Perhaps I haven't understood the concept of the "Scaling Workflow" and hence my confusion ...
I checked out most of the Q&A for designing large format images on Photoshop (like for huge billboards), and the most popular suggestions were to "scale" the image down and design at high resolution.
For instance, if a billboard is to be 20' wide, the vendor may ask you to send a file 20 inches wide. If the billboard will be printed at 40ppi, then your 20" wide file would need to be set to 480ppi to allow for the scaling (12X40).
30 dpi for final output is more than enough for a billboard of that size. It's not unusual for final output to be 12-15 dpi in this context. The usual professional billboard workflow in Photoshop is to build the image at a small scale with high ppi (e.g., 4x6cm @ 300 ppi),
How does this help?
For example, let's assume a billboard's print spec is 35 meters (W) x 15 meters (H) at a resolution of 50 DPI / PPI in CMYK. And the billboard designing will be done with a raster image using Photoshop.
So how does it help if you design it at 350 cm x 150 cm @ 500 ppi (scaled to 1/10, for example) or at its actual size of 3500 cm x 1500 cm @ 50 ppi?
A 350 cm x 150 cm @ 500 PPI will have ~ 68,898 x 29,530 pixels (350 cm = 137.795 inch x 500 PPI = 68897.5 and 150 cm = 59.06 inch x 500 PPI = 29,530).
A 3500 cm x 1500 cm @ 50 PPI will also have 68,898 x 29,528 pixels (3500 cm ~ 1377.953 inch x 50 PPI = 68897.65 and 1500 cm ~ 590.56 inch x 50 PPI = 29,528).
In both cases, the file size in Photoshop is going to be approximately the same (around 4+ GB!), since the amount of pixels in both the Photoshop file will be the same! When the document size is the same for both cases, how can Photoshop use less resource for the "scaled" down size? How does it become easier to design using a "scaled" down version?
To be clear, I understand the theory of why a lower resolution works with large images for bill boards.
My question pertains to the "scaling" designing workflow that was suggested for editing large raster images (mainly in photoshop). I fail to see what benefit this approach offers, and hence my query ...
Very large format artwork, such as for billboards, is always scaled for several reasons.
One is that the major design tools -- Photoshop, Illustrator and InDesign, have maximum document dimensions that can be too small to work the largest sizes at full scale. The common output format, PDF, also has dimension restrictions that won't allow full scale billboard artwork. It's not just a matter of "how many pixels." These applications have limits on how many cm or inches you can specify.
Another simple reason is that the billboard companies you'll be submitting the artwork to specify them that way. They even provide templates for each of the sizes they sell, usually Photoshop documents, sometimes other formats also, but they are always scaled. At the very least, you will always find a detailed spec sheet for every size in the inventory.
Within the application UI, you will often work with rulers and guides. Inches and especially cm tend to be impractically small if you're trying to work with a document that is several meters wide, so scaling makes sense there, too. A 35 cm wide document has a scale that's readable and fairly easy to work with. A 3500 cm ruler, if you could create a document that large, would be much less so.
A billboard spec such as your imagined example is not what you'll meet in the real world. Here is a typical one, for a 14 ft x 48 ft "bulletin" (billboard companies in the U.S. almost always specify height before width):
SCALE / RESOLUTION / LIVE & TRIM AREAS
Built at 9ppi @ 300 document resolution
Live area document size: 5.04â€ x 17.28â€
Overall document size: 5.40â€ x 17.64â€
The word "at" in that second line means "targeting; with the purpose of achieving."
You could build the artwork at 9 ppi if you wanted to, and it's certainly possible to do, but then you'd have to scale the difference between the live area and the overall document size so you'd know where to put your guides. Why bother? Why not use the handy, company-provided template, or at least their spec sheet, and build from there? There is an excellent discussion of billboards and scaling in Chapter 4 of Dan Sorenson's "Photoshop CS2 for Advertising and Marketing" (Peachpit Press).
Working to a company's scaled template has another advantage: it leaves less room for error. At an absolute minimum, the production file you deliver to your billboard provider should be precisely to spec. When the work is time-critical, that can mean the difference between a successful project and a missed deadline.
The easiest, most foolproof way to make sure that happens is to use their template. Failing that, read their spec carefully and follow it to the letter. If it says "8 cm x 16 cm @ 650 ppi", do it exactly that way until you have delivered a few successful projects. With that experience under your belt, you can consider building your own templates that better fit the way you like to work.
Billboards have traditionally been done in Photoshop. I prefer to work in InDesign because it's usually a much faster workflow, so I have made (scaled!) InDesign templates, along with the appropriate color profile and PDF .joboptions file for each provider's equipment.
So there's nothing inherently wrong or right about working on a scaled document, but the practicalities of real-world production, where mistakes can be costly and getting it right is essential, make it important that you know how to work with them and only forego their use when you are certain you know what you're doing.