DBImageDecoderImageCreation.swift

func createFrameImage(at index: Int, downsamplingLevel: DownSamplingLevel = .default, decodingOptions: DecodingOptions = .default) -> CGImage? {

        guard index < frameCount else {
            return nil
        }

        let image: CGImage?
        let options: CFDictionary

        switch decodingOptions.mode {
            case .asynchronous:
                // No need to consider the down sampling when comparing the image native size with sizeForDrawing.
                guard var size = frameSize(at: index) else {
                    return nil
                }

                if let sizeForDrawing = decodingOptions.sizeForDrawing {
                    // Choose the smaller one.
                    if sizeForDrawing.width * sizeForDrawing.height < size.width * size.height {
                        size = sizeForDrawing
                    }
                }

                options = imageSourceAsyncOptions(sizeForDrawing: size, donwsamplingLevel: downsamplingLevel)
                image = CGImageSourceCreateThumbnailAtIndex(imageSource, index, options)

            case .synchronous:
                options = imageSourceOptions(with: downsamplingLevel)
                image = CGImageSourceCreateImageAtIndex(imageSource, index, options)
        }

        return image
    }