Drawing with Quartz - overlap problems

Discussion in 'iOS Programming' started by Jokode, Jan 26, 2009.

  1. Jokode macrumors newbie

    Joined:
    Dec 14, 2008
    #1
    Hi,
    I am not new to iphone coding as I have one app out in the store, so I'm not a complete noob - however please forgive this question as it might be very silly...

    How can I prevent a partially transparent circle and a rectangle that I have drawn in quartz which overlap from 'double' drawing?

    By double drawing I mean the areas which overlap are shaded darker than the areas that dont overlap - because that area has been draw into twice. This wouldnt be a problem if everything is solid color, but I am trying to draw partially transparent and having areas that overlap go dark just makes no sense for my app!

    I really don't know a lot about drawing with a technology such as quartz as I have never really done it before - but I feel like is probably a common problem.

    Is there a function or concept in quartz that I should be using to do this?

    Any help would be very appreciated!

    Joe.
     
  2. Delirium39 macrumors regular

    Joined:
    May 19, 2008
    #2
    It's not double drawing. If you draw two shadows on top of each other, you get a darker shadow. Your first image is drawn on a background color. Your second image is drawn on the background color plus the first image. Adding 50% black to 50% black does not equal 50% black. Make sense?
     
  3. detz macrumors 65816

    Joined:
    Jun 29, 2007
    #3
    Transparent images will show what's behind them, that's what transparency is. I'm not sure what the issue is, if yo make the image in the front semi-transparent it will merge with the colors behind it.
     
  4. Jokode thread starter macrumors newbie

    Joined:
    Dec 14, 2008
    #4
    Thanks for the feedback - after looking over the responses and code again, I think the problem is in my approach.. I'll go back to the drawing board :)
     
  5. jnic macrumors 6502a

    Joined:
    Oct 24, 2008
    Location:
    Cambridge
    #5
    Yes; clipping.
     
  6. CommanderData macrumors regular

    Joined:
    Dec 1, 2007
    #6
    Yep, try to set a clipping area up so that you only draw the half of the circle that does *not* overlap the rectangle. Don't forget to save/restore your context's original state as well!
     

Share This Page