<AWT Dev> thoughts on multiresolution images

[Alan Snyder]

I am writing to share some thoughts based on recent experience using multiresolution images. My experience was not entirely pleasant. I am using JDK 8, although I see no relevant differences in JDK 9.

One of the critical issues using multiresolution images is that the selection of a specific image is not made until the application attempts to draw the image. If the returned image is fully available at that time, then it is drawn with no problem. Otherwise, the image observer is called. Typically, this will call repaint() on a component.

There are two potential problems:

(1) If the component drawing the image is actually a cell renderer, then probably repaint() does nothing. The drawing will be incomplete and may not be fixed.

(2) Otherwise, if the resolution variant was created on the fly and not cached, then when the repainting occurs, a new resolution variant image will be created. Most likely, it will not be fully available, either, so the result is a possibly endless repaint loop.

I don't know of a solution to problem (1). It is not a new problem. However, what is new is that the common workaround of creating an ImageIcon may not work in this case, because only certain platform created multiresolution images are recognized by ImageIcon/MediaTracker/SunToolkit. In the general case, the component does not know which resolution variant is actually needed and thus is unable to wait for its full availability. The approach of waiting for all variants to be available is suboptimal and limiting (see below).

Problem (2) can be solved by caching. Given the importance of caching when arbitrary images might be in use, it is surprising that there is no public support for caching. The MultiResolutionCachedImage class is JDK internal, as is the supporting ImageCache class.

Another problem with multiresolution images is that anything that uses the getSource() method to obtain an ImageProducer will probably not do the right thing. An important example is using FilteredImageSource and an ImageFilter to create a derived image. There is no specific specification of what getSource() should return when invoked on a multiresolution image, but whatever it returns is a single-resolution image and therefore will not be the proper image in some circumstances.

Perhaps getSource() on a multiresolution image should thrown an exception.

There seems to be an assumption that a multiresolution image should contain a "base image" at 1x. I do not see any basis for making that assumption. It seems reasonable to me to create a multiresolution image with a single, higher resolution image. The effective result is a dynamically scaled image, where the scaling factor is determined at the last possible moment, so that no resolution is lost unnecessarily. I also observe that in the future, 1x representations will be less and less useful.

I also question the rationale for the getResolutionVariants() method. This method assumes that a MultiResolutionImage contains a fixed number of variants. I do not see any reason to make that restriction. The resolution variants might be created from a scalable description, such as vector graphics. Even if the number of variants is fixed, if the image is served remotely, it might be very expensive to obtain them all. A lazy approach to creating derived multiresolution images is better.

To work around some of these problems, I created my own API that includes a method similar to the map() method of MultiResolutionCachedImage. It seems to me that a method like this is needed, not just in MultiResolutionImage, but in Image itself, so that applications can write code that works on Images in general, including the MultiResolutionImage variety.

Previously [1], the following code was suggested as a way to create a filtered MultiResolutionImage:

     static Image applyFilter(Image image) {
         // apply a filter to create ligtened image
     }

     static class LigtenedMultiresolutionImage extends AbstractMultiResolutionImage {

         private final Image baseImage;

         public LigtenedMultiresolutionImage(Image baseImage) {
             this.baseImage = baseImage;
         }

         @Override
         public Image getResolutionVariant(float destImageWidth, float destImageHeight) {
             Image rvImage = ((MultiResolutionImage) baseImage).
                     getResolutionVariant(destImageWidth, destImageHeight);
             return applyFilter(rvImage);
         }

         @Override
         public List<Image> getResolutionVariants() {
             List<Image> resolutionvariants = new LinkedList<>();
             for (Image image : ((MultiResolutionImage) baseImage).
                     getResolutionVariants()) {
                 resolutionvariants.add(applyFilter(image));
             }
             return resolutionvariants;
         }

         @Override
         protected Image getBaseImage() {
             return applyFilter(baseImage);
         }
     }

I note that the lack of caching means that this code will be reliable only if applyFilter() returns a fully available image.

My observation is one needs to know a lot more than one might expect to succeed at using multiresolution images.

[1] http://mail.openjdk.java.net/pipermail/2d-dev/2014-June/004638.html