Show a basic storage type to handle a simple RGB raster graphics image, 
and some primitive associated functions.

If possible provide a function to allocate an uninitialised image, 
given its width and height, and provide 3 additional functions:
::* &nbsp; one to fill an image with a plain RGB color,
::* &nbsp; one to set a given pixel with a color,
::* &nbsp; one to get the color of a pixel.


(If there are specificities about the storage or the allocation, explain those.)

''These functions are used as a base for the articles in the category [[Raster_graphics_operations|raster graphics operations]], 
and a basic output function to check the results 
is available in the article [[write ppm file]].''
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