在項目中經常遇到要上傳圖片，如果直接上傳，那么會上傳比較大的圖片，導致費流量，刷新時加載圖片時間過長，手機內存占用率高等問題。
一、先來介紹下概念：
圖片的壓縮其實是倆概念，
1、是 “壓” 文件體積變小，但是像素數不變，長寬尺寸不變，那么質量可能下降，
2、是 “縮” 文件的尺寸變小，也就是像素數減少。長寬尺寸變小，文件體積同樣會減小。
二、解決方法（以上傳頭像為例），先縮再壓：
2.1 矯正圖片方向（照片是有方向的，避免出現“倒立”的情況）
#pragma mark -矯正手機相冊里圖像的方向
- (UIImage*)fixOrientation:(UIImage*)aImage {
// No-op if the orientation is already correct
if(aImage.imageOrientation==UIImageOrientationUp)
returnaImage;
// We need to calculate the proper transformation to make the image upright.
// We do it in 2 steps: Rotate if Left/Right/Down, and then flip if Mirrored.
CGAffineTransformtransform =CGAffineTransformIdentity;
switch(aImage.imageOrientation) {
caseUIImageOrientationDown:
caseUIImageOrientationDownMirrored:
transform =CGAffineTransformTranslate(transform, aImage.size.width, aImage.size.height);
transform =CGAffineTransformRotate(transform,M_PI);
break;
caseUIImageOrientationLeft:
caseUIImageOrientationLeftMirrored:
transform =CGAffineTransformTranslate(transform, aImage.size.width,0);
transform =CGAffineTransformRotate(transform,M_PI_2);
break;
caseUIImageOrientationRight:
caseUIImageOrientationRightMirrored:
transform =CGAffineTransformTranslate(transform,0, aImage.size.height);
transform =CGAffineTransformRotate(transform, -M_PI_2);
break;
default:
break;
}
switch(aImage.imageOrientation) {
caseUIImageOrientationUpMirrored:
caseUIImageOrientationDownMirrored:
transform =CGAffineTransformTranslate(transform, aImage.size.width,0);
transform =CGAffineTransformScale(transform, -1,1);
break;
caseUIImageOrientationLeftMirrored:
caseUIImageOrientationRightMirrored:
transform =CGAffineTransformTranslate(transform, aImage.size.height,0);
transform =CGAffineTransformScale(transform, -1,1);
break;
default:
break;
}
// Now we draw the underlying CGImage into a new context, applying the transform
// calculated above.
CGContextRefctx =CGBitmapContextCreate(NULL, aImage.size.width, aImage.size.height,
CGImageGetBitsPerComponent(aImage.CGImage),0,
CGImageGetColorSpace(aImage.CGImage),
CGImageGetBitmapInfo(aImage.CGImage));
CGContextConcatCTM(ctx, transform);
switch(aImage.imageOrientation) {
caseUIImageOrientationLeft:
caseUIImageOrientationLeftMirrored:
caseUIImageOrientationRight:
caseUIImageOrientationRightMirrored:
CGContextDrawImage(ctx,CGRectMake(0,0,aImage.size.height,aImage.size.width), aImage.CGImage);
break;
default:
CGContextDrawImage(ctx,CGRectMake(0,0,aImage.size.width,aImage.size.height), aImage.CGImage);
break;
}
CGImageRef cgimg =CGBitmapContextCreateImage(ctx);
UIImage *img = [UIImageimageWithCGImage:cgimg];
CGContextRelease(ctx);
CGImageRelease(cgimg);
return img;
}
2.2 拿到上面矯正過的圖片，縮小圖片尺寸，調用下面方法傳入newSize，如（200，200）：
//縮小圖片尺寸
+ (UIImage*)imageWithImageSimple:(UIImage*)image scaledToSize:(CGSize)newSize
{
UIGraphicsBeginImageContext(newSize);
[imagedrawInRect:CGRectMake(0,0,newSize.width,newSize.height)];
UIImage* newImage =UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return newImage;
}
2.3 將2.2的圖片再壓，這個方法可以重復壓
//調整大小
NSData *imageData =UIImageJPEGRepresentation(newImage,rate);
NSUIntegersizeOrigin = [image Datalength];//多少KB
NSUIntegersizeOriginKB = sizeOrigin /1024;//多少KB
2.4 上傳頭像
調用后臺接口，把imageData二進制數據上傳即可
總結：對圖片壓縮處理時，在保證圖片清晰度變化不大時，減小圖片文件大小。方法2.2中的newSize 和 2.3中的rate要以實際效果來設置，我在自己項目中上傳的頭像最終尺寸是200*200像素，大小為4KB左右。
