Skip to main content
basilico51
basilico51
Participant
November 19, 2016
Question

sky landscape in a semicircle

  • November 19, 2016
  • 2 replies
  • 599 views

Good evening.

I would modify a large rectangular photo of a night sky with a semicircular projection.

I have tried with the polar filter but the result is even a round either an ellictic image.

I repeat, I would like to change the picture in a semicircle keeping as a basis the longer side of the rectangular photo.

Thank you.

    This topic has been closed for replies.

    2 replies

    Semaphoric
    Community Expert
    SemaphoricCommunity Expert
    Community Expert
    November 19, 2016

    You can do this via Polar Coordinates. Just be sure the subject is only on the right or left half of the canvass.

        

    After Polar Coordinates:

        

    You could then rotate the results if you like. Unfortunately, the outer edge will look smeared. Stars would become short, arced streaks. You can get around this (sort of) by starting with an image 6.28 times wider than the final, and then scaling the width down to the final size before the Polar Coordinates.

        

    Now the stretching is in the middle.

      basilico51
      basilico51Author
      Participant
      November 20, 2016

      Very much interesting. I do not understand very well this:

      @You can get around this (sort of) by starting with an image 6.28 times wider than the final, and then scaling the width down to the final size before the Polar Coordinates.@

      Please, describe me this last procedure step by step, as the result seems to be satisfying.

      Semaphoric
      Community Expert
      SemaphoricCommunity Expert
      Community Expert
      November 20, 2016

      I used 6.28 because that is 2 x pi. So, I started with a doc like this (Width is 6.28 x the Height):

          

      and then scaled the width so the image would be a square:

          

      Now the image looks "squashed", and when it's Polar Coordinated and "stretched", the outside will look "normal".

      Here's a version with an original aspect ratio of 3.14 (pi), and done as above; in this, the "normal" part is about halfway out from the middle (a compromise):

          

      A couple of notes: The two transformations can really degrade the image quality, so it's best to start with the largest size you can. Also, in these examples, the bottom edge becomes the circumference, the left and right edges combine to make the diameter (two radii), and the top edge is collapsed into the center.

        Chuck Uebele
        Community Expert
        Chuck UebeleCommunity Expert
        Community Expert
        November 19, 2016

        Have you tried transform>warp?

        Terms & ConditionsAccessibility statement
        Using the community Terms of use Privacy Cookie preferences Do not sell or share my personal information ad choicesAdChoices