@FrankSchlegel thank you for ur answer. I just setting up CGContext like this. even I just put Device colorspace, It's same wrong result image. here's the code. (and I try to edit my post but I can only edit my reply ;;) (UIImage *)resizeImage:(UIImage *)originalImage size:(CGSize)size {   @autoreleasepool {     CGContextRef context=nil;     CGColorSpaceRef colorSpace=nil;     CGSize ceiledSize = CGSizeMake(ceil(size.width), ceil(size.height));     colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceSRGB);     CGFloat pixelsWidth = ceiledSize.width;     CGFloat pixelsHeight = ceiledSize.height;     CGFloat bitmapBytesPerRow  = (pixelsWidth * 4);// 1     CGFloat bitmapByteCount   = (bitmapBytesPerRow * pixelsHeight);     uint8_t *bitmapData = calloc( bitmapByteCount, sizeof(uint8_t));     context = CGBitmapContextCreate(bitmapData, ceiledSize.width, ceiledSize.height, 8, bitmapBytesPerRow,                     colorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrderDefault);     CGContextDrawImage(context, CGRectMake(0.0, 0.0, ceiledSize.width, ceiledSize.height), originalImage.CGImage);     CGImageRef cgImage = CGBitmapContextCreateImage(context);     UIImage *resultImage = [[UIImage alloc] initWithCGImage:cgImage];     CGImageRelease(cgImage);     CGContextRelease(context);     CGColorSpaceRelease(colorSpace);     free(bitmapData);     return resultImage;   } }

I try to use vImage functions too. but the result image changed color. (UIImage *)resizeImage:(UIImage *)originalImage size:(CGSize)size { //  CGColorSpaceRef colorspace = CGImageGetColorSpace(originalImage.CGImage); //  CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();   CGColorSpaceRef colorspace = CGColorSpaceCreateWithName(kCGColorSpaceSRGB);       vImage_CGImageFormat format;   format.bitsPerComponent = 8;   format.bitsPerPixel = 32;   format.bitmapInfo = (CGBitmapInfo)(kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrderDefault); //ARGB //  format.bitmapInfo = (CGBitmapInfo)(kCGImageAlphaPremultipliedLast | kCGBitmapByteOrderDefault); //RGBA   format.decode = nil;   format.colorSpace = colorspace;   format.renderingIntent = kCGRenderingIntentDefault;       CGSize ceiledSize = CGSizeMake(ceil(size.width), ceil(size.height));   vImage_Error error = kvImageNoError;         // Create source buffer   vImage_Buffer sourceBuffer;   error = vImageBuffer_InitWithCGImage(&sourceBuffer, &format, nil, originalImage.CGImage, kvImageNoFlags);   if(error != kvImageNoError) {     NSLog(@"ERROR: vImage SOURCE IMAGE INIT");     return NULL;   }       // Create target Buffer   vImage_Buffer targetBuffer;   error = vImageBuffer_Init(&targetBuffer, ceiledSize.height, ceiledSize.width, format.bitsPerPixel, kvImageNoFlags);   if(error != kvImageNoError) {     NSLog(@"ERROR: vImage TARGET IMAGE INIT");     return NULL;   }       // Scale Image   error = vImageScale_ARGB8888(&sourceBuffer, &targetBuffer, nil, kvImageHighQualityResampling);   if(error != kvImageNoError) {     NSLog(@"ERROR: vImage SCALE");     return NULL;   }       // Create Image   CGImageRef targetCGImage = vImageCreateCGImageFromBuffer(&targetBuffer, &format, nil, nil, kvImageNoFlags, &error);   if(error != kvImageNoError) {     NSLog(@"ERROR: vImage CREATE CGImage");     return NULL;   }       UIImage *resizedImage = [UIImage imageWithCGImage:targetCGImage];       free(sourceBuffer.data);   free(targetBuffer.data);   CGColorSpaceRelease(colorspace);       return resizedImage; }

and I just tested that u recommand me. unfortunately, It doesn't work too. if I use something wrong, plz tell me (UIImage *)resizeImage:(UIImage *)originalImage size:(CGSize)size {   CGSize ceiledSize = CGSizeMake(ceil(size.width), ceil(size.height));       UIGraphicsImageRendererFormat *format = [[UIGraphicsImageRendererFormat alloc] init];   format.scale = 0.5;       UIGraphicsImageRenderer *renderer = [[UIGraphicsImageRenderer alloc] initWithSize:ceiledSize format:format];   UIImage *resultImage = [renderer imageWithActions:^(UIGraphicsImageRendererContext * _Nonnull rendererContext) {     CGContextRef context = UIGraphicsGetCurrentContext();     CGContextDrawImage(context, CGRectMake(0.0, 0.0, ceiledSize.width, ceiledSize.height), originalImage.CGImage);   }];       CGContextRelease(context);   CGColorSpaceRelease(colorSpace);       return resultImage; }