Scan Area Options

The four most important well-known options are the ones that define the scan area. The scan area is defined by two points (x/y coordinate pairs) that specify the top-left and the bottom-right corners. This is illustrated in Figure 4.2. Note that the origin of the coordinate system is at the top-left corner of the scan surface as seen by the sensor (which typically is a mirror image of the scan surface seen by the user). For this reason, the top-left corner is the corner for which the abscissa and ordinate values are simultaneously the smallest and the bottom-right corner is the corner for which the abscissa and ordinate values are simulatenously the largest. If this coordinate system is not natural for a given device, it is the job of the backend to perform the necessary conversions.

図 4.2: Scan area options

The names of the four options that define the scan area are given in the table below:

Name	Description
tl-x	Top-left $x$ coordinate value
tl-y	Top-left $y$ coordinate value
br-x	Bottom-right $x$ coordinate value
br-y	Bottom-right $y$ coordinate value

There are several rules that should be followed by front and backends regarding these options:

• Backends must attach a unit of either pixels (SANE_UNIT_PIXEL) or millimeters (SANE_UNIT_MM) to these options. The unit of all four options must be identical.

• Whenever meaningful, a backend should attach a range or a word-list constraint to these options.

• A frontend can determine the size of the scan surface by first checking that the options have range constraints associated. If a range or word-list constraints exist, the frontend can take the minimum and maximum values of one of the x and y option range-constraints to determine the scan surface size.

• A frontend must work properly with any or all of these options missing.