[Qgis-user] Model designer: Using value from a Number input as a parameter in a Raster Calculator expression

[Bo Victor Thomsen]

Thanks Martin,  for the heads-up.

In hindsight, the Model Designer of course needs a "pre-calculate" 
feature like this to incorporate external programs and function 
libraries ( that is not capable of directly read and evaluate a QGIS 
internal variable or expression).

Med venlig hilsen / Best regards

Bo Victor Thomsen

On 28/05/2025 20.05, Martin Weis via QGIS-User wrote:
> Hello,
>
> Try pre-calculated formula:
> 'A* ' || @number
>
> see the model in this gist:
> https://gist.github.com/Marwe/32c41f69e99782b023c34b913b507470
> Worked for me :)
>
> Have fun
> Martin
>
> Am 27.05.25 um 11:02 schrieb Bo Victor Thomsen via QGIS-User:
>> Hi list -
>>
>> I can't find a method to use a Number input as a part of raster 
>> calculator expression in Model Designer.
>>
>> Using QGIS version 3.42, in Model Designer I've:
>>
>>   * Defined a Number Input field: /my_number/
>>   * Defined a Raster Layer selector : /my_raster/
>>   * Defined a Rater Calculator algorithm: /multiplication/
>>
>> Now the crucial question: How to use the number from my number input 
>> in an expression inside Raster Calculator?
>>
>> Ex. for a simple multiplication: /"A at 1" * parameter('my_number')/ or 
>> /"A at 1" * @my_number/
>>
>> Whatever I do, I get an error message like this:
>>
>> Algorithm 'model' starting…
>>
>> Input parameters:
>>
>> { 'my_number' : 22, 'my_raster' : 'D:/gis/gis_analyser/flis_volumen/ 
>> normaliseret.tif', 'resultat' : 'TEMPORARY_OUTPUT' }
>>
>> Prepare algorithm: native:modelerrastercalc_1
>>
>> Running multiplication [1/1]
>>
>> Input Parameters:
>>
>> { CELL_SIZE: None, CREATION_OPTIONS: None, CRS: None, EXPRESSION: 
>> '"A at 1" * parameter(\'my_number\')', EXTENT: None, LAYERS: ['D:/gis/ 
>> gis_analyser/flis_volumen/normaliseret.tif'], OUTPUT: 
>> 'TEMPORARY_OUTPUT' }
>>
>> Error encountered while running multiplication: Error parsing formula.
>>
>> Execution failed after 0.04 seconds
>>
>>
>> The error message indicate an error in my expression: /"A at 1" * 
>> parameter('my_number')/
>>
>> So, how do i declare and use a Number Input inside a raster 
>> calculator expression ??
>>
>>
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