OK I think I found the answer, looking on the Adobe forum for Designer. https://community.adobe.com/t5/substance-3d-designer-discussions/substance-designer-sbsar-change-bitmaps-within-painter-houdini/m-p/13586749#M2271
How do I allow a user to input their own bitmap into a SBSAR that I make? I'd like to make a normalize tool with Designer, which only normalizes RG and passes through B unchanged. Details here: Normalize (-1,1) in Designer ? I can't seem to figure out how to expose an input parameter for the graph which allows a bitmap.…