How can I make Drupal to generate image style from existing style, instead from original image. For example, original image is 3000x2000. I have a style medium (1000x700). And then I need a style thumbnail (100x100). How can I make Drupal to resize 1000x700 -> 100x100 and not 3000x2000 -> 100x100 (for optimization purposes).
Instead try to do this job you can have the same result with create another image style and apply sequential this means first apply
I think you don't consider that in one image style we can multi image effect.
I suggest you for better understanding image style read this article