Article:		Bitmap Image Filters
Subject:		handle alpha channel?
Date:		2002-08-09 17:19:22
From:		mikebeam
Response to: handle alpha channel?
That's part of the solution. The other part has to do with the alpha channel occupying the first 8 bits of a pixel, so you have to offset your red, green, and blue indices by 1. So i had something like the following in the inner for-loop: *p2 = (char)rint((*(p1 + ao) + *(p1 + 1 + ao) + *(p1 + 2 + ao)) / 3); where ao (alpha offset) is determined at the start of the method as: int ao = [srcImageRep hasAlpha] ? 1 : 0; See if that works for you... Mike

Showing messages 1 through 2 of 2.

• ah you had given me support by giving me a problem to solve
  2002-08-10 10:16:59  psheldon [View]
  
  
  I do need some (easy) problems.
  
  I don't get the syntax on right hand side of :
  int ao = [srcImageRep hasAlpha] ? 1 : 0;
  supposedly setting a0 to 1.
  
  Are ? and : c operators and is this rhs c syntax?
  
  My c pdf file from an old codewarrior has awful hypertexting.
• friend Dan Van Bose in Fort Worth explained syntax
  2002-08-11 14:40:41  psheldon [View]
  
  
  c = boolean ? a : b
  
  c evaluates to first symbol after ? if boolean true and second expression if false.
  
  So, this sentence commands skipping by an offset if Alpha is there and mixing 3 colors only.
  
  I understand now.