Android converting touch points to actual bitmap points on an image

Question

Here is my problem:

I have an Android application that displays an image. The image itself is resized to 480 x 640 regardless of size.

The user can click on multiple points of the image. Based on where the user clicks on the image, the bitmap itself has some warping applied to it.

So let's say the original image is 1000 x 2000 (using whole numbers to make it simpler). Once the image is loaded into the ImageView, it scales to display properly in the imageview.

This is obviously different for different phones with different resolutions.

Now when the user clicks on different points, I ultimately want to pass those points to my WCF Service along with the bitmap data to perform some image manipulation.

So the problem for me is how to take the points where the user touched on the phone and convert those to points that are relative to the normal unscaled bitmap.

Summary:

Bitmap is scaled to fit. User Clicks at 100,100. 100,100 is the point relative to the scaled image...not the actual bitmap itself. I'm looking for guidance on how to convert that 100,100 to the point on the actual bitmap.

Thanks in advance for any help you can give.

Answer 1

ok, so the Android ImageView has a default ScaleType of FIT_CENTER, so that means:

public static final Matrix.ScaleToFit CENTER

Compute a scale that will maintain the original src aspect ratio, but will also ensure that src fits entirely inside dst. At least one axis (X or Y) will fit exactly. The result is centered inside dst.

so if you're whole image view has 480x640 to show the image, and for example your image is 1000x2000, then:

2000/640 = scaleFactor = 3.125/1

so width will scale down to 320 leaving 80 pixels on either side empty, so it can maintain the aspect ratio.

//this one will be 80.
int xBuffer= (imageViewWidth - (realImageWidth*scaleFactor))/2;
//this one will be zero in your example    
int yBuffer = (imageViewHeight - (realImageHeight*scaleFactor))/2; 


int imageViewX = 0;//x coord where on the image view it was clicked
int imageViewY = 0;//y coord where on the image view it was clicked

if (imageViewX < xBuffer || imageViewX > imageViewWidth-xBuffer)
{
     //ignore the click, outside of your image.
}
else if (imageViewY < yBuffer || imageViewY > imageViewHeight-yBuffer)
{
     //ignore the click, outside of your image.
}
else
{
    realImageY = imageViewY * scaleFactor;
    realImageX = (imageViewY - 80) * scaleFactor;
    //save click somehow..
    saveClick(realImageX,realImageY);
}