Question

I have a bitmap image that I’m trying to do a hit test on. The hit test works if its just a normal bitmap. But I need to rotate and scale the bitmap and I just can’t seem to figure out the hit test properly.

x and y here are the cursor x and y. I need to check if the cursor (finger press) was clicked inside the manipulated bitmap. The scale seems to work fine, but the rotation doesn’t seem to take affect.

float[] pts = new float[4];
float left = m.getX();
float top = m.getY();
float right = left + mBitmaps.get(i).getWidth();
float bottom = top + mBitmaps.get(i).getHeight();
pts[0] = left;
pts[1] = top;
pts[2] = right;
pts[3] = bottom;

float midx = left + mBitmaps.get(i).getWidth()/2;
float midy = top + mBitmaps.get(i).getHeight()/2;

Matrix matrix = new Matrix();
matrix.setRotate(m.getRotation(), midx, midy);
matrix.setScale(m.getSize(), m.getSize(), midx, midy);

matrix.mapPoints(pts);

if(x >= pts[0] && x <= pts[2] && y >= pts[1] && y <= pts[3])
{
    return i;
}

Answer 1

Your test fails because after rotation the rectangle is no longer aligned to the coordinate axes.

A trick you can do is to transform the cursor position back with the inverse transformation matrix and then compare the transformed position with the original rectangle.

Matrix matrix = new Matrix();
matrix.setRotate(m.getRotation(), midx, midy);
matrix.postScale(m.getSize(), m.getSize(), midx, midy);

Matrix inverse = new Matrix();
matrix.invert(inverse);
pts[0] = x;
pts[1] = y;
inverse.mapPoints(pts);
if(pts[1] >= top && pts[1] <= bottom && pts[0] >= left && pts[0] <= right)
{
    return i;
}