When you create a new painting, you can adjust the 'roughness' of the canvas with the roughness radial control in the paper settings panel. These settings can also be accessed from the 'Tools/Layer Options/Edit Paper Settings' menu option, also from the menu control on the central preview panel (the control to the right of the 'Load tracing paper' button), and also can be set for each layer from the layer stack menu for each layer.
You can also change the scale of the paper texture as well as the colour and transparency and metalicity of the paper.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"