iphone ios 如何创建缩率图

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两篇博文

1。How to create square thumbnails using iPhone SDK / CG Quartz 2D

http://www.nickkuh.com/iphone/how-to-create-square-thumbnails-using-iphone-sdk-cg-quartz-2d/2010/03/

2。iPhone: Creating an image thumbnail

http://johnnytrops.com/blog/wp/2009/02/03/iphone-creating-an-image-thumbnail/

3.Resize a UIImage the right way

http://vocaro.com/trevor/blog/2009/10/12/resize-a-uiimage-the-right-way/

4。如何获取视频的缩率图

http://stackoverflow.com/questions/10209012/grabbing-first-frame-of-a-video-thumbnail-resolution-iphone

1.方法一函数

- (UIImage *)thumbWithSideOfLength:(float)length {NSString *subdir = @”my/images/directory”;NSString *filename = @”myOriginalImage.png”;NSString *fullPathToThumbImage = [subdir stringByAppendingPathComponent:[NSString stringWithFormat:@"%dx%d%@",(int) length, (int) length,filename];NSString *fullPathToMainImage = [subdir stringByAppendingPathComponent:filename];UIImage *thumbnail;NSFileManager *fileManager = [NSFileManager defaultManager];if ([fileManager fileExistsAtPath:fullPathToThumbImage] == YES) {thumbnail = [UIImage imageWithContentsOfFile:fullPathToThumbImage];}else {//couldn’t find a previously created thumb image so create one first…UIImage *mainImage = [UIImage imageWithContentsOfFile:fullPathToMainImage];UIImageView *mainImageView = [[UIImageView alloc] initWithImage:mainImage];BOOL widthGreaterThanHeight = (mainImage.size.width > mainImage.size.height);float sideFull = (widthGreaterThanHeight) ? mainImage.size.height : mainImage.size.width;CGRect clippedRect = CGRectMake(0, 0, sideFull, sideFull);//creating a square context the size of the final image which we will then// manipulate and transform before drawing in the original imageUIGraphicsBeginImageContext(CGSizeMake(length, length));CGContextRef currentContext = UIGraphicsGetCurrentContext();CGContextClipToRect( currentContext, clippedRect);CGFloat scaleFactor = length/sideFull;if (widthGreaterThanHeight) {//a landscape image – make context shift the original image to the left when drawn into the contextCGContextTranslateCTM(currentContext, -((mainImage.size.width – sideFull) / 2) * scaleFactor, 0);}else {//a portfolio image – make context shift the original image upwards when drawn into the contextCGContextTranslateCTM(currentContext, 0, -((mainImage.size.height – sideFull) / 2) * scaleFactor);}//this will automatically scale any CGImage down/up to the required thumbnail side (length) when the CGImage gets drawn into the context on the next line of codeCGContextScaleCTM(currentContext, scaleFactor, scaleFactor);[mainImageView.layer renderInContext:currentContext];thumbnail = UIGraphicsGetImageFromCurrentImageContext();UIGraphicsEndImageContext();NSData *imageData = UIImagePNGRepresentation(thumbnail);[imageData writeToFile:fullPathToThumbImage atomically:YES];thumbnail = [UIImage imageWithContentsOfFile:fullPathToThumbImage];}return thumbnail;}

方法2函数

-(void)imagePickerController:(UIImagePickerController *)picker didFinishPickingImage:(UIImage *)image editingInfo:(NSDictionary *)editingInfo{ // set the image we have chosen to the background of the current view. (OPTIONAL!)imageView.image        = image; // write the file to the TEMP Directory in the iphone, then when the user has completed// selecting their image from the picker, we want to write both the full sized file, and the// thumbnail to the /tmp directory for use later (whatever you want) NSData *pngImage = UIImagePNGRepresentation(image);if([pngImage writeToFile:[NSString stringWithFormat:@"%@/tempImage.png", TEMP_FOLDER] atomically:YES]){ // now that we have saved our image, lets generate a thumbnail from that image, and// save that one as well. // lets essentially make a copy of the selected image.UIImage *myThumbNail    = [[UIImage alloc] initWithData:pngImage]; // begin an image context that will essentially "hold" our new imageUIGraphicsBeginImageContext(CGSizeMake(60.0,60.0)); // now redraw our image in a smaller rectangle.[myThumbNail drawInRect:CGRectMake(0.0, 0.0, 60.0, 60.0)];//[myThumbNail release]; // make a "copy" of the image from the current contextUIImage *newImage    = UIGraphicsGetImageFromCurrentImageContext();UIGraphicsEndImageContext(); // create a new view to place on our screen (OPTIONAL-- testing)UIImageView *thumbView    = [[UIImageView alloc] initWithFrame:CGRectMake(0.0, 0.0, 60.0, 60.0)];thumbView.image    = newImage;[self.view addSubview:thumbView]; // now grab the PNG representation of our imageNSData    *thumbData    = UIImagePNGRepresentation(newImage); // now finally we can write our new thumbnail to the tmp directory on the phone.if([thumbData writeToFile:[NSString stringWithFormat:@"%@/tempImageThumb.png", TEMP_FOLDER] atomically:YES]){NSLog(@"thumb written!");} // close the modal view[picker dismissModalViewControllerAnimated:YES]; }else{// there was an error writing the image file to the tmp folder....NSLog(@" there was an error writing the file to the temp directory");}}