NSImage/NSBitmapImageRep color shifts when creating scaled copy

• Steve Christensen NSImage/NSBitmapImageRep color shifts when creating scaled copy Mar 06 2009, 20:21
  I'm trying to create a scaled-down copy of a large NSBitmapImageRep
  (i.e., 3200x2400 -> 320x240). The smaller image eventually gets
  passed to OpenGL for drawing. What I'm finding is that the copy has
  color shifted. When I draw the copy, it appears to be darker and more
  saturated than the original.
  
  I have tried several ways to get a copy that has the same color
  characteristics as the original but no success so far. My simplest
  version looks like this:
  
  NSRect resizedBounds = {NSZeroPoint, desiredBitmapSize};
  NSImage* resizedImage = [[[NSImage alloc]
  initWithSize:resizedBounds.size] autorelease];
  
  [resizedImage lockFocus];
  [[NSGraphicsContext currentContext]
  setImageInterpolation:NSImageInterpolationHigh];
  [image drawInRect:resizedBounds fromRect:NSZeroRect
  operation:NSCompositeCopy fraction:1.0];
  [resizedImage unlockFocus];
  
  bitmapImage = [NSBitmapImageRep imageRepWithData:[resizedImage
  TIFFRepresentation]];
  
  Am I missing something or is this just how Quartz works? Is there
  another way to get there? I'm not wedded to NSImage and friends if a
  better solution exists elsewhere. This needs to run on 10.4 or later.
  
  Thanks,
  steve
• Kyle Sluder Re: NSImage/NSBitmapImageRep color shifts when creating scaled copy Mar 06 2009, 22:15
  On Fri, Mar 6, 2009 at 2:21 PM, Steve Christensen <punster...> wrote:
  

  > I'm trying to create a scaled-down copy of a large NSBitmapImageRep (i.e.,
  > 3200x2400 -> 320x240). The smaller image eventually gets passed to OpenGL
  > for drawing. What I'm finding is that the copy has color shifted. When I
  > draw the copy, it appears to be darker and more saturated than the original.
  

  
  Sounds like you're falling victim to this problem:
  http://www.4p8.com/eric.brasseur/gamma.html
  
  --Kyle Sluder
• Steve Christensen Re: NSImage/NSBitmapImageRep color shifts when creating scaled copy Mar 06 2009, 22:46
  On Mar 6, 2009, at 1:15 PM, Kyle Sluder wrote:
  
  

  > On Fri, Mar 6, 2009 at 2:21 PM, Steve Christensen <punster...>
  > wrote:
  >> I'm trying to create a scaled-down copy of a large
  >> NSBitmapImageRep (i.e.,
  >> 3200x2400 -> 320x240). The smaller image eventually gets passed to
  >> OpenGL
  >> for drawing. What I'm finding is that the copy has color shifted.
  >> When I
  >> draw the copy, it appears to be darker and more saturated than the
  >> original.
  >
  > Sounds like you're falling victim to this problem:
  > http://www.4p8.com/eric.brasseur/gamma.html
  

  
  In this case what I found out was running into a difference of
  opinion between the ColorSync profile attached to the
  NSBitmapImageRep in the source image and the default one (or lack of
  one) for a newly-created NSBitmapImageRep in the resized image.
  
  I ended up rewriting my code to create a new NSBitmapImageRep with
  the destination size, then copy what I thought were relevant
  properties (NSImageRGBColorTable, NSImageColorSyncProfileData,
  NSImageGamma) from the source NSBitmapImageRep if they exist. Then
  create a context using +[NSGraphicsContext
  graphicsContextWithBitmapImageRep:] using the destination
  NSBitmapImageRep and draw into that.
  
  steve