减少png图像的文件大小

Question

How can I reduce the file size of png image to less than 64KB and without losing the transparent background and maintain 512 X 512 pixels in Swift.

I have tried this code:

   func resizeImageOriginalSize(targetSize: CGSize) -> UIImage {
    var actualHeight: Float = Float(self.size.height)
    var actualWidth: Float = Float(self.size.width)
    let maxHeight: Float = Float(targetSize.height)
    let maxWidth: Float = Float(targetSize.width)
    var imgRatio: Float = actualWidth / actualHeight
    let maxRatio: Float = maxWidth / maxHeight
    var compressionQuality: Float = 0.5
    //50 percent compression

    if actualHeight > maxHeight || actualWidth > maxWidth {
        if imgRatio < maxRatio {
            //adjust width according to maxHeight
            imgRatio = maxHeight / actualHeight
            actualWidth = imgRatio * actualWidth
            actualHeight = maxHeight
        }
        else if imgRatio > maxRatio {
            //adjust height according to maxWidth
            imgRatio = maxWidth / actualWidth
            actualHeight = imgRatio * actualHeight
            actualWidth = maxWidth
        }
        else {
            actualHeight = maxHeight
            actualWidth = maxWidth
            compressionQuality = 0.4
        }
    }
    let rect = CGRect(x: 0.0, y: 0.0, width: CGFloat(actualWidth), height: CGFloat(actualHeight))
    UIGraphicsBeginImageContextWithOptions(rect.size, false, CGFloat(compressionQuality))
    self.draw(in: rect)
    let newImage = UIGraphicsGetImageFromCurrentImageContext()



    UIGraphicsEndImageContext()
    return newImage!
}

but size is not what I expected.

Answer 1

You should check WWDC Session 219 - Image and graphics best practices . First third of it contains useful information on how to optimize graphics, particularly memory footprint.

One of the ways proposed is the Downsampling technique. Here is a snippet:

func downsample(imageAt imageURL: URL, to pointSize: CGSize, scale: CGFloat) -> UIImage {
    let imageSourceOptions = [kCGImageSourceShouldCache: false] as CFDictionary
    let imageSource = CGImageSourceCreateWithURL(imageURL as CFURL, imageSourceOptions)!
    let maxDimensionInPixels = max(pointSize.width, pointSize.height) * scale
    let downsampleOptions =
            [kCGImageSourceCreateThumbnailFromImageAlways: true,
            kCGImageSourceShouldCacheImmediately: true,
            kCGImageSourceCreateThumbnailWithTransform: true,
            kCGImageSourceThumbnailMaxPixelSize: maxDimensionInPixels] as CFDictionary
    let downsampledImage = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, downsampleOptions)!
    return UIImage(cgImage: downsampledImage)
}

Downsampling will reduce image memory size proportionally to the downsampled size change. Further there is always a tradeoff between image quality and memory footprint and the result highly depends on compression algorithm and compression settings.

This stackoverflow thread , and especially this answer provides some tips.

Hope it helps.

Answer 2

我正在压缩到WebP formate，与创建PNG相比，它提供的文件大小比PNG小3倍左右，因为我要创建同时也接受WebP的WhatsApp贴纸包