NSImage losing quality upon writeToFile - objective-c

Question

Basically, I'm trying to create a program for batch image processing that will resize every image and add a border around the edge (the border will be made up of images as well). Although I have yet to get to that implementation, and that's beyond the scope of my question, I ask it because even if I get a great answer here, I still may be taking the wrong approach to get there, and any help in recognizing that would be greatly appreciated. Anyway, here's my question:
Question:
Can I take the existing code I have below and modify it to create higher-quality images saved-to-file than the code currently outputs? I literally spent 10+ hours trying to figure out what I was doing wrong; "secondaryImage" drew the high quality resized image into the Custom View, but everything I tried to do to save the file resulted in an image that was substantially lower quality (not so much pixelated, just noticeably more blurry). Finally, I found some code in Apple's "Reducer" example (at the end of ImageReducer.m) that locks the focus and gets a NSBitmapImageRep from the current view. This made a substantial increase in image quality, however, the output from Photoshop doing the same thing is a bit clearer. It looks like the image drawn to the view is of the same quality that's saved to file, and so both are below Photoshop's quality of the same image resized to 50%, just as this one is. Is it even possible to get higher quality resized images than this?
Aside from that, how can I modify the existing code to be able to control the quality of image saved to file? Can I change the compression and pixel density? I'd appreciate any help with either modifying my code or pointing me in the way of good examples or tutorials (preferably the later). Thanks so much!
- (void)drawRect:(NSRect)rect {
// Getting source image
NSImage *image = [[NSImage alloc] initWithContentsOfFile: #"/Users/TheUser/Desktop/4.jpg"];
// Setting NSRect, which is how resizing is done in this example. Is there a better way?
NSRect halfSizeRect = NSMakeRect(0, 0, image.size.width * 0.5, image.size.height * 0.5);
// Sort of used as an offscreen image or palate to do drawing onto; the future I will use to group several images into one.
NSImage *secondaryImage = [[NSImage alloc] initWithSize: halfSizeRect.size];
[secondaryImage lockFocus];
[[NSGraphicsContext currentContext] setImageInterpolation: NSImageInterpolationHigh];
[image drawInRect: halfSizeRect fromRect: NSZeroRect operation: NSCompositeSourceOver fraction: 1.0];
[secondaryImage unlockFocus];
[secondaryImage drawInRect: halfSizeRect fromRect: NSZeroRect operation: NSCompositeSourceOver fraction: 1.0];
// Trying to add image quality options; does this usage even affect the final image?
NSBitmapImageRep *bip = nil;
bip = [[NSBitmapImageRep alloc] initWithBitmapDataPlanes:NULL pixelsWide: secondaryImage.size.width pixelsHigh: secondaryImage.size.width bitsPerSample:8 samplesPerPixel:4 hasAlpha:YES isPlanar:NO colorSpaceName:NSDeviceRGBColorSpace bytesPerRow:0 bitsPerPixel:0];
[secondaryImage addRepresentation: bip];
// Four lines below are from aforementioned "ImageReducer.m"
NSSize size = [secondaryImage size];
[secondaryImage lockFocus];
NSBitmapImageRep *bitmapImageRep = [[NSBitmapImageRep alloc] initWithFocusedViewRect:NSMakeRect(0, 0, size.width, size.height)];
[secondaryImage unlockFocus];
NSDictionary *prop = [NSDictionary dictionaryWithObject: [NSNumber numberWithFloat: 1.0] forKey: NSImageCompressionFactor];
NSData *outputData = [bitmapImageRep representationUsingType:NSJPEGFileType properties: prop];
[outputData writeToFile:#"/Users/TheUser/Desktop/4_halfsize.jpg" atomically:NO];
// release from memory
[image release];
[secondaryImage release];
[bitmapImageRep release];
[bip release];
}

Answer 1

I'm not sure why you are round tripping to and from the screen. That could affect the result, and it's not needed.
You can accomplish all this using CGImage and CGBitmapContext, using the resultant image to draw to the screen if needed. I've used those APIs and had good results (but I do not know how they compare to your current approach).
Another note: Render at a higher quality for the intermediate, then resize and reduce to 8bpc for the version you write. This will not make a significant difference now, but it will (in most cases) once you introduce filtering.

Answer 2

Finally, one of those "Aha!" moments! I tried using the same code on a high-quality .tif file, and the resultant image was 8 times smaller (in dimensions), rather than than the 50% I'd told it to do. When I tried displaying it would any rescaling of the image, it wound up still 4 times smaller than the original, when it should have displayed at the same height and width. I found out the way I was taking the NSSize from the imported image was wrong. Previously, it read:
NSRect halfSizeRect = NSMakeRect(0, 0, image.size.width * 0.5, image.size.height * 0.5);
Where it should be:
NSBitmapImageRep *imageRep = [NSBitmapImageRep imageRepWithData: [image TIFFRepresentation]];
NSRect halfSizeRect = NSMakeRect(0, 0, [imageRep pixelsWide]/2, [imageRep pixelsHigh]/2);
Apparently it has something to do with DPI and that jazz, so I needed to get the correct size from the BitmapImageRep rather than from image.size. With this change, I was able to save at a quality nearly indistinguishable from Photoshop.