Rotating An Object With Touch Events

Rotating an object in 3D is a neat way of letting your users interact with the scene, but the math can be tricky to get right. In this article, I’ll take a look at a simple way to rotate an object based on the touch events, and how to work around the main drawback of this method.

Simple rotations.

This is the easiest way to rotate an object based on touch movement. Here is example pseudocode:

Matrix.setIdentity(modelMatrix);

... do other translations here ...

Matrix.rotateX(totalYMovement);
Matrix.rotateY(totalXMovement);

This is done every frame.

To rotate an object up or down, we rotate it around the X-axis, and to rotate an object left or right, we rotate it around the Y axis. We could also rotate an object around the Z axis if we wanted it to spin around.

How to make the rotation appear relative to the user’s point of view.

The main problem with the simple way of rotating is that the object is being rotated in relation to itself, instead of in relation to the user’s point of view. If you rotate left and right from a point of zero rotation, the cube will rotate as you expect, but what if you then rotate it up or down 180 degrees? Trying to rotate the cube left or right will now rotate it in the opposite direction!

One easy way to work around this problem is to keep a second matrix around that will store all of the accumulated rotations.

Here’s what we need to do:

  1. Every frame, calculate the delta between the last position of the pointer, and the current position of the pointer. This delta will be used to rotate our accumulated rotation matrix.
  2. Use this matrix to rotate the cube.

What this means is that drags left, right, up, and down will always move the cube in the direction that we expect.

Android Code

The code examples here are written for Android, but can easily be adapted to any platform running OpenGL ES. The code is based on Android Lesson Six: An Introduction to Texture Filtering.

In LessonSixGLSurfaceView.java, we declare a few member variables:

private float mPreviousX;
private float mPreviousY;

private float mDensity;

We will store the previous pointer position each frame, so that we can calculate the relative movement left, right, up, or down. We also store the screen density so that drags across the screen can move the object a consistent amount across devices, regardless of the pixel density.

Here’s how to get the pixel density:

final DisplayMetrics displayMetrics = new DisplayMetrics();
getWindowManager().getDefaultDisplay().getMetrics(displayMetrics);
density = displayMetrics.density

Then we add our touch event handler to our custom GLSurfaceView:

public boolean onTouchEvent(MotionEvent event)
{
	if (event != null)
	{
		float x = event.getX();
		float y = event.getY();

		if (event.getAction() == MotionEvent.ACTION_MOVE)
		{
			if (mRenderer != null)
			{
				float deltaX = (x - mPreviousX) / mDensity / 2f;
				float deltaY = (y - mPreviousY) / mDensity / 2f;

				mRenderer.mDeltaX += deltaX;
				mRenderer.mDeltaY += deltaY;
			}
		}

		mPreviousX = x;
		mPreviousY = y;

		return true;
	}
	else
	{
		return super.onTouchEvent(event);
	}
}

Every frame, we compare the current pointer position with the previous, and use that to calculate the delta offset. We then divide that delta offset by the pixel density and a slowing factor of 2.0f to get our final delta values. We apply those directly to the renderer to a couple of public variables that we have also declared as volatile, so that they can be updated between threads.

Remember, on Android, the OpenGL renderer runs in a different thread than the UI event handler thread, and there is a slim chance that the other thread fires in-between the X and Y variable assignments (there are also additional points of contention with the += syntax). I have left the code like this to bring up this point; as an exercise for the reader I leave it to you to add synchronized statements around the public variable read and write pairs instead of using volatile variables.

First, let’s add a couple of matrices and initialize them:

/** Store the accumulated rotation. */
private final float[] mAccumulatedRotation = new float[16];

/** Store the current rotation. */
private final float[] mCurrentRotation = new float[16];
@Override
public void onSurfaceCreated(GL10 glUnused, EGLConfig config)
{

    ...

    // Initialize the accumulated rotation matrix
     Matrix.setIdentityM(mAccumulatedRotation, 0);
}

Here’s what our matrix code looks like in the onDrawFrame method:

// Draw a cube.
// Translate the cube into the screen.
Matrix.setIdentityM(mModelMatrix, 0);
Matrix.translateM(mModelMatrix, 0, 0.0f, 0.8f, -3.5f);

// Set a matrix that contains the current rotation.
Matrix.setIdentityM(mCurrentRotation, 0);
Matrix.rotateM(mCurrentRotation, 0, mDeltaX, 0.0f, 1.0f, 0.0f);
Matrix.rotateM(mCurrentRotation, 0, mDeltaY, 1.0f, 0.0f, 0.0f);
mDeltaX = 0.0f;
mDeltaY = 0.0f;

// Multiply the current rotation by the accumulated rotation, and then set the accumulated
// rotation to the result.
Matrix.multiplyMM(mTemporaryMatrix, 0, mCurrentRotation, 0, mAccumulatedRotation, 0);
System.arraycopy(mTemporaryMatrix, 0, mAccumulatedRotation, 0, 16);

// Rotate the cube taking the overall rotation into account.
Matrix.multiplyMM(mTemporaryMatrix, 0, mModelMatrix, 0, mAccumulatedRotation, 0);
System.arraycopy(mTemporaryMatrix, 0, mModelMatrix, 0, 16);
  1. First we translate the cube.
  2. Then we build a matrix that will contain the current amount of rotation, between this frame and the preceding frame.
  3. We then multiply this matrix with the accumulated rotation, and assign the accumulated rotation to the result. The accumulated rotation contains the result of all of our rotations since the beginning.
  4. Now that we’ve updated the accumulated rotation matrix with the most recent rotation, we finally rotate the cube by multiplying the model matrix with our rotation matrix, and then we set the model matrix to the result.

The above code might look a bit confusion due to the placement of the variables, so remember the definitions:

public static void multiplyMM (float[] result, int resultOffset, float[] lhs, int lhsOffset, float[] rhs, int rhsOffset)

public static void arraycopy (Object src, int srcPos, Object dst, int dstPos, int length)

Note the position of source and destination for each method call.

Trouble spots and pitfalls
  • The accumulated matrix should be set to identity once when initialized, and should not be reset to identity each frame.
  • Previous pointer positions must also be set on pointer down events, not only on pointer move events.
  • Watch the order of parameters, and also watch out for corrupting your matrices. Android’s Matrix.multiplyMM states that “the result element values are undefined if the result elements overlap either the lhs or rhs elements.” Use temporary matrices to avoid this problem.
WebGL examples

The example on the left uses the simplest method of rotating, while the example on the right uses the accumulated rotations matrix.

Your browser does not support the canvas tag. Your browser does not support the canvas tag.
Further exercises

What are the drawbacks of using a matrix to hold accumulated rotations and updating it every frame based on the movement delta for that frame? What other ways of rotation are there? Try experimenting, and see what else you can come up with!

  

OpenGL ES Roundup, February 12, 2012

a single vanilla ice cream sandwich
Image via Wikipedia

If you haven’t checked it out yet, I recommend taking a look at the new Android Design website. There are a lot of resources and interesting information on developing attractive apps for Ice Cream Sandwich, Android’s newest platform. With Ice Cream Sandwich comes new changes, such as the deprecation of the menu bar.

Roundup

Question for you, dear reader: What do you all think about the current site design? I’d love to hear your thoughts and feedback, both positive and negative.

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OpenGL ES Roundup, October 4, 2011

Diney from db-in.com has a great introduction to shaders up at his site. He annotates his post with useful diagrams and also has a clear and neat introduction to tangent space.

Learning WebGL has a cool roundup of WebGL around the web.

The NeHe OpenGL site is alive! They are starting to post updates on a regular basis again.

Is 3D the future of Android? This article by The Droid Demos takes a look at the emerging 3D screen technologies.

Android and Me has a possible screenshot of the Nexus Prime, Google’s next flagship device which will run Ice Cream Sandwich.

How to Embed Webgl into a WordPress Post

Embedding into WordPress

This information was originally part of WebGL Lesson One: Getting Started, but I thought it would be more useful if I also broke it out into a post on its own.

Embedding WebGL into WordPress can be a little tricky, but if you use the right tags and the HTML editor it can be done without too much trouble! You’ll need to insert a canvas in your post, script includes for any third-party libraries, and a script body for your main script (this can also be an include).

First, place a canvas tag where you would like the graphics to appear:

<pre><canvas id="canvas" width="550" height="375">Your browser does not support the canvas tag. This is a static example of what would be seen.</canvas></pre>

Then, include any necessary scripts:

<pre><script type="text/javascript" src="http://www.learnopengles.com/wordpress/wp-content/uploads/2011/06/webgl-utils.js"></script></pre>

Embed any additional scripts that you need, such as the main body for your WebGL script:

<pre><script type="text/javascript">
/**
* Lesson_one.js
*/

...

</script></pre>

The <pre> tag is important; otherwise WordPress will mangle your scripts and insert random paragraph tags and other stuff inside. Also, once you’ve inserted this code, you have to stick to using the HTML editor, as the visual editor will also mangle or delete your scripts.

If you’ve embedded your scripts correctly, then your WebGL canvas should appear, like below:

Your browser does not support the canvas tag. This is a static example of what would be seen.
WebGL lesson one, example of what would be seen in a browser.

Hope this helps out some people out there!








The Project Code Has Moved to Github!

Hi guys,

It’s been ages since I last posted an update, I know. I went away during the summer and neglected the site upon coming back, and now that I’m busy with school it’s been harder than ever to find the time to find an update. Excuses, excuses, I know. 😉

In any case, many of you were asking for a tutorial and demo on texturing, and this is what I’m going to talk about next. There also seems to be a lot more interest for Android tutorials rather than WebGL tutorials, so I will be focusing more time on Android. Let me know if you guys have other thoughts and suggestions.

The project source code is now moving to GitHub! The project page is located at http://learnopengles.github.com/Learn-OpenGLES-Tutorials/ and the repository is located at https://github.com/learnopengles/Learn-OpenGLES-Tutorials. The old repository at https://code.google.com/p/learn-opengles-tutorials/ will remain, but will no longer be updated going forward.

There was nothing wrong with the Google Code project site, and in fact I prefer the simplicity of Google’s interface, but I also prefer to develop using Git. Once you’ve gotten used to Git, it’s hard to go back to anything else. An advantage of GitHub is that it should be easier for others to fork and contribute to the project if they wish to.

As always, let me know your comments and thoughts. The code for Lesson 4 is already done, so I’ll start writing it up now and hopefully publish that soon!

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