Howted 4000, "Color Filter" Function

flatbed scanners for Howtek

MWesley
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Howted 4000, "Color Filter" Function

Postby MWesley » Sun Oct 06, 2002 7:16 pm

Hi,

I am running SF 5.5 with a Howtek D4000 drum scanner under Windows 2000. In Options under the Special tab there is a "Color Filter" setting. According to the user manual this setting allows you to choose a color that will be ignored during monochrome scanning.

I have three questions. First, is this correct? It would be more useful if the function allowed you to choose which color PMT was used to scan the B&W negative rather than choosing a combination of two PMT's.

Second, is this setting actually controlling the scanner and directing it to use only two PMT's during the scan, is one channel of data being ignored by SF as it scans or is this simply a software function that deletes the chosen channel data after it is received?

Third, when the "Prescan monochrome" box is check the "White" option in "Color Filter" is grayed out. Why?

Basically I am looking for an overall explanation of the function of this setting.

Thanks,
Martin Wesley

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Postby LSI_Flyvbjerg » Mon Oct 07, 2002 6:54 am

Martin.

1. The color use select in "color filter" is not the color that is ignored, but the color that is selected for gray scaning. If you choose red, the red channel is selected, green selects green and blue selects blue. If you choose white, a neutral gray (combination of RGB) is selected. I talk to our documentation team about the typo in the manual.

2. For the D4000, all four methods are build in hardware.

3. The monochrome scan is a very old feature which resulted in quicker prescans on older computers. I?m not sure, whether this checkbox has any significant effect on your PC or not. Futhermore, I simply don?t remember the dependency between monochrome scan and color filter. If you really need to know this, please let me know, I will try to find out.

Eric.

MWesley
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Howtek D4000 and SilverFast

Postby MWesley » Mon Oct 07, 2002 6:14 pm

Eric,

Thank you very much for the information and quick reply. I have some additional questions about how SilverFast interacts with the Howtek.

I know the Howtek is a 12-bit scanner and I assume that the data flowing from the scanner to Silverfast is 12-bit data. I always set SilverFast to output in 16-bit mode and I was wondering how the data is translated from 12-bit to 16-bit. In a raw or HDR scan I assume the 12-bit data is simply placed inside the larger 15-bit space with all values remaining adjacent.

When I make an adjustment in SilverFast, say just a simple setting of the end points in levels, the data is spread out over a larger portion of the 16-bit space. Is the 12-bit data spread out with spaces in between the data or does SilverFast create interpolated data values in between the original 12-bit data?

Are adjustments done in SilverFast, such as curves, gamma, etc. applied to the 12-bit data before it is mapped into the 16-bit space or afterward?

I would appreciate any information on this as I am trying to understand how this all works in an effort to get the very best quality out of my negatives.

Thank you,
Martin Wesley

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Postby LSI_Muenier » Tue Oct 08, 2002 7:50 am

Dear Martin,

Eric asked me to comment on this.

When SilverFast wants to acquire 16 bit data from a scanner and this scanner can only generate 12 bits, then the image data are shifted 4 bits up in each word -- that is, they are multiplied by a factor of 16.

By this, the image data are "normalized" so that the largest value the scanner can generate (2^12 - 1) gets mapped to the largest value representable by 16 bits (2^16 - 1). No interpolation is done or useful.

This normalization to 16 bits is done before any of the image adjustments or corrections (gamma, gradation, color correction) is applied.

Best regards,
Martin Muenier, LaserSoft Imaging AG

MWesley
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Postby MWesley » Wed Oct 09, 2002 4:54 am

Martin,

Thank you for the information. I am getting a better understanding of the scanning process.

Since SilverFast maps (2^12-1), 4095, to (2^16-1), 65535, am I correct in the following?

(2^12-2), 4094 > (2^16-32), 65504
(2^12-3), 4093 > (2^16-48), 65488
(2^12-4), 4092 > (2^16-64), 65472
(2^12-5), 4091 > (2^16-80), 65456
etc.

The 16-bit data would then appear to have gaps 15 units wide between each value. Are these spaces left empty, filled with the higher value or is some other data generated to ?fill in the gaps??

This leads to another question. If an adjustment is applied by SilverFast in 16-bit space, say a simple change of end points in Levels which causes the data to be spread over a larger portion of the 16-bit range, does this then create gaps in the data or does Silverfast fill in the gaps with some value? Likewise if the data is ?compressed? into a smaller portion of 16-bit space then it originally occupied, is data eliminated per a mathematical algorithm or are more complex averaging techniques applied?

I appreciate any information you can give me on this process. I think that if I have a better grasp of how the data is handled at different points in the workflow I can make better decisions about how to scan my negatives.

Sincerely,
Martin Wesley

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Postby LSI_Muenier » Wed Oct 09, 2002 6:19 am

Dear Martin,

please note that the relative gap width of 1/4096 remains the same.
Thus, the normalization does not _generate_ gaps, it rather increases the available accuracy each image entry can be described with. In interpolations (to achieve the requested output size) or image corrections done later in SilverFast this may be used.

Best regards,
Martin Muenier

MWesley
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Postby MWesley » Wed Oct 09, 2002 6:55 pm

Martin,

I didn't mean that gaps were being generated, just trying to understand how the 12-bit data is placed in 16-bit space. I understand that all 4096 shades of gray (sorry as a B&W photographer I tend to think in those terms) coming from the scanner are accurately preserved when SilverFast translates it to 16-bit.

I was just wondering what is done with adjacent values in 16-bit space whether they were left empty or filled with some value. It would make sense to map a single tone in 12-bit to 16 adjacent tones in 16-bit to provide redundancy and greater accuracy as additional adjustments are made to the file in Silverfast or Photoshop.

Best,
Martin Wesley

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Postby LSI_Muenier » Thu Oct 10, 2002 6:54 am

Dear Martin,

I don't know how to map a single 12-bit value to "16 adjacent tones" i 16-bits: I need to choose one certain 16-bit value as the map result.

Best regards,
Martin


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