图片的处理－改尺寸、显圆形、判断图片类型

//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;

}

}

return NO;

}

kPNGSignatureData = [NSData dataWithBytes:kPNGSignatureBytes length:8];