how to select specific frames from a video in MATLAB?

问题

I am working on a project about lip recognition and I have to read a recorded a video at a frame rate of 30 fps so, if I have 70 frames I need just to acquire or select a representative frame every 8 frames as the shortest video which has number of frames in the data set is of 16 frames but my problem is to adjust the for loop every 8 frames,and it can't read any frame is the problem about the video reader??so,please if you have any idea I would be grateful thanks,,

v = VideoReader('1 - 1.avi');
s = floor((size(v,4))/8);
for t =1:s:size(v,4)
img = read(s,i);
y = imresize(img,[100,120];

回答1:

I would take the example for VideoReader and modify the code to explain -

%%// Paramters
sampling_factor = 8;
resizing_params = [100 120];

%%// Input video
xyloObj = VideoReader('xylophone.mpg');

%%// Setup other parameters
nFrames = floor(xyloObj.NumberOfFrame/sampling_factor); %%// xyloObj.NumberOfFrames;
vidHeight = resizing_params(1); %// xyloObj.Height;
vidWidth = resizing_params(1); %// xyloObj.Width;

%// Preallocate movie structure.
mov(1:nFrames) = struct('cdata', zeros(vidHeight, vidWidth, 3, 'uint8'),'colormap',[]);

%// Read one frame at a time.
for k = 1 :nFrames
    IMG = read(xyloObj, (k-1)*sampling_factor+1);
    %// IMG = some_operation(IMG);
    mov(k).cdata = imresize(IMG,[vidHeight vidWidth]);
end

%// Size a figure based on the video's width and height.
hf = figure;
set(hf, 'position', [150 150 vidWidth vidHeight])

%// Play back the movie once at the video's frame rate.
movie(hf, mov, 1, xyloObj.FrameRate);

Basically the only change I have made are for 'nFrames' and the other factors revolving around it. Try changing the 'sampling_factor' and see if that makes sense . Also, I have added the image resizing that you are performing at the end of your code.

回答2:

you can achieve this task by reading frames from video and store it in cell array. From cell array , you can easily read whatever frame you want by customizing for loop as follows.

for i=1:8:n
  frame = cell{i};
  process(frame)
end

cell: it contains all frames in video
process: it is a function to perform your task
n: number of frames in video

If you want to get more information for reading frames from video and store into cell array, visit the following link:

https://naveenideas.blogspot.in/2016/07/reading-frames-from-video.html

来源：https://stackoverflow.com/questions/22317579/how-to-select-specific-frames-from-a-video-in-matlab

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