Sample code for a tutorial (http://howrobotswork.wordpress.com/2014/04/28/how-to-dim-bitmaps-with-renderscript/)

RenderScriptActivity.java

package com.andraskindler.sandbox.activity;

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.support.v8.renderscript.Allocation;
import android.support.v8.renderscript.RenderScript;
import android.widget.FrameLayout;
import android.widget.ImageView;

import com.andraskindler.sandbox.R;
import com.andraskindler.sandbox.ScriptC_dim;

import hugo.weaving.DebugLog;
import rx.Observable;
import rx.Subscriber;
import rx.android.schedulers.AndroidSchedulers;
import rx.functions.Action1;

public class RenderScriptActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        final FrameLayout root = (FrameLayout) getLayoutInflater().inflate(R.layout.activity_main, null);
        setContentView(root);

        final ImageView imageView = (ImageView) root.findViewById(R.id.iv_main);
        Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.test_image);

        dimWithRenderScript(bitmap).subscribe(new Action1<Bitmap>() {
            @Override public void call(Bitmap bitmap) {
                imageView.setImageBitmap(bitmap);
            }
        });
    }

    @DebugLog
    private Observable<Bitmap> dimWithRenderScript(final Bitmap in) {
        return Observable.create(new Observable.OnSubscribe<Bitmap>() {
            @Override public void call(Subscriber<? super Bitmap> subscriber) {
                final RenderScript renderScript = RenderScript.create(RenderScriptActivity.this);
                final ScriptC_dim script = new ScriptC_dim(renderScript);
                script.set_dimmingValue(0.6f);
                final Allocation alloc1 = Allocation.createFromBitmap(renderScript, in, Allocation.MipmapControl.MIPMAP_NONE, Allocation.USAGE_SCRIPT);
                final Allocation alloc2 = Allocation.createTyped(renderScript, alloc1.getType());
                script.forEach_dim(alloc1, alloc2);
                alloc2.copyTo(in);
                renderScript.destroy();
                subscriber.onNext(in);
                subscriber.onCompleted();
            }
        }).subscribeOn(AndroidSchedulers.mainThread());
    }

}