Scanning an image involves optionally focusing the scanner, setting up the exposure time for the CCD, area to be scanned, number of bits per sample, number of samples per pixel, scan resolution, and then reading the raw CCD data into a memory buffer in VueScan.
If single-pass multi-scanning is enabled, each line of data from the CCD is read multiple times and combined (averaged) while being stored in the memory buffer. If multi-pass multi-scanning is enabled, the whole scan area is read multiple times and combined (averaged) in the memory buffer.
There is no scaling or color correction of the raw CCD data in the scanning step. Some scanners either always or sometimes convert 10-bit or 12-bit CCD data to 8 bits before transferring it to VueScan, and then VueScan converts it back to 10-bit or 12-bit CCD data. This is done using the same gamma correction table specified by the sRGB standard.
When the "Device|Scan from" option is set to "Disk", the raw CCD data is read from a tiff or jpeg file and stored in the memory buffer, just as if it had been read directly from a scanner. The "Device|Mode" option is used to indicate the type of scanner that the raw CCD data came from (this helps with color correction in the processing step).
When the preview image is scanned, the exposure time is fixed at 1.0, the area to be scanned is the full scan region, and the scan resolution is set to a value that produces approximately 1 million pixels.
When the full scan is done, the exposure time is either computed from the preview or from the manual setting and the area to be scanned is a subset of the scan region (determined by the cropping). If the scan resolution is "Auto", a scan resolution is chosen that produces approximately 4 million pixels.
When the preview or scan is performed, the raw data is put into a memory buffer. If the "Files|Save raw file" option is enabled and "Files|Raw save with" is set to "Preview" or "Scan", the raw data is written to a .tif file at the same time as it's put into the memory buffer.
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