.  It is important for users to understand that CMYK printing has a smaller color gamut range than RGB which is why colors can appear more saturated and brighter on your monitor than on a printed proof.  If the final use of your digital assets will be for printed marketing materials then saving images as 16 bit RGB files will be of no benefit because the color gamut range of the printed piece will be less than that of a standard RGB file not to mention the color gamut range of a 16 bit file.  Our suggestion is to save RGB images as 8 bit/pixel if your intended end use is for printed marketing materials.

RGB or CMYK: Should your high res images for printed advertising be RGB or CMYK?  Ultimately this depends on your unique set of circumstances but here are some considerations.  An RGB image is comprised of 3 channels (red, green & blue) while a CMYK image is comprised of 4 channels (cyan, magenta, yellow and black).  Since there are 4 channels instead of 3, the CMYK file is a third larger.  While there is a size benefit for using RGB files, the main consideration should be how the digital asset file will be used.  The CMYK image is much different than just splitting the data into 4 channels.  The CMYK image includes a variety of settings that control how the black channel is generated.  Without getting into a technical explanation of color reproduction theory, lets suffice it to say that the CMYK image incorporates attributes that will optimize the image printability for specific printing conditions.  When a digital image is separated into CMYK channels this process includes settings for specific printing conditions.  If you are the only one that uses these images for print and you consistently use the same printing process (paper, line screen, press) then you control those printing conditions.  If you are providing those images to a variety of distributors, agencies and others that will use these images in a variety of printing conditions then you don’t control the printing conditions.  It is generally preferable in this circumstance that you provide RGB images and let the end user perform their own CMYK conversion in accordance with their printing conditions.  If your high resolution images are JPEG files then you will have an added benefit.  RGB JPG files are universally viewable by anybody with a web browser.

Compression: File compression offers the greatest potential to reduce the size of your digital asset files. There are 2 main groups of compression techniques.  These are “lossless” meaning that every byte of data is retained and “lossy” where some amount of data is discarded.  Each of these compression techniques has their pros and cons to consider.

Lossless: The most common techniques that utilize “lossless” compression are to use a Zip program or Stuffit program to compress files.  With this technique your image data is unaltered and the file size is reduced considerable.  Standard TIF and EPS files reduce significantly with this type of compression although the results are not as great as the “lossy” techniques.  The downside to this technique is that individual photos that are zipped or stuffed need to be uncompressed prior to opening them in Photoshop or using them in a desktop publishing program such as QuarkXpress or InDesign.

Lossy: The most popular “lossy” compression technique is JPEG.  There are a wide range of JPEG compression settings which produce a wide range of results.  If your files are being used for printed marketing materials, it is unacceptable for digital images to be compressed with a visually discernable reduction in quality.  Fortunately there are Lossy techniques that result in no visually discernable reduction in quality.  Since JPEG compression provides the most efficient size for image files, it is important to define some key points.

  • Saving a digital asset file as a JPEG file does not reduce the resolution of an image.  We find that many people are confused on this matter since they generally associate smaller file sizes with lower resolution.  JPEG compression does reduce the file space but not the resolution.
  • Copying a file from one computer to another does not alter the quality of the image.  We find that many people are confused about this and believe that simply emailing or transferring the image will affect the image quality.
  • JPEG compression works by combining patches of similar color and saving that as a single value instead of saving the data for each individual pixel.  For this reason, a digital image with fine detail such as a Persian rug would not compress as small as a similar size image of a clear blue sky.  As images are compressed with lower quality settings, you can see the blocks that were combined together (known as artifacts).  As images are compressed with higher quality settings the range of pixels that are combined together is reduced and the “artifacts” can reduced to the point where they are not visually discernable.
  • Lower quality JPEG settings DO result in visually discernable “artifacts” which reduce the quality of your digital image.  However the highest quality JPEG settings do not visually reduce the quality of your image except in the most extreme cases.  These extreme cases generally include “high key” (bright) images that have larger patches of gradation (areas that fade from one color to another).
  • Higher resolution images achieve better compression results.  Since there are more pixels in the image, there are more pixels that can be combined together for compression.  High resolution 300 dpi images achieve a greater compression ratio than lower resolution image version of that image.  Since high res images achieve a high compression ratio, we can afford to apply a lower compression setting which will preserve the greatest image quality.
  • Digital images compressed with the highest quality settings reproduce reliable quality with very few instances of “artifacts”.  Our experience with tens of thousands of images is that they can be compressed/decompressed many times for alterations without any visually discernable loss of quality.

File Formats: For the sake of this discussion, we will focus on the file formats that are best suited for high resolution images intended for printed advertising.  The three most common digital asset file formats for these types of images are JPEG, TIF and EPS files.  Each of these have their merits and each of these have a wide range of format variations.  The file format question is even more complex since both TIF and EPS files can be saved with compression settings applied to them.

The decision of which file format to use depends on the file itself that will be used.  The TIF and EPS digital file formats are unique in that they both support a Photoshop clipping path.  A Photoshop clipping path is a “vector mask” within an image that determines which portions of the image are visible and which areas are transparent when the image is placed into a desktop publishing document such as Quark Xpress or InDesign.  Other terms often used for the clipping path are “silhouettes”, “silos”, “outlines”, “blockouts”.  A Photoshop clipping path is used to show just a portion of the image.  For instance a product may be photographed on a grey background.  A clipping path will allow just the product to be viewable and the background will not appear.  If your files include these clipping paths they need to be saved as either a TIF or EPS file as these are the 2 formats that generally support the clipping path when it is used in a desktop publishing application.  If your digital asset files include clipping paths then they need to be saved as TIF or EPS files for these to be useable.

Our view at Honeycomb Archive is that high resolution files should usually be saved in a compressed format.  This is not to say that you shouldn’t retain a copy of the original uncompressed or RAW image offline, however, advertising workflows will be much more efficient if some type of image compression is utilized.  With this in mind, the JPEG, TIF and EPS files all support various forms of image compression.  When any of these formats are saved in Photoshop, a secondary set of options appears for the user to define the compression settings.  All 3 of these file formats support JPEG compression settings.  This gets a little confusing because both your TIF and EPS files can be saved with JPEG compression applied to them.  Regardless of the file format used, if you decide to save your high resolution marketing images with JPEG compression it should always be high or maximum quality.  High quality JPEG compression will produce slight discernable artifacts although these can usually only be seen upon very close examination.  Maximum quality settings produce a great result with virtually no discernable artifacts.

The ability to save TIF files with JPEG compression is a fairly new feature for Photoshop.  The downside to using this feature is that most older TIF viewers (including older versions of Photoshop) will not be able to read this file.  If you are set on using TIF files as your high resolution file format you may consider using LZW compression.  LZW is a Lossless compression technique that may reduce your file to ½ to ¼ of their uncompressed size.  This is an excellent compression strategy although does not reduce the size as compact as JPEG compression.

EPS files with JPEG compression are our preferred file format for high resolution images.  The EPS format supports the Photoshop clipping path and the EPS file can be saved with maximum quality JPEG compression.  This format has been in place since the earliest versions of Photoshop so anybody with Photoshop can open these files.  Typically, EPS files compressed in this way are 10%-20% of their uncompressed size.  An average high resolution file (8x8 at 300 dpi) may take up about 1-2 megabytes.

Regular JPEG files are also an option for digital images that do not include Photoshop clipping paths.  These JPEG files also need to be saved with the high or maximum quality settings.  If these JPEG files are RGB they can be viewed by anybody with a web browser as all web browsers support viewing of RGB JPEG files (they do not support viewing of CMYK JPEG files).

Why you are likely using compression already!

Although there are certainly pros and cons to image compression, particularly JPEG image compression, this technique has become very common and if performed properly can achieve excellent image quality.  With that being said, there are some who would rather not consider any type of lossy compression to their image files.  If you fall into that camp, please consider this before you give up on all of the efficiency that image compression has to offer. 

Every major commercial printer today uses direct-to-plate technology for making their printing plates.  Every high end catalog, magazine, newspaper circular and direct mail piece has the printing plates produced thru this direct to plate technology.  At the core of this workflow are Adobe Acrobat PDF files.  The central workflow for virtually all of these marketing materials is to produce a PDF file from the QuarkXpress or InDesign file used in the layout process.  One of several factors that optimizes the size of these PDF files is JPEG compression.  So JPEG compression is at the heart of this process is to optimize these PDF files.

Since the majority of printed marketing materials are already produced using these compressed PDF workflows and perhaps even most of your printed marketing is produced using these workflows, it may be worth considering how these benefits can be extended into other areas of your advertising production.



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