How to get RGB-values out of video frames using VideoReader-function. When reading an individual frame using impixel-function I get only 3 times zero for RGB.

2015 6 30
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更新时间：2015 7 3
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videoObject = VideoReader('TypeI_4_1_TestClipped.wmv');
imageData = read(videoObject);
numFrames = get(videoObject, 'NumberOfFrames');
Heig= get(videoObject, 'Height');
Wid= get(videoObject, 'Width');
Rateframe= get(videoObject, 'FrameRate');
time=get(videoObject, 'Duration');
% To cut down the time for this trial height and width are reduced
Wid=200;
Heig=100;
Hii = Heig*(ones(1,Wid));
mov(10).cdata = read(videoObject,10);
P1=impixel(mov(10).cdata,wii,Hii);
% or
imagedata=read(videoObject,10);
P2=impixel(mov(10).cdata,wii,Hii);
% Both P1 and P2 appear to became 200 x 3 full of zeros

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Walter Roberson
Walter Roberson 2015-6-30
What is wii ?
The code is demonstrating that you can read into a "movie" structure or that you can treat the data as "one frame at a time". The same content is going to go into mov(10).cdata and to imagedata so you would expect the same results.
Dinesh Iyer
Dinesh Iyer 2015-7-1
Can you examine the output of VideoReader/read using
imshow(imageData(:, :, :, 2)) % 2 can be replaced by any valid index
before modifying it in your code?
Pekka Koivisto
Pekka Koivisto 2015-7-2
There was more than one error in the code I sent - however it did not affect the problem. Here is what I tried: videoObject = VideoReader('TypeI_4_1_TestClipped.wmv'); imageData = read(videoObject);
numFrames = get(videoObject, 'NumberOfFrames');
Heig= get(videoObject, 'Height');
Wid= get(videoObject, 'Width');
Rateframe= get(videoObject, 'FrameRate');
time=get(videoObject, 'Duration');
% To cut down the time for this trial height and width are reduced
Wid=200;
Heig=100;
Hii = Heig*(ones(1,Wid));
wii=200;
mov(10).cdata = read(videoObject,10);
P1=impixel(mov(10).cdata,wii,Hii);
% or
imagedata=read(videoObject,10);
P2=impixel(imageData,wii,Hii);
% Both P1 and P2 appear to became 200 x 3 full of zeros
Pekka Koivisto
Pekka Koivisto 2015-7-3
I have now solved my problem. It was not in the code but in the video. During the cropping process I had not taken into account that the frame size DOES NOT shrink, but the rest of the frame is black, which is in RGB=0, 0, 0. So now when I focused the pixels correctly I received the real RGB values ! Thanks for everyone participating the problem. With best regards Pekka Koivisto

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Nalini Vishnoi
Nalini Vishnoi 2015-7-2

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Hi Pekka,
I understand that you would like to get the RGB values out of video frames using 'VideoReader' and 'impixel' function. As other users have pointed out there is some ambiguity in your example code, hence I am unable to test it. Please find a small example below demonstrating the use of 'VideoReader' and 'impixel' functions to read the RGB value of a frame:
>> xyloObj = VideoReader('xylophone.mp4');
>> frame = read(xyloObj,10); % reading the 10th frame
>> c = 200;
>> r = 100;
>> rgb = impixel(frame, c, r); % returns the RGB value of column 200 and row 100
>> c = 101:200;
>> r = 1:100;
>> rgb = impixel(frame, c, r); % returns the RGB values of frame(r,c). Here r and c are vectors
I hope the above example helps. You can find more information about how impixel function works in the documentation link here.
Thanks,
Nalini

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Pekka Koivisto
Pekka Koivisto 2015-7-2
Yes in the code I sent wii was not defined HOWEVER that was not the issue here. I copy pasted the code Nalini sent me and tested it with the video (wmv) file I am struggling with (yes it has been checked to be full of other than 0 rgb:s) and once again I received a matrix full of zeros. So unfortunately this code did not help me in my problem. All the best Pekka Koivisto

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