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image_expression_null_value_handling [2022/08/20 20:14] admin |
image_expression_null_value_handling [2022/08/21 13:24] admin [Proper Way of Testing for Image Values -- Guarding Against Null Values] |
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| ===== Proper Way of Testing for Image Values -- Guarding Against Null Values ===== | ===== Proper Way of Testing for Image Values -- Guarding Against Null Values ===== | ||
| - | Lets assume that we want to replace all occurrences of a given valid value in a image with some other valid valid: ''if i1 = 2 then 10 else i1''. Since we are not explicitly guarding the ''if then else'' test against null, the expression is internally equivalent to ''if isnull(i1) then null else if i1 = 2 then 10 else i1''. So, simply using the most compact ''if then else'' definition works as expected. | + | Lets assume that we want to replace all occurrences of a given valid value in a image with some other valid valid: ''if i1 = 2 then 10 else i1''. Since we are not explicitly guarding the ''if then else'' test against null, the expression is internally equivalent to ''if isNull(i1) then null else if i1 = 2 then 10 else i1''. So, simply using the most compact ''if then else'' definition works as expected. |
| - | Now, lets assume the we want do combine two images a landscape map and road map as a single map, where roads in the road map are represent by the value 1 and everything else is represented using the null value. Assuming the ''i1'' is the landscape map and ''i2'' is the road map, we might be tempted to write an expression as ''if i2 = 1 then 1 else i1''. Unfortunately that is not going to work, since the expression is internally equivalent to ''if isnull(i2) then null else if i2 = 1 then 1 else i1'' (we not guarding against null values). As you can see, all occurrences of null values on the road map will be replicated in the output map. | + | Now, lets assume the we want do combine two images, a landscape map and road map, as a single map, where roads in the road map are represent by the value 1 and everything else is represented using the null value. Assuming the ''i1'' is the landscape map and ''i2'' is the road map, we might be tempted to write an expression as ''if i2 = 1 then 1 else i1''. Unfortunately that is not going to work, since the expression is internally equivalent to ''if isNull(i2) then null else if i2 = 1 then 1 else i1'' (we not guarding against null values). As you can see, all occurrences of null values on the road map will be replicated in the output map. |
| - | So, the proper way of doing the calculation is using a ''isnull( ix)'' to protect the ''if then else'' test against the null: ''if not isnull(i2) and i2 = 1 then 1 else i2''. Now we only test for the value in the road map if we are sure that value is not going to contaminate the whole ''if then else'' turning the result into null. | + | So, the proper way of doing the calculation is using a ''isNull( iX )'' to protect the ''if then else'' test against the null: ''if not isNull(i2) and i2 = 1 then 1 else i2''. Now we only test for the value in the road map if we are sure that value is not going to contaminate the whole ''if then else'', turning the result into null. |