In the project, I need to convert data between OpenCV image class and iOS image class. The best way to do it is via Objective-C category methods. Objective-C categories provide a means to add methods to a class, and any methods that we add through a category become part of the class definition.
So we can have an UIImage-OpenCVExtensions.h file
#import <UIKit/UIKit.h>
#import "opencv2/opencv.hpp"
@interface UIImage (OpenCVExtensions)
// Creates an IplImage in gray, BGR or BGRA format. It is the caller's responsibility to cvReleaseImage() the return value.
- (IplImage *)createIplImageWithNumberOfChannels:(int)channels;
// Returns a UIImage by copying the IplImage's bitmap data.
- (id)initWithIplImage:(IplImage *)iplImage;
- (id)initWithIplImage:(IplImage *)iplImage orientation:(UIImageOrientation)orientation;
// Returns an affine transform that takes into account the image orientation when drawing a scaled image
- (CGAffineTransform)transformForOrientationDrawnTransposed:(BOOL *)drawTransposed;
@end
And the implementation is like this:
#import "UIImage-OpenCVExtensions.h"
static inline void premultiplyImage(IplImage *img, BOOL reverse);
static void releaseImage(void *info, const void *data, size_t size);
@implementation UIImage (OpenCVExtensions)
- (IplImage *)createIplImageWithNumberOfChannels:(int)channels
{
NSAssert(channels == 1 || channels == 3 || channels == 4, @"Invalid number of channels");
CGImageRef cgImage = [self CGImage];
BOOL drawTransposed;
CGAffineTransform transform = [self transformForOrientationDrawnTransposed:&drawTransposed];
CvSize cvsize = cvSize(drawTransposed ? CGImageGetHeight(cgImage) : CGImageGetWidth(cgImage),
drawTransposed ? CGImageGetWidth(cgImage) : CGImageGetHeight(cgImage));
IplImage *iplImage = cvCreateImage(cvsize, IPL_DEPTH_8U, (channels == 3) ? 4 : channels); // CG can only write into 4 byte aligned bitmaps
CGBitmapInfo bitmapInfo = kCGImageAlphaNone;
if (channels == 3) {
bitmapInfo = kCGImageAlphaNoneSkipFirst | kCGBitmapByteOrder32Little; // BGRX. CV_BGRA2BGR will discard the uninitialized alpha channel data.
} else if (channels == 4) {
bitmapInfo = kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little; // BGRA. Must unpremultiply the image.
}
CGColorSpaceRef colorSpace = (channels == 1) ? CGColorSpaceCreateDeviceGray() : CGColorSpaceCreateDeviceRGB();
CGContextRef bitmapContext = CGBitmapContextCreate(iplImage->imageData,
iplImage->width,
iplImage->height,
iplImage->depth,
iplImage->widthStep,
colorSpace,
bitmapInfo);
CGColorSpaceRelease(colorSpace);
// Rotate and/or flip the image if required by its orientation
CGContextConcatCTM(bitmapContext, transform);
// Copy the source bitmap into the destination, ignoring any data in the uninitialized destination
CGContextSetBlendMode(bitmapContext, kCGBlendModeCopy);
// Drawing CGImage to CGContext
CGRect rect = CGRectMake(0.0, 0.0, CGImageGetWidth(cgImage), CGImageGetHeight(cgImage));
CGContextDrawImage(bitmapContext, rect, cgImage);
CGContextRelease(bitmapContext);
// Unpremultiply the alpha channel if the source image had one (since otherwise the alphas are 1)
CGImageAlphaInfo alphaInfo = CGImageGetAlphaInfo(cgImage);
if (channels == 4 && (alphaInfo != kCGImageAlphaNone && alphaInfo != kCGImageAlphaNoneSkipFirst && alphaInfo != kCGImageAlphaNoneSkipLast)) {
premultiplyImage(iplImage, YES);
}
// Convert BGRA images to BGR
if (channels == 3) {
IplImage *temp = cvCreateImage(cvGetSize(iplImage), IPL_DEPTH_8U, channels);
cvCvtColor(iplImage, temp, CV_BGRA2BGR);
cvReleaseImage(&iplImage);
iplImage = temp;
}
return iplImage;
}
- (id)initWithIplImage:(IplImage *)iplImage
{
return [self initWithIplImage:iplImage orientation:UIImageOrientationUp];
}
- (id)initWithIplImage:(IplImage *)iplImage orientation:(UIImageOrientation)orientation
{
// CGImage requries either 8-bit or 32-bit aligned images
IplImage *formattedImage;
if (iplImage->nChannels == 3) {
formattedImage = cvCreateImage(cvGetSize(iplImage), IPL_DEPTH_8U, 4);
cvCvtColor(iplImage, formattedImage, CV_BGR2BGRA);
} else if (iplImage->nChannels == 4) {
formattedImage = cvCloneImage(iplImage);
premultiplyImage(formattedImage, NO);
} else {
formattedImage = cvCloneImage(iplImage);
}
CGDataProviderRef provider = CGDataProviderCreateWithData(formattedImage, formattedImage->imageData, formattedImage->imageSize, releaseImage);
CGBitmapInfo bitmapInfo = (iplImage->nChannels == 1) ? kCGImageAlphaNone : (kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little);
CGColorSpaceRef colorSpace = (formattedImage->nChannels == 1) ? CGColorSpaceCreateDeviceGray() : CGColorSpaceCreateDeviceRGB();
CGImageRef cgImage = CGImageCreate(formattedImage->width,
formattedImage->height,
formattedImage->depth,
formattedImage->depth * formattedImage->nChannels,
formattedImage->widthStep,
colorSpace,
bitmapInfo,
provider,
NULL,
false,
kCGRenderingIntentDefault);
CGColorSpaceRelease(colorSpace);
CGDataProviderRelease(provider);
self = [self initWithCGImage:cgImage scale:1.0 orientation:orientation];
CGImageRelease(cgImage);
return self;
}
static inline void premultiplyImage(IplImage *img, BOOL reverse)
{
NSCAssert(img->depth == IPL_DEPTH_8U, @"depth not IPL_DEPTH_8U");
uchar *row = (uchar *)img->imageData;
for (int i = 0; i < img->height; i++) {
for (int j = 0; j < img->width; j+= img->nChannels) {
uchar alpha = row[j + 3];
if (alpha != UCHAR_MAX && (!reverse || alpha != 0)) {
for (int k = 0; k < 3; k++) {
if (reverse) {
row[j + k] = ((int)row[j + k] * UCHAR_MAX + alpha / 2 - 1) / alpha;
} else {
row[j + k] = ((int)row[j + k] * alpha + UCHAR_MAX / 2 - 1) / UCHAR_MAX;
}
}
}
}
row += img->widthStep;
}
}
static void releaseImage(void *info, const void *data, size_t size)
{
IplImage *image = (IplImage *)info;
cvReleaseImage(&image);
}
- (CGAffineTransform)transformForOrientationDrawnTransposed:(BOOL *)drawTransposed
{
UIImageOrientation imageOrientation = [self imageOrientation];
CGAffineTransform transform = CGAffineTransformIdentity;
CGSize size = [self size]; // already transposed by UIImage
switch (imageOrientation) {
case UIImageOrientationDown: // EXIF orientation 3
case UIImageOrientationDownMirrored: // EXIF orientation 4
transform = CGAffineTransformTranslate(transform, size.width, size.height);
transform = CGAffineTransformRotate(transform, M_PI);
break;
case UIImageOrientationLeft: // EXIF orientation 6
case UIImageOrientationLeftMirrored: // EXIF orientation 5
transform = CGAffineTransformTranslate(transform, size.width, 0);
transform = CGAffineTransformRotate(transform, M_PI_2);
break;
case UIImageOrientationRight: // EXIF orientation 8
case UIImageOrientationRightMirrored: // EXIF orientation 7
transform = CGAffineTransformTranslate(transform, 0, size.height);
transform = CGAffineTransformRotate(transform, -M_PI_2);
break;
default:
break;
}
switch (imageOrientation) {
case UIImageOrientationUpMirrored: // EXIF orientation 2
case UIImageOrientationDownMirrored: // EXIF orientation 4
transform = CGAffineTransformTranslate(transform, size.width, 0);
transform = CGAffineTransformScale(transform, -1.0, 1.0);
break;
case UIImageOrientationLeftMirrored: // EXIF orientation 5
case UIImageOrientationRightMirrored: // EXIF orientation 7
transform = CGAffineTransformTranslate(transform, size.height, 0);
transform = CGAffineTransformScale(transform, -1.0, 1.0);
break;
default:
break;
}
if (drawTransposed) {
switch (imageOrientation) {
case UIImageOrientationLeft:
case UIImageOrientationLeftMirrored:
case UIImageOrientationRight:
case UIImageOrientationRightMirrored:
*drawTransposed = YES;
break;
default:
*drawTransposed = NO;
}
}
return transform;
}
@end
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