Question

如何在不丢失透明背景的情况下将png图像的文件大小减小到64KB以下，并在Swift中保持512 X 512像素。

我尝试了以下代码：

   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!
}

但是大小不是我期望的。

Answer 1

您应该选中WWDC Session 219 - Image and graphics best practices。它的前三分之一包含有关如何优化图形的有用信息，尤其是内存占用量。

提出的一种方法是下采样技术。这是一个片段：

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)
}

下采样将与下采样的大小变化成比例地减小图像内存大小。此外，在图像质量和内存占用之间总是要权衡取舍，而结果很大程度上取决于压缩算法和压缩设置。

此stackoverflow thread，尤其是this answer提供了一些技巧。

希望有帮助。

Answer 2

我正在压缩为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

减少png图像的文件大小

2 个答案: