SS, you may not have seen my reply in the other thread, so here it is again + some new material specific to your question about up-rez'ing:
re: aspect ratio and up-rez'ing questions
Aspect Ratio
As you correctly observed, if your original 1920 x 1080 px image is simply scaled up to 1414 mm in the long dimension, then (ignoring the small border), the short dimension will be only 795 mm, which is a good bit narrower than the 1m of the poster.
You have four choices:
a) Add white space (or any color you want) to the short dimension so that it comes out to be exactly 1m (minus any unprintable border) when scaled up;
b) Crop a bit off the long dimension so that when the short dimension is scaled up to be 1 m (minus any unprintable border), the long dimension is 1414 mm (minus any unprintable border);
c) The difference in aspect ratio between what you want and what you have is not extreme, so you could probably scale the two dimensions slightly differently and most people wouldn't notice (especially if there are only graphics and text on the poster); or,
d) Most printing shops will do either (a) or (b) for you, so can let them decide and make an appropriate decision.
If you want to be absolutely sure that this is done correctly (eg, because the original image is already cropped very tightly), then do it yourself. You don't need to put crop marks. You simply crop it yourself to the correct aspect ratio, then up-rez to a size that is a cm or so slightly smaller than the overall dimensions of the poster to allow for an unprintable border.
Up rez'ing
You should be more concerned with is the fact that the starting pixel dimensions are quite low for a poster of this size. This means that even using the best available up-rez'ing software or RIP, your poster will be somewhat soft. This will be very noticeable if someone looks at it from a couple of feet away or less. However, if the poster is only going to be viewed from several feet away or further, most people will not notice this softness. It's your call whether you want to use this image and accept a bit of softness or use a different image.
If you decide you must use this particular image and it's not available in a larger size (meaning, larger pixel dimensions), then you need to decide how to up-rez it so it doesn't look pixellated when printed at poster dimensions.
Any firm that prints posters will run into this issue on probably 90% of the files sent to them. If they are a reputable place, they will have their own software to take care of this. It usually goes under the name of RIP (Raster Image Processing) software. In almost all cases, I would let the vendor perform the up-rez'ing step for you.
That being said, I am very concerned about you using the firm that gave you the bizarre advice about setting the CMYK values of the poster to one particular value. Either (a) you talked to someone who absolutely did not know what they were talking about, (b) they assumed you are an expert in pre-press, so they were speaking in shorthand, or (c) you misunderstood them.
In all of these scenarios, this is obviously not a match made in heaven, and, as I suggested in the other thread, I would go to a poster printer that deals with the general public.
If you are dead-set on using this particular firm and not looking into any of the hundreds of other firms that make hundreds of posters per day, I would speak to the original firm carefully about up-rez'ing, pixellation, etc. and make sure that they don't think you are going to do it.
Software such as Genuine Fractals (now, "Perfect Resize" by onOne Software) or the algorithms built into Photoshop for up-rez'ing should only used if you have to DIY (do-it-yourself). The usual situation for this is if you are posting an image on the web or printing at home.
HTH,
Tom M
