TWO RAW OPTIONS
HDRi & RGBI
-- Archival Scan Formats?
By MIKE PASINI
The Imaging Resource Digital Photography Newsletter
Review Date: February 2010
Wine may age well but film deteriorates. Film can tear, bend or break. It can be scratched. Mold, mildew and fungi can grow on it, particularly in hot and humid climates. Its color dyes are fugitive, fading slowly with age.
So leave the bottle corked and crack open the scanner. There's no time like the present to preserve your memories by scanning your old negatives and slides.
The trouble is that there are so many decisions to make. What resolution? Which scanner? What software? Should you use defect removal to clean up the scratches?
It can give you a worse headache than the whole bottle of wine.
Both VueScan and SilverFast can save a scan in a format that postpones processing -- and all those decisions except which resolution to scan. The formats are conceptually similar but not interchangeable.
VueScan. Since version 7, the Professional edition of Ed Hamrick's VueScan has offered what it refers to as a 64-bit RGBI image format in 64 bits. Color data is scanned in 16-bit channels for red, green and blue data (48 bits) plus a 16-bit channel for the infrared scan that documents the dust and scratches on the film.
VueScan can save the RGBI format as a TIFF but without a preview.
SilverFast. LaserSoft Imaging President and CEO Karl-Heinz Zahorsky recently wrote to us describing his company's nearly identical concept called HDRi (High Density Range infrared). It's available in SilverFast v6.6.1 and later.
Under the hood, HDRi is also a four-channel image format conforming to the TIFF specification. For a color scan, the usual red, green and blue data are available as 16-bit channels (for a subtotal of 48 bits) but LaserSoft has added an additional 16-bit grayscale channel to file (64 bits total). If you're scanning in black and white, that's a 32-bit HDR file (a 16-bit channel for grayscale tone information plus the 16-bit channel for infrared defect data).
HDRi files also record a gamma value to help image processing programs render the expected contrast and files larger than 30 megabytes include an embedded preview.
Advantages. By separating the defect information from the color data (which, it must be noted, is not itself pristine), HDRi and RGBI allow you to postpone the defect removal and other image processing operations just as your camera's Raw format allows you to postpone all the post-processing that creates a JPEG.
The main advantage of that is simply that you can return to the Raw scan data stored in the archival file (instead of rescanning) years after the scan when smarter defect removal software can do a better job on your original data. So even if, in the intervening years, your film has deteriorated, you've got the scan data from better days.
The scanning operation itself is a bit faster, too, in that you don't have to wait for the application to process the infrared data before continuing with the next image. You can also batch process the Raw files into whatever file format you need with whatever settings you need.
And finally, scanning to Raw files minimizes film handling. You don't have to access the film, clean it and rescan just to process it a bit differently.
To make this approach feasible, you have to have observe a few caveats.
Resolution. The first is that you have to have scanned your originals at high resolution. Some practitioners insists 2700 dpi is sufficient. Others draw the line at 4000 dpi. You can go as high as 7200 dpi on your desktop today but you quickly run into constraints dealing with the large files higher resolutions create.
The key to this calculation is knowing what your most demanding output device requires. How many dpi does the device require to print at the largest size you want? In some cases, the "device" may be a commercial printer or a publication. Worst case is running your 25mm image across two pages as a double-truck bleed while requiring 300 dpi from your image. Though that high a resolution isn't strictly necessary, it can still be the entry bar for your image.
So you have to scan to Raw format with sufficient resolution that any particular subsequent file conversion will have enough data. You can down-sample but you can't up-sample.
The guilt-free way to calculate this is simply to set resolution to the physical limit of your scanner. But with some scanners offering as much as 7200 dpi, you may quickly wonder if you aren't overdoing it.
File Processing. Once you have scanned to Raw, however, your processing options are very limited because image editing software (like Photoshop) doesn't know what to do with the infrared channel of either the RGBI or HDRi file.
If you open the Raw TIFF in Photoshop, for example, the Channels display will only indicate the composite and red, green and blue channels. The infrared channel is discarded.
So if you do any image editing on the file and save it, you will lose the defect removal information.
Your only alternative for reprocessing the Raw data is to use the tools the scanning software application provides, as we detail below in the Worflow Comparison section. For VueScan, that means using VueScan itself. For SilverFast, that means using HDR, another application entirely.
Metadata Editing. Unlike the Raw files from a camera, Raw scan files do not enjoy the benefits of metadata editing. Adobe Camera Raw, Lightroom, Aperture, Bibble, Capture One, none of them can read the file and store edits in the file header or a side car.
When you reprocess the Raw scan, you can't do any metadata editing. The applications that reprocess do pixel editing, subsequently saving the edited image as a new file in the format you've specified.
In VueScan's case, you can specify the DNG format, which would incorporate the defect removal processing while deferring the image processing (if you choose). In this case, you would have the equivalent of a camera Raw file. But then, you could have started from there, skipping the RGBI format all together.
So why not incorporate the infrared channel in a DNG file?
"DNG has no provision to carry three RGB channels plus the extra grayscale 16 bit channel," Karl-Heinz explained. "This is easy in the standard TIFF format. Also we can keep the file format compatible with e.g. Photoshop, so no conflicts or crashes, as Photoshop would just disregard the extra channel with the dust and scratch removal information."
Driver Issue. VueScan only operates as a standalone application, with all options available. But SilverFast can be run from within Photoshop, for example, using the File|Import command.
However, if you prefer to work that way, you can't scan a Raw format because Photoshop (or any other image editing running the plug-in) can't handle the infrared channel, so the option is disabled.
The workaround is to run SilverFast in standalone mode, which does display all the options.
Scanner Capability. Finally, your scanner has to support infrared scanning. Not all do (most notably the Microtek M1). If your scanner doesn't do infrared scanning, it won't be able to create the grayscale defect removal channel.
LaserSoft keeps a list of compatible scanners.
And for film scanning, you really should use a scanner that also supports the multiexposure mode in SilverFast or VueScan, which greatly extends the dynamic range supported scanners can capture by scanning once for the highlights and again for the shadow detail.
Being able to set the scanner's exposure for those two scans is probably the most significant scanner feature. It eliminates the need for multisampling and extends the actual dynamic range of the scanner, which tends to be less than the 4.0 or so film exhibits.
Working with Raw scans is a two-step dance. The first step is capturing the Raw files from the original film frames. The second is processing or re-processing that Raw data into more common file formats, typically by batch processing them.
Input. Scanning the originals as Raw scans is primarily a matter of setting the file type in the application of your choice.
Profiling. Creating an ICC profile for Ektachrome and Kodachrome on your slide scanner is a prerequisite for capturing the highest quality data, a priority with an archive scan.
VueScan can indeed build an ICC profile but you'll have to acquire the IT8 targets yourself. One respected source is Wolfe.
SilverFast's Ai IT8 Studio option (an upgrade from the bundled SE version and part of the Archive Suite) includes both Ektachrome and Kodachrome IT8 targets. But the SilverFast targets include a barcode that points SilverFast to the companion data file for the target's color patches. This makes it very simple to calibrate your scanner, essentially just loading the appropriate target and pressing the Calibration button in the Preview window, as we outlined in our brief review of the feature.
VueScan. In VueScan, the key Input panel setting is Bits per Pixel, which you should set for "64 bit RGBI." In the Output panel enable TIFF and set the TIFF file type to 64 bit RGBI.
Hamrick recommends setting the Crop|Preview area (see the advanced options in the Input panel) to Default with Crop|Crop Size set to Maximum.
SilverFast. In Silverfast, they key scan setting is the Scan Type in the Control window, which you should set to either 32 bit HDRi Grayscale or 64 bit HDRi Color.
Then use SilverFast's Auto Frame Finding feature to find the parts of the Prescan that are images and not background.
You should also enable the Multiexposure option in the Preview window to maximize the density range you can capture.
Next use the SilverFast JobManager to batch scan the multiple selections into 64-bit HDRi format. If your scanner only scans one image at time, no need to batch scan. The JobManager essentially takes your order for batch scanning, including any edits you may want to perform on each image without taking the time to preview each image.
Processing. Both VueScan and SilverFasts provide batch processing of their own Raw scans.
VueScan. To reprocess Raw scans made in VueScan, you return to VueScan, setting the Input|Source option to File and Input|Files to point to the first image in the set. You can then set your processing options as you normally would before setting Input|Batch to all the files you want to process. Hamrick also recommends locking the exposure and film base color for the set.
You can set the output file to any of several types, including JPEG, TIFF and DNG.
SilverFast. To process SilverFast's Raw HDRi files, you open them in HDR Studio, the second component to the Archive Suite.
HDR Studio provides a Light Table overview of the images, allows you to optimize them with SilverFast tools and can batch process and output the images with JobManager. You do have to save the images in an output format, rather than record the edits in a sidecar file or in the Exif metadata of the main file.
LaserSoft has produced a short movie explaining the Archive Suite workflow.
It is amusing at least that, as we celebrate the twentieth anniversary of Photoshop, there isn't an image editing application that knows what to do with an infrared channel in a TIFF file.
And it's also amusing that the two leading, after-market film scanning applications have devised their own similar if not identical Raw scan file formats.
And while both approaches work well, we find it hard to commit to either in our own workflow. We are either looking at high-quality images for which we need a particular output and can therefore scan for that single purpose or we are looking at mountains of family photos that really only require screen versions to be catalogued and enjoyed as slide shows, say on the HDTV. For the later a good lab with a production scanner is sufficient.
But ask us if we'd like a DNG archive of our film images with defect removal information and we'd mull that one over a bit. It's the proprietary Raw format requiring specialized tools that disinclines us to invest the time in capturing it.
While we ourselves can't commit to the format, we can endorse both approaches. They deliver what they promise, each in their own way.
We're particularly charmed with SilverFast's calibration capability and its included targets. We're likewise charmed with VueScan's single-application solution.
But we have a feeling there's more to this story. Stay tuned!
Questions, comments or controversy on this article? Click this link to see what other Imaging Resource readers have had to say about Archival Scan Formats, or add comments of your own!
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