Resizing Large Resolution Images Producing 1000x1000 Pixels Size when Size is set to 500x500 Pixels

问题

Im using the following extension method to resize an image.When it comes to large resolution images the output size remains 1000x1000 pixels even when I set the output size to 500x500 pixels

extension NSImage {
    func resizeImage(width: CGFloat, _ height: CGFloat) -> NSImage {
        let img = NSImage(size: CGSize(width:width, height:height))

        img.lockFocus()
        let ctx = NSGraphicsContext.current
        ctx?.imageInterpolation = .high
        self.draw(in: NSMakeRect(0, 0, width, height), from: NSMakeRect(0, 0, size.width, size.height), operation: .copy, fraction: 1)
        img.unlockFocus()

        return img
    }

What im I doing wrong? Please advice

UPDATE:

//SAVING 



for x in fileArray  {


            var image = NSImage(contentsOf:x)!
            let imageURL=outdir+"/"+"xxx"
            image=image.resizeImage(width: CGFloat(rwidth), CGFloat(rheight))
            saveimage(xdata: image, imageURL: imageURL, format: fileformat)

            }

func saveimage(xdata:NSImage,imageURL:String,format:String) -> Bool
    {


        let bMImg = NSBitmapImageRep(data: (xdata.tiffRepresentation)!)
        switch format {
        case ".png":
            let filepath=URL(fileURLWithPath: imageURL+".png")
            let dataToSave = bMImg?.representation(using: NSBitmapImageRep.FileType.png, properties: [NSBitmapImageRep.PropertyKey.compressionFactor : 1])
            do
            {
                try  dataToSave?.write(to: filepath)
                return true

            } catch
            {
                return false
            }
        case ".jpg":
            let filepath=URL(fileURLWithPath: imageURL+".jpg")
             let dataToSave = bMImg?.representation(using: NSBitmapImageRep.FileType.jpeg, properties: [NSBitmapImageRep.PropertyKey.compressionFactor : 1])
            do
            {
                try  dataToSave?.write(to:filepath)
                return true

            } catch
            {
               return false
            }
        case ".tif":
            let filepath=URL(fileURLWithPath: imageURL+".tif")
            let dataToSave = bMImg?.representation(using: NSBitmapImageRep.FileType.tiff, properties: [NSBitmapImageRep.PropertyKey.compressionFactor : 1])
            do
            {
                try  dataToSave?.write(to:filepath)
                return true

            } catch
            {
                return false
            }
        case ".bmp":
            let filepath=URL(fileURLWithPath: imageURL+".bmp")
            let dataToSave = bMImg?.representation(using: NSBitmapImageRep.FileType.bmp, properties: [NSBitmapImageRep.PropertyKey.compressionFactor : 1])
            do
            {
                try  dataToSave?.write(to:filepath)
                return true

            } catch
            {
                return false
            }
        case ".gif":
             let filepath=URL(fileURLWithPath: imageURL+".gif")
            let dataToSave = bMImg?.representation(using: NSBitmapImageRep.FileType.gif, properties: [NSBitmapImageRep.PropertyKey.compressionFactor : 1])
            do
            {
                try  dataToSave?.write(to:filepath)
                return true

            } catch
            {
                return false
            }

        default:

            return true
        }

    }

回答1:

The problem is that you are not taking into account the scale of the image you're creating.

You have a Retina screen. When you call lockFocus and draw into the resulting graphics context, the graphics context has double-resolution. Thus, a size of 500x500 results in an underlying bitmap of 1000x1000. That doesn't matter for purposes of display; to the extent that it does matter, it's a good thing.

But when you proceed to save the image data to disk, you throw away the knowledge that this is a double-resolution image, and you simply save the bitmap — which is 1000x1000.

You should look at Apple's documentation on how to deal with high-resolution images, e.g. https://developer.apple.com/library/content/documentation/GraphicsAnimation/Conceptual/HighResolutionOSX/APIs/APIs.html

来源：https://stackoverflow.com/questions/50648837/resizing-large-resolution-images-producing-1000x1000-pixels-size-when-size-is-se