Sparrow Forum

1. 

   alex_h
   Moderator

   Hi Daniel,

   A freelancer on a previous sparrow project hacked my working copy of the SPGLTexture class to allow support for 1024px square pvrtc textures. This works fine.

   Now on a new project I want to use pvrtc textures with alpha, but I'm having some problems getting this to work.
   In SPGLTexture I set mPremultipliedAlpha to TRUE for pvrtc textures. This should tell SPRenderSupport to use the GL_SRC_ALPHA blend function for that texture. For some reason though, my texture then doesn't display with alpha.
   However, if I temporarily modify SPRenderSupport to use the GL_SRC_ALPHA blend function for all textures, the pvrtc texture does display correctly. Unfortunately, with this setting my 24bit pngs with alpha then get a funny outline where there should be a smooth alpha gradient.

   Do you have any idea what I should do to get both pvrtc and png files to display alpha transparency correctly? Maybe there is something simple that I am missing?

   cheers,

   Alex

   Posted 1 year ago #
2. 

   Daniel
   Primary Feather

   Hi Alex!

   Aaaaah, the premultiplied alpha issue. This cost me some nerves during the implementation

   Hm, I would think that you need to set mPremultipliedAlpha to NO. As you sure know, the whole premultiplied alpha thing is caused by Xcode, which preprocesses all png images by removing the alpha channel and instead applying the alpha value directly to the RGB channels. (The reason is that this makes displaying them faster, and in UIKit, it does not lead to any problems.)

   However, this is only done for PNG images -- at least as far as I know.

   And setting it to "NO" would then make Sparrow use "GL_SRC_ALPHA" for blending -- different to what you write (I just checked the code). So perhaps you just mixed that up?

   Daniel

   Posted 1 year ago #
3. 

   alex_h
   Moderator

   Fantastic! it works - thanks a lot for your help. You're right, I had somehow got the boolean the wrong way around.

   cheers,

   Alex

   Posted 1 year ago #
4. 

   Daniel
   Primary Feather

   Great, glad to hear that =)

   Daniel

   Posted 1 year ago #

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