So, keep in mind we didn’t really want to implement arbitrary image resizing on the OpenMV Cam because you really don’t have a lot of RAM. It’s not a desktop PC where you can easily malloc large memory buffers.
Anyway, if you’d like to do this use the sensor.alloc_extra_fb() method (see the docs under sensor) and alloc all the extra frame buffers you need. The buffers are allocated via a stack like structure so they don’t use up memory in the main heap space.
Once you do that you can draw on the frame buffers via set_pixel().
For an example of using these extra buffers see the frame differencing scripts.