Rotate video from camera preview

n Matlab I can get a preview from a camera with the command: preview(obj,himage) that puts the preview stream into himage.

In my code I have an axes that contains the video preview but I would like to rotate the video by 90 degrees. This might be very stupid but I cannot figure out how is done.

My code is the following:

prevHaxes = axes('Parent', mainFig);

previewImage= imagesc(placeholderImg,'Parent', prevHaxes); %Placeholder for when the stream is off

preview(video_obj, previewImage);

I have tried to use imrotate but I got the following error:

"Expected input number 1, input image, to be one of these types:

numeric, logical

Instead its type was matlab.graphics.primitive.Image."

So my question is, is there a way to get the video preview from a camera but to visualize it rotated by an angle?

Answer 1

[PaoloB]

All right,

found a way and it is posted here for future reference. Assuming "placeholderImg" is the image where the preview will be displayed, it is possible to set a function that will intercept all frames. The function is UpdatePreviewWindowFcn. So before starting the preview, one sets:

setappdata(placeholderImg,'UpdatePreviewWindowFcn',@mypreview_fcn);

and defines mypreview_fcn as

function mypreview_fcn(obj, event, himage)
    rotImg = rot90(event.Data);
    set(himage, 'cdata', rotImg);
end

That is it.

[Jonathan]

Could you give an detail on how to call this function? Thx.

[Ian Hunter]

I would also find that helpful. How do I replace the standard preview() call with the my preview_fcn call such that my figure will be updated with the fetched, then transformed frame. Thanks. -Ian

Answer 2

[Guoheng Zhao]

Another way, not as elegant but simpler, is to set the axis view angle and direction.

[Rob Campbell]

This possibly might be faster, too. Could be a performance hit with the callback. There definitely is when updating an imaging from a camera using a listener and callback instead of using "preview".

Answer 3

[Steve Beguin]

Thanks PaoloB,

This is the most elegant solution I have seen! It works well in my GUI with multiple axes

Here are some hints on how I implemented it:

%////////////////////////////////

global cameraObj ;

global horizontal_flip;

horizontal_flip = 0;

global vertical_flip;

vertical_flip = 0;

global rotation;

rotation = 0;

cameraObj = videoinput('winvideo',selection); %selection is the camera device ID (in my case works for microscope cameras as well as webcams and anything connected through the manyCam software)

axes(handles.axesCamera); %sets the focus on the desired axis of the GUI

vidRes = cameraObj.VideoResolution;

nBands = cameraObj.NumberOfBands;

hImage = image( zeros(vidRes(2), vidRes(1), nBands) );

axis off;

PreviewHandle = preview(cameraObj,hImage); % the handle of the preview object

% //////////////////////////  then in specific callback for radiobutton or pushbutton here is the code

% --- Executes on button press in radiobuttonHorizontalFlip.

function radiobuttonHorizontalFlip_Callback(hObject, eventdata, handles)

global PreviewHandle;
global horizontal_flip;

if get(hObject,'Value')

      horizontal_flip = 1;

else

      horizontal_flip = 0;

end

setappdata(PreviewHandle,'UpdatePreviewWindowFcn',@mypreview_fcn);

% --- Executes on button press in radiobuttonVerticalFlip.

function radiobuttonVerticalFlip_Callback(hObject, eventdata, handles)

global PreviewHandle;

global vertical_flip;

if get(hObject,'Value')

      vertical_flip = 1;

else

      vertical_flip = 0;

end

setappdata(PreviewHandle,'UpdatePreviewWindowFcn',@mypreview_fcn);

% --- Executes on button press in pushbuttonRotatePlus90deg.

function pushbuttonRotatePlus90deg_Callback(hObject, eventdata, handles)

global PreviewHandle;

global rotation;

rotation = rotation + 1;

setappdata(PreviewHandle,'UpdatePreviewWindowFcn',@mypreview_fcn);

% --- Executes on button press in pushbuttonRotateMinus90deg.

function pushbuttonRotateMinus90deg_Callback(hObject, eventdata, handles)

global PreviewHandle;

global rotation;

rotation =  rotation - 1;

setappdata(PreviewHandle,'UpdatePreviewWindowFcn',@mypreview_fcn);

function mypreview_fcn(obj, event, himage)

global horizontal_flip;

global vertical_flip;

global rotation;

Img = event.Data;

if horizontal_flip

      Img = fliplr(Img);

end

if vertical_flip

      Img = flipud(Img);

end

Img = rot90(Img,rotation);

set(himage, 'cdata', Img);

%////////////////////

Hope it will be helpful for someone trying to achieve the same thing

Cheers,

Steve