51 lines
2.2 KiB
Plaintext
51 lines
2.2 KiB
Plaintext
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DIVISION BY ZERO in grid units:
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- There are two main conversion scenarios in grid:
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+ converting a unit to a final location/dimension for drawing
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(or for determining the boundary of a drawn object)
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- The grid graphics package converts all incoming units to INCHES
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(then uses R graphics engine to convert to device coordinates)
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- Almost all conversions to INCHES involve multiplication only
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- The exception is "native" units, which require a division by
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the range of the native scale. This creates the possibility
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of division by zero if the range of the native scale is zero
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(i.e., the scale limits are identical).
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SOLUTION: make it an error to set a scale with identical limits.
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+ converting from one unit to another (within a viewport context)
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- Conversions, via convertUnit(), involve two steps:
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conversion from the original unit TO inches and then
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conversion FROM inches to the final unit.
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- Any division-by-zero in the conversion TO inches is avoided
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by the solution above.
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- Division-by-zero in the conversion FROM inches can occur
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IF the viewport has zero physical width or height
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AND the final unit is relative ("npc" or "native").
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SOLUTION: (i) IF original unit is also relative ...
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(a) IF original unit is plain unit AND we are NOT
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converting 'x' to 'y' (or vice versa) catch this
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as special case and make it work.
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(b) OTHERWISE (original unit is complex or we are
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converting 'x' to 'y' or vice versa)
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return zero.
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(ii) IF original unit is absolute, throw an error
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(because the conversion does not make sense) UNLESS the original
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unit value is zero, in which case return zero.
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- NOTE that conversion FROM anything TO an absolute final unit
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does not risk division-by-zero, even when viewport has zero
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physical width or height. (The result is either zero, if
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the original unit is relative, or an identity operation [in
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terms of the physical result] if the original unit is absolute.)
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