hasFrame and readFrame are not working in synchrony.

It is interesting that hasFrame and readFrame doesn't work in synchrony. Here is the example:

atFrame = 0;
while hasFrame(videoObject)
    atFrame = atFrame +1;
    
    if atFrame < myConstant % Say myConstant be 500
        continue
    end
    
    thatFrame = readFrame(videoObject);
    imshow(thatFrame)
    pause
end

Then what you will see is the first frame of the video object but not 500th frame. hasFrame looked for 500 times consequently into the video object to verify there still contained at least one image in the video object but readFrame didn't synchronize with hasFrame.

Is there any way to start video scan with hasFrame and make the video object being read at that point?

Answer 1

[darova]

Just get numer of frames

xyloObj = VideoReader('xylophone.mp4');
n = xyloObj.NumberOfFrames;
for i = 1:n
    fr = read(xyloObj,i);
    imshow(fr)
    pause(.5)
end

[Neon Argentus]

xyloObj.NumberOfFrames is not a good way to get number of frames with variable frame rate videos. It's thus I use hasFrame+readFrame combination to scan the video from the beginning and read all possible frames step by step. In order the read function to know it is on the frame i, it needs to execute either readFramesUntilEnd() or readFramesInIndexRange().

I'm looking for starting a scan from the beginning and let the reading in the middle of the scan.

[darova]

• xyloObj.NumberOfFrames is not a good way to get number of frames with variable frame rate videos.

Why?

[Neon Argentus]

VideoReader, read function and Duration*FrameRate formula may compute different number of frames. To be sure how many frames exist, the best way, to my opinion, is scan the video and verify each frame exists then do processing on them. I have some videos that VideoReader.NumFrames and result of the hasFrame-readFrame scan differs by six.

[darova]

Aha. Thank you

Answer 2

[Walter Roberson]

hasFrame() does not trigger reading a frame. You can test hasFrame() any number of times you want without it changing the frame number.

atFrame = 0;
while hasFrame(videoObject)
    atFrame = atFrame +1;
    
    thatFrame = readFrame(videoObject);    
    
    if atFrame < myConstant % Say myConstant be 500
        continue
    end
    
    imshow(thatFrame)
    pause
end

You would use this method when you were sure that you wanted to proceed by way of frame number, regardless of what time that frame occurred at. If you wanted to proceed to a particular time, then you should instead set the object CurrentTime property.

Note: for backwards compatibility, it is still possible to use read(videoObject, frame_number) . This is not recommended because read() can end up triggering it to read the entire video just to get an accurate frame count (the old read() interface was defined to set the number of frames field to be exactly accurate; the readFrame() interface is only defined to set the number of frames field to highest frame number you have read so far.)