moving crop window before live preview of camera

[Suisse]

I am looking for this for a long time. Any suggestion will be appreciated. 

I am detecting the face of a person in the camera preview with google MLKit.

It means I have for each frame of the camera the boundaries and scale of the face.

face_found = [x,y,w,h]; 

I now want to only display the face of the user, since I have every argument I need to crop the face out of the frame and only show this region of interest (ROI). 

How can I achieve this goal? And if its not possible with CameraX, is there something else I can use for this? How is snapchat doing such tasks? 

[Charcoal Chen]

To achieve the goal, you may need to render the camera preview with opengl processing. CameraX has a coretestapp sample project implemented in that way. Maybe you can try to add an updateFaceRect() function in OpenGLRenderer.java and call it when a new face location rectangle is found to make the preview output result only show target rectangle area:

// Update  the face location rectangle

void updateFaceRect(RectF faceRect) {

    // Update the preview crop rectangle to be the found face location rectangle

    mPreviewCropRect = faceRect;

    // Trigger to update transformations to only show the face location rectangle area

    mMvpDirty = true;

[Xi Zhang (张熹)]

If you prefer not to use OpenGL, you can also achieve cropping by calling setTranslationX/setTranslationY/setScaleX/setScaleY/setRotation on the view finder. You just need to make sure a TextureView is used to display the preview. 

However if you are already using OpenGL, or if you need a skew or non-affine transformation, then please see Charcoal's solution.

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