When is PointResize NOT Reversible?

[zerowalker]

Not sure if this only rules for Pixel games, but as you may know, PointResize can be used to increase the size, and then if you want you can decrease it and it will be identical.

However, to do this you must follow certain rules, and i am not completely sure of those rules, though i think i know it, but rather ask here.

For example, if i want a resolution from 240x160 to come as close to a height of 1200, do i have to take 160 and simply multiply it until it get's close?

Meaning it can never reach 1200 exactly, and i can only choose from 1120/1280?

Or is there some workaround?


Conclusion Below:

PointResize is Only reversible when it's multiplied by an integer. 120x1.5 = Wrong, 120x2 = Correct.

However there is a way to bypass that according to Gavino:

Originally Posted by Gavino

PointResize can also be reversed (as all the original pixels are still there), but it requires using the 'offset' parameters src_top and src_left.

For example, starting with a 320x240 clip,
PointResize(800, 600) # 2.5x upscale, AB->AAABB
is not recovered by
PointResize(320, 240) # AAABB->AA
but can be losslessly reversed by
PointResize(320, 240, 1, 1) # AAABB->AB

The actual offsets required are different for each case and can be tricky to work out.
For RGB inputs, you also have to remember that Avisynth stores them upside down, so the vertical offset may be different.

Leaving PointResize to be quite Flexible, though you would need to do the manual calculation to reverse it, (perhaps it could be made Automatic if it knew the Original Size?).


Thanks for all information