Loading an image as a matrix

[AlexFink]

I want to load an image as a matrix, and perform sparse matrix arithmetic.
The relevant libraries I found for this are:

Image operations (load, resize):
GitHub - nothings/stb: stb single-file public domain libraries for C/C++

Sparse matrix arithmetic:
GSL - GNU Scientific Library - GNU Project - Free Software Foundation

Are there other libraries that can save the pain of adding a translation between these two libraries formats (the loaded image, and the matrix type)?

thanks