Women in Technology

Article:		Bitmap Image Filters
Subject:		handle alpha channel?
Date:		2002-08-08 21:12:34
From:		psheldon
"p1 = srcData + 3 * (y * w + x);" in IAGrayscaleFilter.m I believe does not as needs n in place of 3, I think. I must be missing something because I don't see a change needed. Or maybe I learned enough to be of use in this article. ;-)

Showing messages 1 through 3 of 3.

• handle alpha channel?
  2002-08-09 17:19:22  Michael Beam | [View]
  
  
  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
• 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.