NSBitmapImageRep - how can I remove or reverse an alpha channel from tiff?

Question

Below is some code I cobbled together to save a square preview of an image. I am looking to figure out how to remove or inverse an alpha channel mask from a tiff image, and could really use some help....
// I have been using 256.0 and 0.4 for the last 2 arguments, though feel free test with whatever.
- (NSString *)savePreview: (NSString *)filePath: (NSString *)saveFilePath: (NSString *)previewSize: (NSString *)compressionRatio {
NS_DURING
NSAutoreleasePool *memoryPool = [[NSAutoreleasePool alloc] init];
NSImage *myImage = [[NSImage alloc] initWithContentsOfFile: filePath];
if (myImage == nil) {
[myImage release];
NSLog(@"! Error loading image");
return @"! Error loading image";
NSImageRep *myRep = [[myImage representations] objectAtIndex: 0];
float hres = [myRep pixelsWide]/[myImage size].width;
float vres = [myRep pixelsHigh]/[myImage size].height;
float newWidth = [previewSize floatValue];
float newHeight = newWidth;
NSImage *thumbImage = [[NSImage alloc] initWithSize: NSMakeSize(newWidth, newHeight)];
NSAffineTransform *at = [NSAffineTransform transform];
[myImage setScalesWhenResized: YES];
float heightFactor = newHeight/[myImage size].height;
float widthFactor = newWidth/[myImage size].width;
float scale;
if(heightFactor > widthFactor) {
scale = widthFactor;
} else {
scale = heightFactor;
[at scaleBy: scale];
[thumbImage lockFocus];
[[NSGraphicsContext currentContext] setImageInterpolation: NSImageInterpolationHigh]; // NSImageInterpolationNone, NSImageInterpolationLow, NSImageInterpolationHigh
[myImage setSize: [at transformSize: [myImage size]]];
[myImage compositeToPoint: NSMakePoint((newWidth-[myImage size].width)/2 , (newHeight-[myImage size].height)/2) operation: NSCompositeCopy];
NSBitmapImageRep *bitmapImageRep = [[NSBitmapImageRep alloc] initWithFocusedViewRect: NSMakeRect(0, 0, [thumbImage size].width, [thumbImage size].height)];
[thumbImage unlockFocus];
[myImage release];
[thumbImage release];
NSData *bitmapData = nil;
NSNumber *factor = [NSNumber numberWithFloat: [compressionRatio floatValue]]; // The compression factor. Used only for JPEG files. The value is a float between 0.0 and 1.0, with 0.0 being the lowest and 1.0 being the highest. It’s set when reading in and used when writing out.
NSDictionary *props = [NSDictionary dictionaryWithObject: factor forKey: NSImageCompressionFactor];
bitmapData = [bitmapImageRep representationUsingType: NSJPEGFileType properties: props];
[bitmapData writeToFile: saveFilePath atomically: YES];
[bitmapImageRep release];
[memoryPool release];
return @"YES";
NS_HANDLER
NSLog(@"Exception occured : %@", localException);
return @"NO";
NS_ENDHANDLER
}

Answer 1

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