If the scan resolution is Auto, a scan resolution is chosen that produces approximately 4 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 preview area (determined by the cropping). When the preview image is scanned, the exposure time is fixed at 1.0, the area to be scanned is the full preview area, and the scan resolution is set to a value that produces approximately 1 million pixels. When the Input | Source option is set to “File”, 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. This is done using the same gamma correction table specified by the sRGB standard.

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. There is no scaling or color correction of the raw CCD data in the scanning step. If multi-pass multi-scanning is enabled, the whole scan area is read multiple times and combined (averaged) in the memory buffer.

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. 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 VueScan’s memory buffer. The Save button starts from the raw CCD data in memory and only performs the processing step. The Preview button and Scan button perform both the scanning and processing steps. These two things are done in steps, and it’s useful to understand each of the steps when scanning and processing. VueScan does two different things: it scans an image, and then it processes the raw data from the scanner to produce a color-corrected image.

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