Develop Reference

Rotation and translation at a specific point

I need to rotate an image at a specific point and then translate it to a specific point on the screen.
The point I want to rotate it at is in the center of the picture.
The traslation works, but the rotation doesn't.
I have a Vector of bitmap and I'm using Canvas and Matrix.
Code:
for (Bitmap image:images)
{
//rotation
double angle=Math.toDegrees(rotation);
Matrix matrix=new Matrix();
matrix.postRotate((float)angle,finalMap.getWidth()/2-1,0);
//transform
matrix.setTranslate(position.x,position.y);
//print on screen
c.drawBitmap(image,matrix, paint);
}

Try changing your rotation/translation calls like this (in exactly this order):
matrix.setTranslate(position.x,position.y);
matrix.preRotate((float)angle,finalMap.getWidth()/2-1,0);
The reason it doesn't work the way you have it currently, is that your setTranslate() call is discarding the rotation that you previously did and just replacing it with a translation performed on an identity matrix. Matrix transformation methods starting with the "set" prefix will just apply the transformation as if nothing happened before them.
If you want to read more, this is a useful answer: https://stackoverflow.com/a/8197896/2464728

What does 'the rotation does not' mean? How is it failing to do as you expect?
I would initially wonder about the use of 0 for the y rotation point.

Bitmap coordinates after canvas.scale

I need help at getting the correct coordinates when touching the canvas.
I have an image that is 1240x1756
The user can scroll around on this bitmap. It is the bitmap attached to the canvas. I do this by translating the canvas. canvas.translate()
The user can also place a new bitmap and move that anywhere around on the canvas. up to 256x256.
Up until this point I have no problem calculating the touch coordinates at scale factor of 1.0. I take the screen touch coords and add the offset of the image and then check to see if the 256x256 moveable bitmap intersects the touch coords.
However my issue is getting the correct coordinates after the canvas has been scaled. 0.1-1.0 are my minimum and maximum scaling values.
Can anyone point me in the right direction for a working algorithm?

You should be able to just multiply the pixel offset by 1/scale factor.

Getting a scale factor of a Bitmap after rotating it with a matrix

I've got the following problem which I tried to solve the whole day.
I load a Bitmap picture with his corresponding height and width into an ImageView.
Then I use a matrix for moving,scaling and rotating the image.
For scaling I'm using the postScale method.
For moving I'm using the postTranslate method.
Only for Rotating I'm using the preRotate method.
Now I need to get the factor I scaled the image with, because I later need this factor in another program.
Using the MSCALE_X and MSCALE_Y values of the matrix only fits until I did no rotation. If I rotated the image, the scale values don't fit anymore (because the matrix was multiplied with the formula which is shown in the api).
Now my Question is:
How can I still get the scale factor of the image after rotating it?
For the rotation factor (degrees) it is simple, because I store it within an extra variable which is icremented/decremented while rotating.
But for the scale factor it does not work, because if I first scale an image down to 50% and then rescale it up to 150% then I scaled it with a factor of 3 but the original scaling factor is only 1.5).
Another example is. Even if I did not rescale the picture it even changes its scaling factor if I rotate it.
//Edit:
Finally I solved the problem on my own :) (doing a bit math and then I figured something interesting (or lets say obvious) out).
Here my solution:
I figured out that the values MSCALE_X and MSCALE_Y are calculated by using the cosinus function (yeah the basic math...). (Using 0° rotation leads to the correct scalingWidth and scalingHeight within X and Y). (90 and 270° results in a scalingWidth/Height of 0 and 180° results in a scalingWidth/Height multiplied by -1).
This leads me to the idea to write the following function:
This function saves the current matrix within a new matrix. Then it rotates the new matrix to the startstate (0°). Now we can read the non violated values MSCALE_X and MSCALE_Y in our matrix (which are the correct scaling factors now)

I had the same problem. This is a simple way and the logic is sound (in my mind).
For: xScale==yScale
float scale = matrix.mapRadius(1f) - matrix.mapRadius(0f);
For: xScale != yScale
float[] points={0f,0f,1f,1f};
matrix.mapPoints(points);
float scaleX=points[2]-points[0];
float scaleY=points[3]-points[1];
If you are not translating, you may be able to get away with just with 1f vector/point. I've tested the xScale==yScale (mapRadius) variant and it seems to work.

I had a similar problem with an app I'm writing. I couldn't see an obvious an simple solution so I just created a RectF object that had the same initial coords of the bitmap. Then, everytime I adjusted the matrix, I'd apply the transformation to the RectF as well (using Matrix.mapRect() ). This worked perfectly for me. It also allowed me to keep track of the absolute position of the edges of the bitmap.

Billboarding in Android OpenGL ES 1.0

I'm trying to get billboarding to work, but having trouble with the last step.
After following these directions from NeHe's tutorials (http://nehe.gamedev.net/data/articles/article.asp?article=19) I have my look, right, and up vectors, and I have already translated the modelview matrix to the centerpoint of the billboard by using glTranslatef().
float[] m = {right.x,right.y,right.z,0f,
up.x,up.y,up.z,0f,
look.x,look.y,look.z,0f,
pos.x,pos.y,pos.z,1f}; //pos is the centerpoint position
gl.glMultMatrixf(m, 0);
When I try to create a multiplication matrix out of these like so, the billboards are displayed all over the place in the wrong positions and orientations.
I guess my problem is that I don't know how to correctly create and multiply the matrix. I tried doing this instead, but then half the lines (the ones that need to rotate counter-clockwise) are rotated in the wrong direction:
//normal is the vector that the billboard faces before any manipulations.
float angle = look.getAngleDeg(normal); //returns angle between 0 and 180.
gl.glRotatef(angle, up.x,up.y,up.z);

Got it, using my second method. Calculating the angle between vectors (arccos of the dot product) can only give an angle between 0 and 180, so half the time you want to negate the angle so the rotation is in the opposite direction.
This is easy to check...since I already have the right vector, I can just check if the angle between the right vector and the normal is acute. If it's acute, then you want to negate the original angle.

3D rotation while object being translated

I've been playing with Android animation framework and I found the following 3D rotation sample code:
http://developer.android.com/resources/samples/ApiDemos/src/com/example/android/apis/animation/Transition3d.html
http://developer.android.com/resources/samples/ApiDemos/src/com/example/android/apis/animation/Rotate3dAnimation.html
It does pretty much what I want but I want the ImageView to rotate while it's being translated from point A to point B and the it should rotate along it's own center(which is moving) instead of the center of the container of screen.
Does anyone know how to do that?
-Rachel

Well it's pretty close to what you posted. Essentially you're multiplying the rotational matrix by the translation matrix. That's essentially what happens under the covers. Android hides that detail from you with it's API:
Matrix matrix = transformation.getMatrix();
matrix.rotate( rotateX, rotateY );
matrix.postTranslate( transX, transY );
Rotate first then translate will rotate the image around it's own axis first before translating it.