A picasso circle image transformation. Optimized version of: https://gist.github.com/julianshen/5829333. Use shader.setLocalMatrix() method to draw circle bitmap not from source bitmap left-top. So, we no need to create square bitmap!

CircleImageTransformation.java

/**
 * Created by codezjx on 2016/5/4.
 */
public class CircleImageTransformation implements Transformation {

    /**
     * A unique key for the transformation, used for caching purposes.
     */
    private static final String KEY = "circleImageTransformation";

    @Override
    public Bitmap transform(Bitmap source) {

        int minEdge = Math.min(source.getWidth(), source.getHeight());
        int dx = (source.getWidth() - minEdge) / 2;
        int dy = (source.getHeight() - minEdge) / 2;

        // Init shader
        Shader shader = new BitmapShader(source, Shader.TileMode.CLAMP, Shader.TileMode.CLAMP);
        Matrix matrix = new Matrix();
        matrix.setTranslate(-dx, -dy);   // Move the target area to center of the source bitmap
        shader.setLocalMatrix(matrix);

        // Init paint
        Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
        paint.setShader(shader);

        // Create and draw circle bitmap
        Bitmap output = Bitmap.createBitmap(minEdge, minEdge, source.getConfig());
        Canvas canvas = new Canvas(output);
        canvas.drawOval(new RectF(0, 0, minEdge, minEdge), paint);

        // Recycle the source bitmap, because we already generate a new one
        source.recycle();

        return output;
    }

    @Override
    public String key() {
        return KEY;
    }
}