Load large images with Picasso and custom Transfor

问题:

I'm getting an Out Of Memory exception using Picasso when loading "large" images (> 1.5MB) from Android Gallery (using startActivityForResult).

I'm using a custom Target object because I need to preprocess the Bitmap when it is ready and also I'm using a custom Transform object to scale the Bitmap.

The problem is that the method public Bitmap transform(Bitmap source) on my Transform object is never called because the Out Of Memory Exception, so I don't get the oportunity to resample the image.

But, if I use the .resize(maxWidth, maxHeight) method, then it loads the image OK. I supossed that the Transform object was for that purpose too, but it seems that the transform method is called after the resize, and, if I don't call resize, then it will end in an Out of Memory..

The problem is that with resize I need to specify both width and height, but I need to scale and keep the aspect ratio.

Consider that the images will be selected from user Gallery, so they can be bigger or smaller, portrait, squared or landscape, etc, so I need my own Transformation object to perform the logic that needs my application.

回答1:

I found a solution..

In my Transform object I needed to scale the image (keeping aspect ratio) to 1024 x 768 max.

Transform object was never called unless I call .resize(width, height) to resample down the image.

For keeping aspect ratio and using resize I call .centerInside(). This way image will be scaled resample to fit width, height).

The value that I give to .resize(width, height) is Math.ceil(Math.sqrt(1024 * 768)). This way I'm sure to have an image higher enough for my custom Transform object, and also avoid Out Of Memory exception

Update: Full example

Following this example you will get a image that fits inside MAX_WIDTH and MAX_HEIGHT bounds (keeping aspect ratio)

private static final int MAX_WIDTH = 1024;
private static final int MAX_HEIGHT = 768;

int size = (int) Math.ceil(Math.sqrt(MAX_WIDTH * MAX_HEIGHT));

// Loads given image
Picasso.with(imageView.getContext())
    .load(imagePath)
    .transform(new BitmapTransform(MAX_WIDTH, MAX_HEIGHT))
    .skipMemoryCache()
    .resize(size, size)
    .centerInside()
    .into(imageView);

And this is my custom BitmapTransform class:

import android.graphics.Bitmap;
import com.squareup.picasso.Transformation;

/**
 * Transformate the loaded image to avoid OutOfMemoryException
 */
public class BitmapTransform implements Transformation {

    private final int maxWidth;
    private final int maxHeight;

    public BitmapTransform(int maxWidth, int maxHeight) {
        this.maxWidth = maxWidth;
        this.maxHeight = maxHeight;
    }

    @Override
    public Bitmap transform(Bitmap source) {
        int targetWidth, targetHeight;
        double aspectRatio;

        if (source.getWidth() > source.getHeight()) {
            targetWidth = maxWidth;
            aspectRatio = (double) source.getHeight() / (double) source.getWidth();
            targetHeight = (int) (targetWidth * aspectRatio);
        } else {
            targetHeight = maxHeight;
            aspectRatio = (double) source.getWidth() / (double) source.getHeight();
            targetWidth = (int) (targetHeight * aspectRatio);
        }

        Bitmap result = Bitmap.createScaledBitmap(source, targetWidth, targetHeight, false);
        if (result != source) {
            source.recycle();
        }
        return result;
    }

    @Override
    public String key() {
        return maxWidth + "x" + maxHeight;
    }

}

回答2:

There's a very good article on the Android Developer website which helped me alot when I had the same problem with loading large images.

This article explains really well what causes the error and how to solve it. There are also other articles (see menu) to, for example, cache images.