image scaling CGDataProviderCopyData - ios

Question

update 1
I have found a promising apple doc heredescribing CGDataProviderCopyData. I think this does what I originally asked about by taking a drawing from a context and extracting the pixel values.
The example code uses CGImageGetDataProvider and some other features that I do not understand so that I cannot figure out how to implement their functions. How do I take information from the variable con or from its context and get access to the pixels?
update 1
update 0
Maybe I am asking the wrong question here. CGContextDrawImage scales the image from 104 by 104 to 13 by 13 in my case, but then CGContextDrawImage displays the image. Maybe I need to find the part of CGContextDrawImage which just does the scaling.
I have found initWithData:scale: in the "UIImage Class Reference". But I don't know how to supply the data for that method. The scale I want is 0.25 .
- (id)initWithData:(NSData *)data scale:(CGFloat)scale
Can someone tell me how to supply the (NSData *)data for my app?
update 0
//
// BSViewController.m
#import "BSViewController.h"
#interface BSViewController ()
#end
#implementation BSViewController
- (IBAction) chooseImage:(id) sender{
[self.view addSubview:self.imageView];
UIImage* testCard = [UIImage imageNamed:#"ipad 7D.JPG"];
CGSize sz = [testCard size];
CGImageRef num = CGImageCreateWithImageInRect([testCard CGImage],CGRectMake(532, 0, 104, 104));
UIGraphicsBeginImageContext(CGSizeMake( 250,650));
CGContextRef con = UIGraphicsGetCurrentContext();
CGContextDrawImage(con, CGRectMake(0, 0, 13, 13) ,num);
UIGraphicsEndImageContext();
self.workingImage = CFBridgingRelease(num);
CGImageRelease(num);
I am working on the transition from above to below.
More specifically I need to feed imageRef the correct input. I want to give the imageRef a 13 by 13 image, but when I give imageRef num it gets a 104 by 104 image, and when I give imageRef con it gets a 0 by 0 image. (Another tentative approach is mentioned at the bottom.)
The code below is Brandon Trebitowski's
CGImageRef imageRef = num;
NSUInteger width = CGImageGetWidth(imageRef);
NSUInteger height = CGImageGetHeight(imageRef);
NSLog(#"the width: %u", width);
NSLog(#"the height: %u", height);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
unsigned char *rawData = (unsigned char*) calloc(height * width * 4, sizeof(unsigned char));
NSUInteger bytesPerPixel = 4;
NSUInteger bytesPerRow = bytesPerPixel * width;
NSUInteger bitsPerComponent = 8;
CGContextRef context = CGBitmapContextCreate(rawData, width, height,
bitsPerComponent, bytesPerRow, colorSpace,
kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGColorSpaceRelease(colorSpace);
CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
CGContextRelease(context);
NSLog(#"Stop 3");
// Now your rawData contains the image data in the RGBA8888 pixel format.
int byteIndex = (bytesPerRow * 0) + 0 * bytesPerPixel;
for (int ii = 0 ; ii < width * height ; ++ii)
{
int outputColor = (rawData[byteIndex] + rawData[byteIndex+1] + rawData[byteIndex+2]) / 3;
rawData[byteIndex] = (char) (outputColor);
rawData[byteIndex+1] = (char) (outputColor);
rawData[byteIndex+2] = (char) (outputColor);
byteIndex += 4;
}
}
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
}
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
#end
I have also experimented with defining self.workingImage one of the following two ways and supplying that to imageRef.
self.workingImage = num;
self.workingImage = (__bridge UIImage *)(num);
CGImageRef imageRef = [self.workingImage CGImage];

Answer 1

I changed 2 lines and added 3 lines and got the results I wanted.
The main change was to use UIGraphicsBeginImageContextWithOptions instead of UIGraphicsBeginImageContext so that the rescaling could be done before the image was drawn.
#implementation BSViewController
- (IBAction) chooseImage:(id) sender{
[self.view addSubview:self.imageView];
UIImage* testCard = [UIImage imageNamed:#"ipad 7D.JPG"];
CGSize sz = [testCard size];
CGImageRef num = CGImageCreateWithImageInRect([testCard CGImage],CGRectMake(532, 0, 104, 104));
UIGraphicsBeginImageContextWithOptions(CGSizeMake( 104,104), NO, 0.125); // Changed
CGContextRef con = UIGraphicsGetCurrentContext();
CGContextDrawImage(con, CGRectMake(0, 0, 104, 104) ,num); // Changed
UIImage* im = UIGraphicsGetImageFromCurrentImageContext(); // Added
UIGraphicsEndImageContext();
self.workingImage = CFBridgingRelease(num);
CGImageRelease(num);
UIImageView* iv = [[UIImageView alloc] initWithImage:im]; // Added
[self.imageView addSubview: iv]; // Added
CGImageRef imageRef = [im CGImage];
NSUInteger width = CGImageGetWidth(imageRef);
NSUInteger height = CGImageGetHeight(imageRef);
etc.