图片的处理－改尺寸、显圆形、判断图片类型

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//1.改变图像的尺寸，方便上传服务器
- (UIImage ) scaleFromImage: (UIImage ) image toSize: (CGSize) size
{
UIGraphicsBeginImageContext(size);
[image drawInRect:CGRectMake(0, 0, size.width, size.height)];
UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return newImage;
}

//2.保持原来的长宽比，生成一个缩略图
- (UIImage )thumbnailWithImageWithoutScale:(UIImage )image size:(CGSize)asize
{
UIImage *newimage;
if (nil == image) {
newimage = nil;
}
else{
CGSize oldsize = image.size;
CGRect rect;
if (asize.width/asize.height > oldsize.width/oldsize.height) {
rect.size.width = asize.height*oldsize.width/oldsize.height;
rect.size.height = asize.height;
rect.origin.x = (asize.width - rect.size.width)/2;
rect.origin.y = 0;
}
else{
rect.size.width = asize.width;
rect.size.height = asize.width*oldsize.height/oldsize.width;
rect.origin.x = 0;
rect.origin.y = (asize.height - rect.size.height)/2;
}
UIGraphicsBeginImageContext(asize);
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetFillColorWithColor(context, [[UIColor clearColor] CGColor]);
UIRectFill(CGRectMake(0, 0, asize.width, asize.height));//clear background
[image drawInRect:rect];
newimage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
}
return newimage;
}
3.显示圆形头像
[imgeView.layer setCornerRadius:CGRectGetHeight([self.img bounds]) / 2]; //修改半径，实现头像的圆形化
imgeView.layer.masksToBounds = YES;

4判断是否为PNG图片
png／jpg图片十六进制表示形式的前8位不同，可作为区分的标志；png图片前8位包含137 80 78 71 13 10 26 10（UIImagePNGRepresentation和UIImageJPEGRepresentation用来转成png或jpg格式）
static unsigned char kPNGSignatureBytes[8] = {0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A};
static NSData *kPNGSignatureData = nil;

BOOL ImageDataHasPNGPreffix(NSData *data);

BOOL ImageDataHasPNGPreffix(NSData *data) {
NSUInteger pngSignatureLength = [kPNGSignatureData length];
if ([data length] >= pngSignatureLength) {
if ([[data subdataWithRange:NSMakeRange(0, pngSignatureLength)] isEqualToData:kPNGSignatureData]) {
return YES;
}
}