Codename One fast image scaling algorithm to generate thumbnails from captured photos. This optimization makes sense on Android and Simulator (Java SE) only. See: https://github.com/codenameone/CodenameOne/issues/3090

MyApplication.java

package net.informaticalibera.test.imagescaling;

import com.codename1.capture.Capture;
import com.codename1.components.SpanLabel;
import com.codename1.io.FileSystemStorage;
import static com.codename1.ui.CN.*;
import com.codename1.ui.Form;
import com.codename1.ui.Dialog;
import com.codename1.ui.Label;
import com.codename1.ui.plaf.UIManager;
import com.codename1.ui.util.Resources;
import com.codename1.io.Log;
import com.codename1.ui.Toolbar;
import java.io.IOException;
import com.codename1.ui.layouts.BoxLayout;
import com.codename1.io.Util;
import com.codename1.ui.Button;
import com.codename1.ui.CN;
import com.codename1.ui.EncodedImage;
import com.codename1.ui.Image;
import com.codename1.ui.events.ActionListener;
import com.codename1.ui.util.ImageIO;
import java.io.InputStream;
import java.io.OutputStream;

/**
 * This file was generated by <a href="https://www.codenameone.com/">Codename
 * One</a> for the purpose of building native mobile applications using Java.
 */
public class MyApplication {

    private Form current;
    private Resources theme;

    public void init(Object context) {
        // use two network threads instead of one
        updateNetworkThreadCount(2);

        theme = UIManager.initFirstTheme("/theme");

        // Enable Toolbar on all Forms by default
        Toolbar.setGlobalToolbar(true);

        // Pro only feature
        Log.bindCrashProtection(true);

        addNetworkErrorListener(err -> {
            // prevent the event from propagating
            err.consume();
            if (err.getError() != null) {
                Log.e(err.getError());
            }
            Log.sendLogAsync();
            Dialog.show("Connection Error", "There was a networking error in the connection to " + err.getConnectionRequest().getUrl(), "OK", null);
        });
    }

    public void start() {
        if (current != null) {
            current.show();
            return;
        }
        Form hi = new Form("Hi World", BoxLayout.y());
        Button button1 = new Button("Open camera");
        Button button2 = new Button("Open gallery");
        SpanLabel summary = new SpanLabel();
        Label label1 = new Label();
        Label label2 = new Label();
        Label label3 = new Label();
        Label label4 = new Label();
        Label label5 = new Label();
        Label label6 = new Label();
        hi.addAll(button1, button2, summary, label1, label2, label3, label4, label5, label6);
        hi.show();

        ActionListener actionEvt = evt -> {
            String tempPath = (String) evt.getSource();
            if (tempPath != null && FileSystemStorage.getInstance().exists(tempPath)) {
                try {
                    // copy the temp file in the home
                    String capturedPhoto = FileSystemStorage.getInstance().getAppHomePath() + "captured.jpg";
                    InputStream temp = FileSystemStorage.getInstance().openInputStream(tempPath);
                    OutputStream dest = FileSystemStorage.getInstance().openOutputStream(capturedPhoto);
                    Util.copy(temp, dest);

                    InputStream source = FileSystemStorage.getInstance().openInputStream(capturedPhoto);
                    Image original = EncodedImage.create(source);
                    int newWidth = CN.convertToPixels(30);
                    int newHeight = newWidth * original.getHeight() / original.getWidth();

                    // resizes using the fastScaleImage method
                    long startTime1 = System.currentTimeMillis();
                    Image thumbnail1 = fastScaleImage(original, newWidth, newHeight);
                    long endTime1 = System.currentTimeMillis();
                    long time1 = endTime1 - startTime1;
                    label1.setIcon(thumbnail1);

                    // resizes using the Image.scale method
                    long startTime2 = System.currentTimeMillis();
                    Image thumbnail2 = original.scaled(newWidth, newHeight);
                    long endTime2 = System.currentTimeMillis();
                    long time2 = endTime2 - startTime2;
                    label2.setIcon(thumbnail2);

                    // resizes using the ImageIO.save method
                    long startTime3 = System.currentTimeMillis();
                    String thumbPath = FileSystemStorage.getInstance().getAppHomePath() + "thumb.jpg";
                    InputStream source3 = FileSystemStorage.getInstance().openInputStream(capturedPhoto);
                    OutputStream response = FileSystemStorage.getInstance().openOutputStream(thumbPath);
                    ImageIO.getImageIO().save(source3, response, ImageIO.FORMAT_JPEG, newWidth, newHeight, 0.9f);
                    long endTime3 = System.currentTimeMillis();
                    InputStream thumbStream = FileSystemStorage.getInstance().openInputStream(thumbPath);
                    Image thumbnail = EncodedImage.create(thumbStream);
                    long time3 = endTime3 - startTime3;
                    label3.setIcon(thumbnail);

                    // Test of make an image bigger
                    // resizes using the fastScaleImage method
                    long startTime4 = System.currentTimeMillis();
                    Image thumbnail4 = fastScaleImage(thumbnail, original.getWidth(), original.getHeight());
                    long endTime4 = System.currentTimeMillis();
                    long time4 = endTime4 - startTime4;
                    label4.setIcon(thumbnail4);

                    // resizes using the Image.scale method
                    long startTime5 = System.currentTimeMillis();
                    Image thumbnail5 = thumbnail.scaled(original.getWidth(), original.getHeight());
                    long endTime5 = System.currentTimeMillis();
                    long time5 = endTime5 - startTime5;
                    label5.setIcon(thumbnail5);

                    // resizes using the ImageIO.save method
                    long startTime6 = System.currentTimeMillis();
                    String newImgPath = FileSystemStorage.getInstance().getAppHomePath() + "newImg.jpg";
                    ImageIO.getImageIO().saveAndKeepAspect(thumbPath, newImgPath, ImageIO.FORMAT_JPEG, original.getWidth(), original.getHeight(), 0.9f, false, true);
                    long endTime6 = System.currentTimeMillis();
                    InputStream newImgStream = FileSystemStorage.getInstance().openInputStream(newImgPath);
                    Image thumbnail6 = EncodedImage.create(newImgStream);
                    long time6 = endTime6 - startTime6;
                    label6.setIcon(thumbnail6);

                    summary.setText("RESIZING TO A SMALLER IMAGE\n\nfastScaleImage time: " + time1 + "\n" + "Image.scaled() time: " + time2 + "\n" + "ImageIO.save() time: " + time3 + "\n\n"
                            + "RESIZING TO A BIGGER IMAGE\n\nfastScaleImage time: " + time4 + "\n" + "Image.scaled() time: " + time5 + "\n" + "ImageIO.save() time: " + time6);

                    hi.revalidate();
                } catch (IOException ex) {
                    Log.p("Error loading the photo", Log.ERROR);
                    Log.e(ex);
                }
            }
        };

        button1.addActionListener(l -> {
            Capture.capturePhoto(actionEvt);
        });

        button2.addActionListener(l -> {
            CN.openGallery(actionEvt, CN.GALLERY_IMAGE);
        });
    }

    public void stop() {
        current = getCurrentForm();
        if (current instanceof Dialog) {
            ((Dialog) current).dispose();
            current = getCurrentForm();
        }
    }

    public void destroy() {
    }

    /**
     * <p>
     * Fast scaling algorithm, optimized to resize a captured photo to a
     * thumbnail: in this use case, it's about 3x ~ 10x faster that
     * Image.scaled() and IOImage.save() ON ANDROID AND SIMULATOR ONLY, with an
     * equivalent quality. On iOS, the fastest algorithm is IOImage.save()</p>
     * <p>
     * CAUTION: Don't use to enlarge an image, because it will be slower than
     * other methods.</p>
     * <p>
     * Speed comparation, see
     * <a href="https://gist.github.com/jsfan3/4d2d55f0d5136ec88d36257401d41854">testing
     * code</a></p>
     *
     *
     * @param original Image
     * @param newWidth in pixels
     * @param newHeight in pixels
     * @return new scaled Image (it will be an EncodedImage instance)
     */
    public static Image fastScaleImage(Image original, int newWidth, int newHeight) {
        int[] rawInput = original.getRGB(); // ARGB array
        int[] rawOutput = new int[newWidth * newHeight];
        int originalW = original.getWidth();
        int originalH = original.getHeight();

        int YD = (originalH / newHeight) * originalW - originalW;
        int YR = originalH % newHeight;
        int XD = originalW / newWidth;
        int XR = originalW % newWidth;
        int outOffset = 0;
        int inOffset = 0;

        for (int y = newHeight, YE = 0; y > 0; y--) {
            for (int x = newWidth, XE = 0; x > 0; x--) {
                rawOutput[outOffset++] = rawInput[inOffset];
                inOffset += XD;
                XE += XR;
                if (XE >= newWidth) {
                    XE -= newWidth;
                    inOffset++;
                }
            }
            inOffset += YD;
            YE += YR;
            if (YE >= newHeight) {
                YE -= newHeight;
                inOffset += originalW;
            }
        }
        return EncodedImage.createFromRGB(rawOutput, newWidth, newHeight, true);
    }

}