Changing Adaptive Media's Image Scaling

As described in the Adaptive Media user guide, Adaptive Media scales images to match the image resolutions defined by the Liferay DXP administrator. The default scaling is usually suitable, but you can also customize it to your needs. Before doing so, however, you should understand how this scaling works.

Understanding Image Scaling in Adaptive Media

Adaptive Media contains an extension point that lets you replace the way it scales images. The AMImageScaler interface defines Adaptive Media’s image scaling logic. Out of the box, Adaptive Media provides two implementations of this interface:

  • AMDefaultImageScaler: The default image scaler. It’s always enabled and uses java.awt for its image processing and scaling.

  • AMGIFImageScaler: A scaler that works only with GIF images. It depends on the installation of the external tool gifsicle in the Liferay DXP instance. This scaler must be enabled in Control PanelSystem Settings.

You must register image scalers in Liferay DXP’s OSGi container using the AMImageScaler interface. Each scaler must also set the mime.type property to the MIME type it handles. For example, if you set a scaler’s MIME type to image/jpeg, then that scaler can only handle image/jpeg images. If you specify the special MIME type *, the scaler can process any image. Note that the AMDefaultImageScaler is registered using mime.type=*, while the AMGIFImageScaler is registered using mime.type=image/gif. Both scalers, like all scalers, implement AMImageScaler.

You can add as many image scalers as you need, even for the same MIME type. Even so, Adaptive Media uses only one scaler per image, using this process to determine the best one:

  1. Select only the image scalers registered with the same MIME type as the image.

  2. Select the enabled scalers from those selected in the first step (the AMImageScaler method isEnabled() returns true for enabled scalers).

  3. Of the scalers selected in the second step, select the scaler with the highest service.ranking.

If these steps return no results, they’re repeated, but the first step uses the special MIME type *. Also note that if an image scaler is registered for specific MIME types and has a higher service.ranking, it’s more likely to be chosen than if it’s registered for the special MIME type * or has a lower service.ranking.

Creating an Image Scaler

Now that you know how Adaptive Media scales images, you’ll learn how to customize this scaling. As an example, you’ll see a sample image scaler that customizes the scaling of PNG images.

Follow these steps to create a custom image scaler:

  1. Create your scaler class to implement AMImageScaler. You must also annotate your scaler class with @Component, setting mime.type properties for each of the scaler’s MIME types, and registering an AMImageScaler service. If there’s more than one scaler for the same MIME type, you must also set the @Component annotation’s service.ranking property. For your scaler to take precedence over other scalers of the same MIME type, its service ranking property must be higher than that of the other scalers. If service.ranking isn’t set, it defaults to 0.

    For example, this sample image scaler scales PNG and x-PNG images, and has a service ranking of 100:

        immediate = true,
        property = {"mime.type=image/png", "mime.type=image/x-png", "service.ranking:Integer=100"},
        service = {AMImageScaler.class}
    public class SampleAMPNGImageScaler implements AMImageScaler {...

    This requires these imports:

    import org.osgi.service.component.annotations.Component;
  2. Implement the isEnabled() method to return true when you want to enable the scaler. In many cases, you always want the scaler enabled, so you can simply return true in this method. This is the case with the example SampleAMPNGImageScaler:

    public boolean isEnabled() {
        return true;

    This method gets more interesting when the scaler depends on other tools or features. For example, the isEnabled() method in AMGIFImageScaler determines whether gifsicle is enabled. This scaler must only be enabled when the tool it depends on, gifsicle, is also enabled:

    public boolean isEnabled() {
        return _amImageConfiguration.gifsicleEnabled();
  3. Implement the scaleImage method. This method contains the scaler’s business logic, and must return an AMImageScaledImage instance. For example, the example scaleImage implementation in SampleAMPNGImageScaler uses AMImageConfigurationEntry to get the maximum height and width values for the scaled image, and FileVersion to get the image to scale. The scaling is done with the help of a private inner class, assuming that the methods _scalePNG, _getScalePNGHeight, _getScalePNGWidth, and _getScalePNGSize implement the actual scaling:

    public AMImageScaledImage scaleImage(FileVersion fileVersion,
        AMImageConfigurationEntry amImageConfigurationEntry) {
        Map<String, String> properties = amImageConfigurationEntry.getProperties();
        int maxHeight = GetterUtil.getInteger(properties.get("max-height"));
        int maxWidth = GetterUtil.getInteger(properties.get("max-width"));
        try {
            InputStream inputStream = 
                _scalePNG(fileVersion.getContentStream(false), maxHeight, maxWidth);
            int height = _getScalePNGHeight();
            int width = _getScalePNGWidth();
            long size = _getScalePNGSize();
            return new AMImageScaledImageImpl(inputStream, height, width, size);
        catch (PortalException pe) {
            throw new AMRuntimeException.IOException(pe);
    private class AMImageScaledImageImpl implements AMImageScaledImage {
        public int getHeight() {
            return _height;
        public InputStream getInputStream() {
            return _inputStream;
        public long getSize() {
            return _size;
        public int getWidth() {
            return _width;
        private AMImageScaledImageImpl(InputStream inputStream, int height, 
            int width, long size) {
            _inputStream = inputStream;
            _height = height;
            _width = width;
            _size = size;
        private final int _height;
        private final InputStream _inputStream;
        private final long _size;
        private final int _width;

    This requires these imports:

    import com.liferay.portal.kernel.exception.PortalException;
    import com.liferay.portal.kernel.repository.model.FileVersion;
    import com.liferay.portal.kernel.util.GetterUtil;
    import java.util.Map;

Great! Now you know how to write your own image scalers.

Displaying Adapted Images in Your App

Finding Adapted Images

Adapting Your Media Across Multiple Devices

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