Translate/scale bitmap within boundaries?

Question

When I research approaches for touch pan/zoom on an image, I generally find effective, simple code--but nothing that does quite what I want. The image needs to never show a blank space between the edge of the actual image (bitmap) and its View. If the bitmap is 200x100 and the View is 50x50, the user should only be able to zoom out to 100x50, allowing them to slide the image horizontally, but not vertically.

My code does this well when moving (translating) the image--until the image is zoomed. Then something is thrown off; I can move the bitmap far enough to see gaps around it. It's probably something simple and obvious related to factoring pixel measurements by the current scale factor, but I can't find it. I suspect it has to do with the calculations of maxX and maxY in onDraw() below. Any ideas?

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.Drawable;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.view.ScaleGestureDetector;
import android.widget.ImageView;

/**
 * Most code from
 * http://android-developers.blogspot.com/2010/06/making-sense-of-multitouch.html
 * 
 * @author Chad Schultz
 * 
 */
public class PanZoomImageView extends ImageView {

    public static final String TAG = PanZoomImageView.class.getName();

    private static final int INVALID_POINTER_ID = -1;

    // The ‘active pointer’ is the one currently moving our object.
    private int mActivePointerId = INVALID_POINTER_ID;

    private Bitmap bitmap;

    private ScaleGestureDetector mScaleDetector;
    private float mScaleFactor = 1.f;
    private float minScaleFactor;

    private float mPosX;
    private float mPosY;

    private float mLastTouchX, mLastTouchY;

    private boolean firstDraw = true;

    private boolean panEnabled = true;
    private boolean zoomEnabled = true;

    public PanZoomImageView(Context context) {
        super(context);
        setup();
    }

    public PanZoomImageView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        setup();
    }

    public PanZoomImageView(Context context, AttributeSet attrs) {
        super(context, attrs);
        setup();
    }

    private void setup() {
        mScaleDetector = new ScaleGestureDetector(getContext(), new ScaleListener());
    }

    @Override
    public void setImageBitmap(Bitmap bmp) {
        super.setImageBitmap(bmp);
        bitmap = bmp;
        firstDraw = true;
    }

    @Override
    public void setImageDrawable(Drawable drawable) {
        super.setImageDrawable(drawable);
        bitmap = ((BitmapDrawable) drawable).getBitmap();
        firstDraw = true;
    }

    public void onDraw(Canvas canvas) {
        Log.v(TAG, "onDraw()");
        if (bitmap == null) {
            Log.w(TAG, "nothing to draw - bitmap is null");
            super.onDraw(canvas);
            return;
        }

        if (firstDraw 
                && (bitmap.getHeight() > 0) 
                && (bitmap.getWidth() > 0) 
                && (canvas.getHeight() > 0) 
                && (canvas.getWidth() > 0)) {
            //Don't let the user zoom out so much that the image is smaller
            //than its containing frame
            float minXScaleFactor = (float) canvas.getWidth() / (float) bitmap.getWidth();
            float minYScaleFactor = (float) canvas.getHeight() / (float) bitmap.getHeight();
            minScaleFactor = Math.max(minXScaleFactor, minYScaleFactor);
            Log.d(TAG, "minScaleFactor: " + minScaleFactor);
            firstDraw = false;
        }
        mScaleFactor = Math.max(mScaleFactor, minScaleFactor);
        Log.d(TAG, "mScaleFactor: " + mScaleFactor);

        //Save the canvas without translating (panning) or scaling (zooming)
        //After each change, restore to this state, instead of compounding
        //changes upon changes
        canvas.save();

        int maxX, minX, maxY, minY;
        //How far can we move the image horizontally without having a gap between image and frame?
        maxX = (int) (mScaleFactor * (bitmap.getWidth() / 2) - (canvas.getWidth() / 2));
        minX = -1 * maxX;
        //How far can we move the image vertically without having a gap between image and frame?
        maxY = (int) (mScaleFactor * (bitmap.getHeight() / 2) - (canvas.getWidth() / 2));
        minY = -1 * maxY;
        //Do not go beyond the boundaries of the image
        if (mPosX > maxX) {
            mPosX = maxX;
        }
        if (mPosX < minX) {
            mPosX = minX;
        }
        if (mPosY > maxY) {
            mPosY = maxY;
        }
        if (mPosY < minY) {
            mPosY = minY;
        }

        Log.d(TAG, "canvas width: " + canvas.getWidth() + " canvas height: "
                + canvas.getHeight());
        Log.d(TAG, "bitmap width: " + bitmap.getWidth() + " height: " + bitmap.getHeight());
        Log.d(TAG, "translating mPosX: " + mPosX + " mPosY: " + mPosY);

        if (zoomEnabled) {
            Log.d(TAG, "zooming to scale factor of " + mScaleFactor);
            canvas.scale(mScaleFactor, mScaleFactor);
        } else {
            Log.d(TAG, "zooming disabled");
        }

        if (panEnabled) {
            Log.d(TAG, "panning to " + mPosX + "," + mPosY); 
            canvas.translate(mPosX, mPosY);
        } else {
            Log.d(TAG, "panning disabled");
        }

        super.onDraw(canvas);
        canvas.restore(); //clear translation/scaling
    }

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        // Let the ScaleGestureDetector inspect all events.
        mScaleDetector.onTouchEvent(ev);

        final int action = ev.getAction();
        switch (action & MotionEvent.ACTION_MASK) {
            case MotionEvent.ACTION_DOWN: {
                final float x = ev.getX();
                final float y = ev.getY();

                mLastTouchX = x;
                mLastTouchY = y;
                mActivePointerId = ev.getPointerId(0);
                break;
            }

            case MotionEvent.ACTION_MOVE: {
                final int pointerIndex = ev.findPointerIndex(mActivePointerId);
                final float x = ev.getX(pointerIndex);
                final float y = ev.getY(pointerIndex);

                // Only move if the ScaleGestureDetector isn't processing a gesture.
                if (!mScaleDetector.isInProgress()) {
                    float dx = x - mLastTouchX;
                    float dy = y - mLastTouchY;

                    //Adjust for zoom factor. Otherwise, the user's finger moving 10 pixels
                    //at 200% zoom causes the image to slide 20 pixels instead of perfectly
                    //following the user's touch
                    dx /= mScaleFactor;
                    dy /= mScaleFactor;

                    mPosX += dx;
                    mPosY += dy;

                    invalidate();
                }

                mLastTouchX = x;
                mLastTouchY = y;

                break;
            }

            case MotionEvent.ACTION_UP: {
                mActivePointerId = INVALID_POINTER_ID;
                break;
            }

            case MotionEvent.ACTION_CANCEL: {
                mActivePointerId = INVALID_POINTER_ID;
                break;
            }

            case MotionEvent.ACTION_POINTER_UP: {
                final int pointerIndex = (ev.getAction() & MotionEvent.ACTION_POINTER_INDEX_MASK) >> MotionEvent.ACTION_POINTER_INDEX_SHIFT;
                final int pointerId = ev.getPointerId(pointerIndex);
                if (pointerId == mActivePointerId) {
                    // This was our active pointer going up. Choose a new
                    // active pointer and adjust accordingly.
                    final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
                    mLastTouchX = ev.getX(newPointerIndex);
                    mLastTouchY = ev.getY(newPointerIndex);
                    mActivePointerId = ev.getPointerId(newPointerIndex);
                }
                break;
            }
        }

        return true;
    }

    private class ScaleListener extends
            ScaleGestureDetector.SimpleOnScaleGestureListener {
        @Override
        public boolean onScale(ScaleGestureDetector detector) {
            mScaleFactor *= detector.getScaleFactor();
            // Don't let the object get too small or too large.
            mScaleFactor = Math.max(0.1f, Math.min(mScaleFactor, 5.0f));
            Log.d(TAG, "detector scale factor: " + detector.getScaleFactor() + " mscalefactor: " + mScaleFactor);

            invalidate();
            return true;
        }
    }

    //Currently zoomEnabled/panEnabled can only be set programmatically, not in XML

    public boolean isPanEnabled() {
        return panEnabled;
    }

    public void setPanEnabled(boolean panEnabled) {
        this.panEnabled = panEnabled;
    }

    public boolean isZoomEnabled() {
        return zoomEnabled;
    }

    public void setZoomEnabled(boolean zoomEnabled) {
        this.zoomEnabled = zoomEnabled;
    }

}

Answer 1

Here's what I eventually came up with on my own after a great deal of painful experimentation--learning some interesting things along the way about how Bitmaps are handled in Android. This code is far from perfect, but it suits my purposes--hopefully it will help others as well.

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.Drawable;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.view.ScaleGestureDetector;
import android.view.View;

/**
 * @author Chad Schultz
 * @version 1
 */
public class PanZoomView extends View {

    public static final String TAG = PanZoomView.class.getName();

    private static final int INVALID_POINTER_ID = -1;

    // The ‘active pointer’ is the one currently moving our object.
    private int mActivePointerId = INVALID_POINTER_ID;

    private Bitmap bitmap;
    private float viewHeight;
    private float viewWidth;
    float canvasWidth, canvasHeight;

    private ScaleGestureDetector mScaleDetector;
    private float mScaleFactor = 1.f;
    private float minScaleFactor;

    private float mPosX;
    private float mPosY;

    private float mLastTouchX, mLastTouchY;

    private boolean firstDraw = true;

    private boolean panEnabled = true;
    private boolean zoomEnabled = true;

    public PanZoomView(Context context) {
        super(context);
        setup();
    }

    public PanZoomView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        setup();
    }

    public PanZoomView(Context context, AttributeSet attrs) {
        super(context, attrs);
        setup();
    }

    private void setup() {
        mScaleDetector = new ScaleGestureDetector(getContext(), new ScaleListener());
    }

    public void setBitmap(Bitmap bmp) {
        setImageBitmap(bmp);
    }

    public void setImageBitmap(Bitmap bmp) {
        bitmap = bmp;
        resetZoom();
        resetPan();
        firstDraw = true;
        invalidate();
    }

    public Bitmap getImageBitmap() {
        return bitmap;
    }

    public Bitmap getBitmap() {
        return getImageBitmap();
    }

    public void resetZoom() {
        mScaleFactor = 1.0f;
    }

    public void resetPan() {
        mPosX = 0f;
        mPosY = 0f;
    }

    public void setImageDrawable(Drawable drawable) {
        setImageBitmap(((BitmapDrawable) drawable).getBitmap());
    }

    public BitmapDrawable getImageDrawable() {
        BitmapDrawable bd = new BitmapDrawable(getContext().getResources(), bitmap);
        return bd;
    }

    public BitmapDrawable getDrawable() {
        return getImageDrawable();
    }

    public void onDraw(Canvas canvas) {
//      Log.v(TAG, "onDraw()");

        if (bitmap == null) {
            Log.w(TAG, "nothing to draw - bitmap is null");
            super.onDraw(canvas);
            return;
        }

        if (firstDraw 
                && (bitmap.getHeight() > 0) 
                && (bitmap.getWidth() > 0)) {
            //Don't let the user zoom out so much that the image is smaller
            //than its containing frame
            float minXScaleFactor = (float) viewWidth / (float) bitmap.getWidth();
            float minYScaleFactor = (float) viewHeight / (float) bitmap.getHeight();
            minScaleFactor = Math.max(minXScaleFactor, minYScaleFactor);
            Log.d(TAG, "minScaleFactor: " + minScaleFactor);
            mScaleFactor = minScaleFactor; //start out "zoomed out" all the way

            mPosX = mPosY = 0;
            firstDraw = false;

        }
        mScaleFactor = Math.max(mScaleFactor, minScaleFactor);

        canvasHeight = canvas.getHeight();
        canvasWidth = canvas.getWidth();
//      Log.d(TAG, "canvas density: " + canvas.getDensity() + " bitmap density: " + bitmap.getDensity());

//      Log.d(TAG, "mScaleFactor: " + mScaleFactor);

        //Save the canvas without translating (panning) or scaling (zooming)
        //After each change, restore to this state, instead of compounding
        //changes upon changes
        canvas.save();
        int maxX, minX, maxY, minY;
        //Regardless of the screen density (HDPI, MDPI) or the scale factor, 
        //The image always consists of bitmap width divided by 2 pixels. If an image
        //is 200 pixels wide and you scroll right 100 pixels, you just scrolled the image
        //off the screen to the left.
        minX = (int) (((viewWidth / mScaleFactor) - bitmap.getWidth()) / 2);
        maxX = 0;
        //How far can we move the image vertically without having a gap between image and frame?
        minY = (int) (((viewHeight / mScaleFactor) - bitmap.getHeight()) / 2);
        maxY = 0;
        Log.d(TAG, "minX: " + minX + " maxX: " + maxX + " minY: " + minY + " maxY: " + maxY);
        //Do not go beyond the boundaries of the image
        if (mPosX > maxX) {
            mPosX = maxX;
        }
        if (mPosX < minX) {
            mPosX = minX;
        }
        if (mPosY > maxY) {
            mPosY = maxY;
        }
        if (mPosY < minY) {
            mPosY = minY;
        }

//      Log.d(TAG, "view width: " + viewWidth + " view height: "
//              + viewHeight);
//      Log.d(TAG, "bitmap width: " + bitmap.getWidth() + " height: " + bitmap.getHeight());
//      Log.d(TAG, "translating mPosX: " + mPosX + " mPosY: " + mPosY);

//      Log.d(TAG, "zooming to scale factor of " + mScaleFactor);
        canvas.scale(mScaleFactor, mScaleFactor);

//      Log.d(TAG, "panning to " + mPosX + "," + mPosY); 
        canvas.translate(mPosX, mPosY);

        super.onDraw(canvas);
        canvas.drawBitmap(bitmap, mPosX, mPosY, null);
        canvas.restore(); //clear translation/scaling
    }

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        // Let the ScaleGestureDetector inspect all events.
        if (zoomEnabled) {
            mScaleDetector.onTouchEvent(ev);
        }

        if (panEnabled) {
            final int action = ev.getAction();
            switch (action & MotionEvent.ACTION_MASK) {
                case MotionEvent.ACTION_DOWN: {
                    final float x = ev.getX();
                    final float y = ev.getY();

                    mLastTouchX = x;
                    mLastTouchY = y;
                    mActivePointerId = ev.getPointerId(0);
                    break;
                }

                case MotionEvent.ACTION_MOVE: {
                    final int pointerIndex = ev.findPointerIndex(mActivePointerId);
                    final float x = ev.getX(pointerIndex);
                    final float y = ev.getY(pointerIndex);

                    // Only move if the ScaleGestureDetector isn't processing a gesture.
                    if (!mScaleDetector.isInProgress()) {
                        float dx = x - mLastTouchX;
                        float dy = y - mLastTouchY;

                        //Adjust for zoom factor. Otherwise, the user's finger moving 10 pixels
                        //at 200% zoom causes the image to slide 20 pixels instead of perfectly
                        //following the user's touch
                        dx /= (mScaleFactor * 2);
                        dy /= (mScaleFactor * 2);

                        mPosX += dx;
                        mPosY += dy;

                        Log.v(TAG, "moving by " + dx + "," + dy + " mScaleFactor: " + mScaleFactor);

                        invalidate();
                    }

                    mLastTouchX = x;
                    mLastTouchY = y;

                    break;
                }

                case MotionEvent.ACTION_UP: {
                    mActivePointerId = INVALID_POINTER_ID;
                    break;
                }

                case MotionEvent.ACTION_CANCEL: {
                    mActivePointerId = INVALID_POINTER_ID;
                    break;
                }

                case MotionEvent.ACTION_POINTER_UP: {
                    final int pointerIndex = (ev.getAction() & MotionEvent.ACTION_POINTER_INDEX_MASK) >> MotionEvent.ACTION_POINTER_INDEX_SHIFT;
                    final int pointerId = ev.getPointerId(pointerIndex);
                    if (pointerId == mActivePointerId) {
                        // This was our active pointer going up. Choose a new
                        // active pointer and adjust accordingly.
                        final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
                        mLastTouchX = ev.getX(newPointerIndex);
                        mLastTouchY = ev.getY(newPointerIndex);
                        mActivePointerId = ev.getPointerId(newPointerIndex);
                    }
                    break;
                }
            }
        }
        return true;
    }

    private class ScaleListener extends
            ScaleGestureDetector.SimpleOnScaleGestureListener {
        @Override
        public boolean onScale(ScaleGestureDetector detector) {
            mScaleFactor *= detector.getScaleFactor();
            // Don't let the object get too small or too large.
            mScaleFactor = Math.max(0.1f, Math.min(mScaleFactor, 5.0f));
//          Log.d(TAG, "detector scale factor: " + detector.getScaleFactor() + " mscalefactor: " + mScaleFactor);

            invalidate();
            return true;
        }
    }

    //Currently zoomEnabled/panEnabled can only be set programmatically, not in XML

    public boolean isPanEnabled() {
        return panEnabled;
    }

    public void setPanEnabled(boolean panEnabled) {
        this.panEnabled = panEnabled;
    }

    public boolean isZoomEnabled() {
        return zoomEnabled;
    }

    public void setZoomEnabled(boolean zoomEnabled) {
        this.zoomEnabled = zoomEnabled;
    }

    /**
     * Calls getCroppedBitmap(int outputWidth, int outputHeight) without
     * scaling the resulting bitmap to any specific size.
     * @return
     */
    public Bitmap getCroppedBitmap() {
        return getCroppedBitmap(0, 0);
    }

    /**
     * Takes the section of the bitmap visible in its View object
     * and exports that to a Bitmap object, taking into account both
     * the translation (panning) and zoom (scaling).
     * WARNING: run this in a separate thread, not on the UI thread!
     * If you specify that a 200x200 image should have an outputWidth
     * of 400 and an outputHeight of 50, the image will be squished
     * and stretched to those dimensions.
     * @param outputWidth desired width of output Bitmap in pixels
     * @param outputHeight desired height of output Bitmap in pixels
     * @return the visible portion of the image in the PanZoomImageView
     */
    public Bitmap getCroppedBitmap(int outputWidth, int outputHeight) {
        int origX = -1 * (int) mPosX * 2;
        int origY = -1 * (int) mPosY * 2;
        int width = (int) (viewWidth / mScaleFactor);
        int height = (int) (viewHeight / mScaleFactor);
        Log.e(TAG, "origX: " + origX + " origY: " + origY + " width: " + width + " height: " + height + " outputWidth: " + outputWidth + " outputHeight: " + outputHeight + "getLayoutParams().width: " + getLayoutParams().width + " getLayoutParams().height: " + getLayoutParams().height);
        Bitmap b = Bitmap.createBitmap(bitmap, origX, origY, width, height);

        if (outputWidth > 0 && outputWidth > 0) {
            //Use the exact dimensions given--chance this won't match the aspect ratio
            b = Bitmap.createScaledBitmap(b, outputWidth, outputHeight, true);
        }

        return b;
    }

      @Override
        protected void onSizeChanged(int w, int h, int oldw, int oldh) {
            super.onSizeChanged(w, h, oldw, oldh);
            viewHeight = h;
            viewWidth = w;
        }
}

Answer 2

mScaleFactor = Math.max(0.1f, Math.min(mScaleFactor, 5.0f));

This way you can scale up to 5 times, calculate the maximum scale based on the size you want and add to this instead of using 5 all the time.

Also when I worked with pinch zoom on a project I find it easier to use if you use absolute values instead of multiplying. Just get the distance of the fingers on the first touch and when moving the fingers, calculate the distance and then the scale based on the first distance. This way it follows the fingers better and works better when you limit the minimum and maximum scale.

Answer 3

This post is quite old and already answered, however I found another solution which worked even better for me then the accepted answer.

    protected override void OnDraw(Canvas canvas)
    {
        // Calculate the boundaries of the canvas
        var minX = (int)((_viewWidth / _scaleFactor) - canvas.Width);
        var minY = (int)((_viewHeight / _scaleFactor) - canvas.Height);

        if (_posX > 0)
            _posX = 0;
        else if (_posX < minX)
            _posX = minX;
        if (_posY > 0)
            _posY = 0;
        else if (_posY < minY)
            _posY = minY;

        // Change image position
        canvas.Scale(_scaleFactor, _scaleFactor);
        canvas.Translate(_posX, _posY);

        base.OnDraw(canvas);
    }

    protected override void OnSizeChanged(int w, int h, int oldw, int oldh)
    {
        base.OnSizeChanged(w, h, oldw, oldh);

        _viewHeight = h;
        _viewWidth = w;
    }

Note that this code is written in Xamarin.Android, however converting this to Java will be easy.

This keeps the image perfectly within it's boundaries.