Matlab coder "All inputs must be constant"

Hi,

I tried to generate my code to Mex and to C++ using the Matlab coder.

In my m.file I want to do the following lines:

h = round(a*b + b0);

bw_blobs = imclose((clr_cand)>0,ones(h));

When "a" and "b0" are constants and "b" is variable which change between eatch running.

When i tried to generate the code I got the next error message:

"All inputs must be constant"

how can I solve this error?

Please your advise.

Thanks,

Itai

Answer 1

[Abhishek]

Hi @itai,

The error you’re seeing— “All inputs must be constant” — occurs because, in code generation with MATLAB Coder, certain functions like “ones()” require their input to be a compile-time constant.

In the provided code, since “h” depends on the variable “b”, which in turn is variable, leads to this error.

Documentation for “coder.mustBeConst” confirms this argument:

"coder.mustBeConst(value)” validates that the function input value is a constant during code generation, also referred to as a compile-time constant in this context. During code generation, this function throws an error if value is not a compile-time constant.

So, using "coder.mustBeConst" would only enforce the constant requirement and cause code generation to fail.

As a workaround, the following things can be done:

1. Predefine the maximum possible size: If the maximum value of “h” can be known before code-generation, then an array of that size can be allocated.
2. Using “coder.varsize()”: If the size of the variable “h” can vary, but it has bounds, then an array of variable-size nature can be defined for the purpose of code-generation, in this way:

coder.varsize('bw_blobs', [MIN_H, MAX_H]);  

where, “MIN_H” and “MAX_H” stores the lower and upper bound for the variable “h” respectively.

The above workaround helps in a way to handle the code generation process without any error.

Please refer to the following documentation to know more about “coder.mustBeConst()”: https://www.mathworks.com/help/releases/R2024b/coder/ref/coder.mustbeconst.html

Hope this helps resolve the error.