ios - 在 iOS 中将 UIImage 读入缓冲区并写回 UIImage 时，原始文件和输出文件不匹配

Question

使用下面的代码，我从 ALAsset 获取 UIImage

  ALAssetRepresentation * assetRepresentation = [asset defaultRepresentation]; // Big Picture
                imageRef = [assetRepresentation fullResolutionImage];

                if (imageRef)
                {
                    UIImage* image = [UIImage imageWithCGImage: imageRef];
                }

现在我使用下面的代码从这个 UIImage 创建一个缓冲区:

  + (unsigned char *) convertUIImageToBitmapRGBA8:(UIImage *) image {

    CGImageRef imageRef = image.CGImage;

    // Create a bitmap context to draw the uiimage into
    CGContextRef context = [self newBitmapRGBA8ContextFromImage:imageRef];

    if(!context) {
        return NULL;
    }

    size_t width = CGImageGetWidth(imageRef);
    size_t height = CGImageGetHeight(imageRef);

    CGRect rect = CGRectMake(0, 0, width, height);

    // Draw image into the context to get the raw image data
    CGContextDrawImage(context, rect, imageRef);

    // Get a pointer to the data
    unsigned char *bitmapData = (unsigned char *)CGBitmapContextGetData(context);

    // Copy the data and release the memory (return memory allocated with new)
    size_t bytesPerRow = CGBitmapContextGetBytesPerRow(context);
    size_t bufferLength = bytesPerRow * height;

    unsigned char *newBitmap = NULL;

    if(bitmapData) {
        newBitmap = (unsigned char *)malloc(sizeof(unsigned char) * bytesPerRow * height);

        if(newBitmap) { // Copy the data
            for(int i = 0; i < bufferLength; ++i) {
                newBitmap[i] = bitmapData[i];
            }
        }

        free(bitmapData);

    } else {
        NSLog(@"Error getting bitmap pixel data\n");
    }

    CGContextRelease(context);

    return newBitmap;
}

现在，我使用以下代码将缓冲区转换回 UIImage:

 + (UIImage *) convertBitmapRGBA8ToUIImage:(unsigned char *) buffer withWidth:(int) width withHeight:(int) height
{
    size_t bufferLength = width * height * 4;
    CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, buffer, bufferLength, NULL);
    size_t bitsPerComponent = 8;
    size_t bitsPerPixel = 32;
    size_t bytesPerRow = 4 * width;

    CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();

    if(colorSpaceRef == NULL) {
        NSLog(@"Error allocating color space");
        CGDataProviderRelease(provider);
        return nil;
    }
    CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault | kCGImageAlphaPremultipliedLast;
    CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;

    CGImageRef iref = CGImageCreate(width,
                                    height,
                                    bitsPerComponent,
                                    bitsPerPixel,
                                    bytesPerRow,
                                    colorSpaceRef,
                                    bitmapInfo,
                                    provider,        // data provider
                                    NULL,                // decode
                                    YES,                        // should interpolate
                                    renderingIntent);

    uint32_t* pixels = (uint32_t*)malloc(bufferLength);

    if(pixels == NULL) {
        NSLog(@"Error: Memory not allocated for bitmap");
        CGDataProviderRelease(provider);
        CGColorSpaceRelease(colorSpaceRef);
        CGImageRelease(iref);
        return nil;
    }

    CGContextRef context = CGBitmapContextCreate(pixels,
                                                 width,
                                                 height,
                                                 bitsPerComponent,
                                                 bytesPerRow,
                                                 colorSpaceRef,
                                                 bitmapInfo);

    if(context == NULL) {
        NSLog(@"Error context not created");
        free(pixels);
    }

    UIImage *image = nil;

    if(context)
    {

        CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, width, height), iref);

        CGImageRef imageRef = CGBitmapContextCreateImage(context);

        // Support both iPad 3.2 and iPhone 4 Retina displays with the correct scale
        if([UIImage respondsToSelector:@selector(imageWithCGImage:scale:orientation:)]) {
            float scale = [[UIScreen mainScreen] scale];
            image = [UIImage imageWithCGImage:imageRef scale:scale orientation:UIImageOrientationUp];
        } else {
            image = [UIImage imageWithCGImage:imageRef];
        }

        CGImageRelease(imageRef);
        CGContextRelease(context);
    }

    CGColorSpaceRelease(colorSpaceRef);
    CGImageRelease(iref);
    CGDataProviderRelease(provider);

    if(pixels) {
        free(pixels);
    }
    return image;
}

+ (CGContextRef) newBitmapRGBA8ContextFromImage:(CGImageRef) image {
    CGContextRef context = NULL;
    CGColorSpaceRef colorSpace;
    uint32_t *bitmapData;

    size_t bitsPerPixel = 32;
    size_t bitsPerComponent = 8;
    size_t bytesPerPixel = bitsPerPixel / bitsPerComponent;

    size_t width = CGImageGetWidth(image);
    size_t height = CGImageGetHeight(image);

    size_t bytesPerRow = width * bytesPerPixel;
    size_t bufferLength = bytesPerRow * height;

    colorSpace = CGColorSpaceCreateDeviceRGB();

    if(!colorSpace) {
        NSLog(@"Error allocating color space RGB\n");
        return NULL;
    }

    // Allocate memory for image data
    bitmapData = (uint32_t *)malloc(bufferLength);

    if(!bitmapData) {
        NSLog(@"Error allocating memory for bitmap\n");
        CGColorSpaceRelease(colorSpace);
        return NULL;
    }

    //Create bitmap context

    context = CGBitmapContextCreate(bitmapData,
                                    width,
                                    height,
                                    bitsPerComponent,
                                    bytesPerRow,
                                    colorSpace,
                                    kCGImageAlphaPremultipliedLast);    // RGBA
    if(!context) {
        free(bitmapData);
        NSLog(@"Bitmap context not created");
    }

    CGColorSpaceRelease(colorSpace);

    return context; 
}

然后..保存到相册:

 +(void) saveImageToPhotoAlbum : (UIImage*) image
{
    if( image != nil)
    {
        NSData* imageData =  UIImagePNGRepresentation(image);     // get png representation
        UIImage* pngImage = [UIImage imageWithData:imageData];

        UIImageWriteToSavedPhotosAlbum(pngImage, self, nil, nil);
    }
    else
    {
        NSLog(@"Couldn't save to Photo Album due to invalid image..");
    }
 }

我的通话如下

void *imageData = [self convertUIImageToBitmapRGBA8 : image];



UIImage* uiImage = [self convertBitmapRGBA8ToUIImage : imageData withWidth:width withHeight: height];

[self saveImageToPhotoAlbum:uiImage];

当我这样做时..文件大小发生变化并且两个文件似乎不一样..

e,g;如果原始文件大小为 33KB，经过此过程后，它会更改为 332KB.. 这里出了什么问题？

Answer 1

是的，将图像加载到 UIImage 中然后调用 UIImagePNGRepresentation(甚至 UIImageJPEGRepresentation)可能会导致文件大小发生变化。从文件大小猜测，原始图像看起来像是 JPEG。 PNG 文件通常比压缩后的 JPEG 文件大，即使压缩程度适中也是如此。如果您想要大小相当的文件，请尝试使用各种质量设置(例如 compressionQuality 为 0.99 或 0.9 或 0.8)的 UIImageJPEGRepresentation。

顺便说一句，只是将图像加载到 UIImage 中的练习可能会导致原始 Assets 发生一些变化(如果没有别的，剥离元数据，可能会改变颜色空间等) .).