Automatic downsampling (e.g. scan at 7200, save at 3600)

[Antonio Marques]

Hallo an alle,

I'd like to propose another feature. As is well known, a scanner's effective resolution is never the same as its nominal one, because besides the number of samples read by inch one must take into account the sensor's quality, the optics, the inherent errors, the noise, etc. In practice any scanner will create images with a real resolution that is less than the number of pixels. In scanners like the Nikons, this is negligible - the real resolution is more than 90% the nominal one. But almost everything cheaper than a Nikon will have a large difference between the real and nominal resolutions. And it doesn't always help to scan at a lower DPI than the maximum, because some of the factors that make the real resolution lower than the nominal one also apply at lower DPI. Case in point: A scanner that yields an effective 3600 DPI when scanning at 7200, may offer only some 3200 when scanning at 3600. For a normal 36x24 image, that means, in pixel terms, that a 70 megapixel image only has the same information as a 17 megapixel one, but if you ask the scanner for a 17 megapixel image, the one you get only has the same information as a 14 megapixel one. And these are actually good values, as many scanners don't even meet them (the disparity in sizes is because the number of pixels increases proportionaly to the square of the DPI).

There is also the film's own resolution. Good 35mm film is said to have the equivalent of a resolution of about 4000 DPI, but not all film reaches that. But there is a number of reasons why you should want to scan at the highest DPI your scanner is able to - in short, of all the factors that can reduce image resolution, that is the one that you can control, and if you don't maximise it, then you may be losing information needlessly. Just like if you plan a street, you'll want to make the lanes larger than the cars that will be circulating in them, or if you have to catch a train you'll want to be at the station some minutes before the train arrives, not exactly in the last moment before it departs.

But after your image has been transferred from your scanner to Silverfast, there is little point in handling 70 megapixels with only 17 mp worth in them. You may not want to reduce them to 17 outright, but you may wish to settle on a value that is reasonable taking into account the scanner's resolution, the film's quality and the cost of processing and storing such huge data. So it would be very nice to have an 'Output DPI' option that you could use to automatically reduce the data size to a useful amount. For instance, you could say the scanning DPI would be 7200 but the output DPI would be 3600 or 4000 or even 4800. The software would automatically downsample the information coming from the scanner, with a futher option to do software image corrections either before the downsampling (maybe more accurate?) or after (certainly much faster). The resulting file would have only the desired final resolution. This would apply even to HDRi files - it would be the user's choice, after all. A 7200->3600 downsampled HDRi file could be better than an original 3600 one. Maybe Silverfast could even suggest, based on the scanner's model and the downsampling algorithm, what would be the optimal output formats for a given scanning DPI. It would also be very nice if SIlverfast indicated somewhere which is/are the scanner's native resolution(s).

I think this would be a very nice feature useful for almost everyone, but I realise there are workarounds for it. I just wanted to propose it. Sorry if this has been proposed before, I looked for it and didn't find it.

Vielen Dank,
Antonio Marques

[LSI_Morales]

Dear Antonio,

First of all thanks for your contribution

As is well known, a scanner's effective resolution is never the same as its nominal one, because besides the number of samples read by inch one must take into account the sensor's quality, the optics, the inherent errors, the noise, etc. In practice any scanner will create images with a real resolution that is less than the number of pixels. In scanners like the Nikons, this is negligible - the real resolution is more than 90% the nominal one. But almost everything cheaper than a Nikon will have a large difference between the real and nominal resolutions. And it doesn't always help to scan at a lower DPI than the maximum, because some of the factors that make the real resolution lower than the nominal one also apply at lower DPI. Case in point: A scanner that yields an effective 3600 DPI when scanning at 7200, may offer only some 3200 when scanning at 3600. For a normal 36x24 image, that means, in pixel terms, that a 70 megapixel image only has the same information as a 17 megapixel one, but if you ask the scanner for a 17 megapixel image, the one you get only has the same information as a 14 megapixel one. And these are actually good values, as many scanners don't even meet them (the disparity in sizes is because the number of pixels increases proportionally to the square of the DPI).

.

Although absolutely right, this not a well known issue. And that is the reason why some scanner manufacturers inflate the numbers of the "nominal" or even the interpolated resolution to make their product appear better than they actually are (selling tactics I guess).

But there is a number of reasons why you should want to scan at the highest DPI your scanner is able to - in short, of all the factors that can reduce image resolution, that is the one that you can control, and if you don't maximize it, then you may be losing information needlessly.

I partially agree with you, but there are some people who are not interested in having the highest pixel count possible but they are only interested in printing postcard size or scan for the web or even tv sets. In those cases the highest physical resolution of the scanner will be overkill.

So it would be very nice to have an 'Output DPI' option that you could use to automatically reduce the data size to a useful amount. For instance, you could say the scanning DPI would be 7200 but the output DPI would be 3600 or 4000 or even 4800. The software would automatically downsample the information coming from the scanner, with a further option to do software image corrections either before the downsampling (maybe more accurate?) or after (certainly much faster).

Actually SilverFast already has a similar system, but things are a bit more complicated than that. Summarizing, the scanner itself (and sometimes the combination between scanner and driver) provide certain steps to set the scanner's physical resolution, those steps are used by SilverFast (represented by the steps in the resolution slider). Usually when you decide an output size and choose any resolution, SilverFast automatically chooses the next highest physical resolution to perform the scan and then downsample to the desired resolution.

This real physical resolution (or resolution step within the scanner) as well as the resolution of the file after downsampling can be seen by pressing the keys f5 or f6 (for Windows) and shift or ctrl (for mac).

Cheers