Skip to content

Rough Book

random musings

Menu
  • About Me
  • Contact
  • Projects
    • bAdkOde
    • CherryBlossom
    • FXCalendar
    • Sulekha
Menu

Implementing pinch-zoom and pan/drag in an Android view on the canvas

Posted on December 4, 2011July 17, 2020 by vivin

Now that we've identified the different states we are in, we can go on and start to implement the dragging/panning functionality. Panning essentially equates to a translation operation on the canvas. By translating the canvas, we control what part of the zoomed-in canvas we display on the device screen. So what information do we need to translate? Well, think about what we expect to see when we pan. We place our finger on the screen and then we move it by a certain amount in a certain direction. We essentially expect the image to move by that same amount, in that same direction as well. So the direction and magnitude that we need to translate the image by, can be gathered from the coordinate where we first pressed the screen, through each point until we take our finger off the screen. Getting all this information is pretty easy. We can get the X and Y coordinate of the finger using event.getX() and event.getY(). So all we need to do is get the X and Y coordinate when we first press our finger on the screen, and translate the image as we move our finger across the screen, by getting the finger's X and Y coordinates each time the finger moves. We can do that like this:

public class ZoomView extends View {

    ...
    ...

    //These two variables keep track of the X and Y coordinate of the finger when it first
    //touches the screen
    private float startX = 0f;
    private float startY = 0f;

    //These two variables keep track of the amount we need to translate the canvas along the X
    //and the Y coordinate
    private float translateX = 0f;
    private float translateY = 0f;

    ...
    ...

    @Override
    public boolean onTouchEvent(MotionEvent event) {

        switch (event.getAction() & MotionEvent.ACTION_MASK) {
            case MotionEvent.ACTION_DOWN:
                mode = DRAG;

                //We assign the current X and Y coordinate of the finger to startX and startY. So these variables now
                //hold the X and Y coordinate of the finger as it first touches the screen.                
                startX = event.getX();
                startY = event.getY();
                ...
                break;

            case MotionEvent.ACTION_MOVE:
                //We calculate the values of translateX and translateY by finding the difference between the X/Y coordinate
                //and the starting X/Y coordinate. Since this event is fired every time the finger moves, we're constantly
                //updating the values of these two coordinates
                translateX = event.getX() - startX;
                translateY = event.getY() - startY;
                break;

            ...
            ...
        }

        ...
        ...
    }
}
Digiprove sealCopyright protected by Digiprove © 2020 Vivin PaliathSome Rights Reserved
Original content here is published under these license terms: X 
License Type:Attribution
License Abstract:You may copy this content, create derivative work from it, and re-publish it, provided you include an overt attribution to the author(s).
License URL:http://creativecommons.org/licenses/by/3.0/
Pages: 1 2 3 4 5 6 7 8

58 thoughts on “Implementing pinch-zoom and pan/drag in an Android view on the canvas”

Comments navigation

Older comments
  1. Catzie says:
    January 16, 2018 at 10:21 pm

    I appreciate how clear you tried to explain everything. Thank you!

    Reply
  2. Antoine Bon says:
    April 24, 2018 at 9:12 am

    I want to do a “zoomable paint”, I mean a paint that I can zoom/zoom out and pan/drag the canvas and then draw on it.

    I have a problem that I can’t solve: when I draw while the canvas is zoomed, I retrieve the X and Y coordinate and effectively drawing it on the canvas. But these coordinates are not correct because of the zoomed canvas.

    I tried to correct these (multiply by (zoomHeigh/screenHeight)) but I can’t find a way to retrieve where I must draw on the original/none-zoomed screen

    Reply
  3. Pingback: android - Vue avec panoramique / traînée horizontale et verticale et pincement-zoom
  4. Pingback: android - Vista horizontal y vertical de la cacerola/de arrastre y pinch-zoom
  5. Monika says:
    August 15, 2020 at 7:30 am

    Whenever I try to implement above method the screen jitters on account of zoom effect but does not zoom essentially. I am drawing circle points on canvas and on zooming I want them to scale further apart. I don’t know if I tend to custom draw those circle points based on dimensions of zoomed canvas why is the overall effect a mere jitter ?

    Reply

Comments navigation

Older comments

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

Meta

  • Log in
  • Entries feed
  • Comments feed
  • WordPress.org

Archives

  • February 2023
  • April 2020
  • February 2020
  • January 2020
  • December 2019
  • November 2019
  • September 2019
  • August 2019
  • July 2019
  • June 2019
  • May 2019
  • March 2019
  • February 2019
  • January 2019
  • December 2018
  • November 2018
  • September 2018
  • August 2018
  • July 2018
  • June 2018
  • May 2018
  • April 2018
  • March 2018
  • February 2018
  • January 2018
  • December 2017
  • November 2017
  • October 2017
  • June 2017
  • March 2017
  • November 2016
  • August 2016
  • July 2016
  • June 2016
  • February 2016
  • August 2015
  • July 2014
  • June 2014
  • March 2014
  • December 2013
  • November 2013
  • September 2013
  • July 2013
  • June 2013
  • March 2013
  • February 2013
  • January 2013
  • October 2012
  • July 2012
  • June 2012
  • January 2012
  • December 2011
  • November 2011
  • October 2011
  • September 2011
  • July 2011
  • June 2011
  • May 2011
  • February 2011
  • January 2011
  • December 2010
  • November 2010
  • October 2010
  • September 2010
  • July 2010
  • June 2010
  • May 2010
  • April 2010
  • March 2010
  • January 2010
  • December 2009
  • November 2009
  • October 2009
  • September 2009
  • August 2009
  • July 2009
  • May 2009
  • April 2009
  • March 2009
  • February 2009
  • January 2009
  • December 2008
  • November 2008
  • October 2008
  • August 2008
  • March 2008
  • February 2008
  • November 2007
  • July 2007
  • June 2007
  • May 2007
  • March 2007
  • December 2006
  • October 2006
  • September 2006
  • August 2006
  • June 2006
  • April 2006
  • March 2006
  • January 2006
  • December 2005
  • November 2005
  • October 2005
  • September 2005
  • August 2005
  • July 2005
  • June 2005
  • May 2005
  • April 2005
  • February 2005
  • October 2004
  • September 2004
  • August 2004
  • July 2004
  • June 2004
  • May 2004
  • April 2004
  • March 2004
  • February 2004
  • January 2004
  • December 2003
  • November 2003
  • October 2003
  • September 2003
  • July 2003
  • June 2003
  • May 2003
  • March 2003
  • February 2003
  • January 2003
  • December 2002
  • November 2002
  • October 2002
  • September 2002
  • August 2002
  • July 2002
  • June 2002
  • May 2002
  • April 2002
  • February 2002
  • September 2001
  • August 2001
  • April 2001
  • March 2001
  • February 2001
  • January 2001
  • December 2000
  • November 2000
  • October 2000
  • August 2000
  • July 2000
  • June 2000
  • May 2000
  • March 2000
  • January 2000
  • December 1999
  • November 1999
  • October 1999
  • September 1999
©2023 Rough Book | Built using WordPress and Responsive Blogily theme by Superb
All original content on these pages is fingerprinted and certified by Digiprove