Using bitmapData to modify an image - objective-c

Question

So I've got a method that successfully gets the color of a pixel.
//arrColT is an NSMutableArray<NSColor *>
NSInteger width = [bitIn pixelsWide];
NSInteger height = [bitIn pixelsHigh];
NSInteger rowBytes = [bitIn bytesPerRow];
unsigned char* pixels = [bitIn bitmapData];
int row, col;
//For every row,
for (row = 0; row < height; row++)
{
unsigned char* rowStart = (unsigned char*)(pixels + (row * rowBytes));
unsigned char* nextChannel = rowStart;
//For every pixel in a row,
for (col = 0; col < width; col++)
{
//Get its color.
unsigned char red, green, blue, alpha;
red = *nextChannel;
int intRed = (int)red;
nextChannel++;
green = *nextChannel;
int intGreen = (int)green;
nextChannel++;
blue = *nextChannel;
int intBlue = (int)blue;
nextChannel++;
alpha = *nextChannel;
int intAlpha = (int)alpha;
nextChannel++;
NSColor *colHas = [NSColor colorWithCalibratedRed:(float)intRed/255 green: (float)intGreen/255 blue: (float)intBlue/255 alpha:(float)intAlpha/255];
for (int i = 0; i<[arrColT count]; i++) {
//If the target color is equal to the current color, replace it with the parallel replace color...somehow.
if([colHas isEqualTo:arrColT[i]]){
}
}
}
}
The question is, how do I get color data back into the bitmapData?
With hope,
radzo73

Answer 1

See Technical Q&A QA1509, which basically advises
create pixel buffer (by creating CGContextRef of predefined format and drawing your image to that);
manipulate the bytes within that pixel buffer as you want; and
create resulting image with CGBitmapContextCreateImage.
E.g.
- (UIImage *)convertImage:(UIImage *)image
{
// get image
CGImageRef imageRef = image.CGImage;
// prepare context
CGBitmapInfo bitmapInfo = kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big; // bytes in RGBA order
NSInteger width = CGImageGetWidth(imageRef);
NSInteger height = CGImageGetHeight(imageRef);
CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate(NULL, width, height, 8, 4 * width, colorspace, bitmapInfo);
// draw image to that context
CGRect rect = CGRectMake(0, 0, width, height);
CGContextDrawImage(context, rect, imageRef);
uint8_t *buffer = CGBitmapContextGetData(context);
// ... manipulate pixel buffer bytes here ...
// get image from buffer
CGImageRef outputImage = CGBitmapContextCreateImage(context);
UIImage *result = [UIImage imageWithCGImage:outputImage scale:image.scale orientation:image.imageOrientation];
// clean up
CGImageRelease(outputImage);
CGContextRelease(context);
CGColorSpaceRelease(colorspace);
return result;
}
For example, here is a B&W conversion routine:
/** Convert the image to B&W as a single (non-parallel) task.
*
* This assumes the pixel buffer is in RGBA, 8 bits per pixel format.
*
* #param buffer The pixel buffer.
* #param width The image width in pixels.
* #param height The image height in pixels.
*/
- (void)convertToBWSimpleInBuffer:(uint8_t *)buffer width:(NSInteger)width height:(NSInteger)height
{
for (NSInteger row = 0; row < height; row++) {
for (NSInteger col = 0; col < width; col++) {
NSUInteger offset = (col + row * width) * 4;
uint8_t *pixel = buffer + offset;
// get the channels
uint8_t red = pixel[0];
uint8_t green = pixel[1];
uint8_t blue = pixel[2];
uint8_t alpha = pixel[3];
// update the channels with result
uint8_t gray = 0.2126 * red + 0.7152 * green + 0.0722 * blue;
pixel[0] = gray;
pixel[1] = gray;
pixel[2] = gray;
pixel[3] = alpha;
}
}
}
If you want to improve performance, you can use dispatch_apply and stride through your image buffer in parallel:
/** Convert the image to B&W, using GCD to split the conversion into several concurrent GCD tasks.
*
* This assumes the pixel buffer is in RGBA, 8 bits per pixel format.
*
* #param buffer The pixel buffer.
* #param width The image width in pixels.
* #param height The image height in pixels.
* #param count How many GCD tasks should the conversion be split into.
*/
- (void)convertToBWConcurrentInBuffer:(uint8_t *)buffer width:(NSInteger)width height:(NSInteger)height count:(NSInteger)count
{
dispatch_queue_t queue = dispatch_queue_create("com.domain.app", DISPATCH_QUEUE_CONCURRENT);
NSInteger stride = height / count;
dispatch_apply(height / stride, queue, ^(size_t idx) {
size_t j = idx * stride;
size_t j_stop = MIN(j + stride, height);
for (NSInteger row = j; row < j_stop; row++) {
for (NSInteger col = 0; col < width; col++) {
NSUInteger offset = (col + row * width) * 4;
uint8_t *pixel = buffer + offset;
// get the channels
uint8_t red = pixel[0];
uint8_t green = pixel[1];
uint8_t blue = pixel[2];
uint8_t alpha = pixel[3];
// update the channels with result
uint8_t gray = 0.2126 * red + 0.7152 * green + 0.0722 * blue;
pixel[0] = gray;
pixel[1] = gray;
pixel[2] = gray;
pixel[3] = alpha;
}
}
});
}

Answer 2

Well it depends on what bitIn is, but something like this should do the trick.
First, create a context. You can do this once and keep the context if you are going to make a lot of changes to the data and need a lot of pictures.
self.ctx = CGBitmapContextCreate (
[bitIn bitmapData],
[bitIn pixelsWide],
[bitIn pixelsHigh],
[bitIn bitsPerPixel],
[bitIn bytesPerRow],
[bitIn colorSpace],
[bitIn bitmapInfo] );
Here I freely used bitIn hoping it will provide all the missing pieces. Then you can assemble it all later, even after further changes to the data, using something like
CGImageRef cgImg = CGBitmapContextCreateImage ( self.ctx );
UIImage * img = [UIImage imageWithCGImage:cgImg];
CGImageRelease ( cgImg );
Now you can make more changes to the data and do the same to get a new image.