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How to extract frames of a video

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Hi everyone! How can I extract frames of a video in matlab if I have used mmreader to read the video.I have used frame2im command but its not working.
  2 Comments
chao dong
chao dong on 30 May 2016
I also need to extract pictures from video files, but it seems to me that format is critical towards this process.

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Accepted Answer

Walter Roberson
Walter Roberson on 22 Sep 2012
obj = mmreader(FileName);
video = obj.read();
Then, frame #K is video(:,:,:,K)
  14 Comments
Hazem El-Alfy
Hazem El-Alfy on 17 Mar 2021
Great rebuttal of my earlier comment. I would rather not delete it for others to understand how not to address this issue.

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More Answers (6)

Image Analyst
Image Analyst on 22 Sep 2012
See my demo for extracting and measuring an AVI file (the standard MATLAB rhinos.avi movie):
% Demo macro to extract frames and get frame means from an avi movie
% and save individual frames to separate image files.
% Also computes the mean gray value of the color channels.
clc; % Clear the command window.
close all; % Close all figures (except those of imtool.)
imtool close all; % Close all imtool figures.
clear; % Erase all existing variables.
workspace; % Make sure the workspace panel is showing.
fontSize = 14;
% Change the current folder to the folder of this m-file.
% (The line of code below is from Brett Shoelson of The Mathworks.)
if(~isdeployed)
cd(fileparts(which(mfilename)));
end
% Open the rhino.avi demo movie that ships with MATLAB.
folder = fullfile(matlabroot, '\toolbox\images\imdemos');
movieFullFileName = fullfile(folder, 'rhinos.avi');
% Check to see that it exists.
if ~exist(movieFullFileName, 'file')
strErrorMessage = sprintf('File not found:\n%s\nYou can choose a new one, or cancel', movieFullFileName);
response = questdlg(strErrorMessage, 'File not found', 'OK - choose a new movie.', 'Cancel', 'OK - choose a new movie.');
if strcmpi(response, 'OK - choose a new movie.')
[baseFileName, folderName, FilterIndex] = uigetfile('*.avi');
if ~isequal(baseFileName, 0)
movieFullFileName = fullfile(folderName, baseFileName);
else
return;
end
else
return;
end
end
try
movieInfo = aviinfo(movieFullFileName)
mov = aviread(movieFullFileName);
% movie(mov);
% Determine how many frames there are.
numberOfFrames = size(mov, 2);
numberOfFramesWritten = 0;
% Prepare a figure to show the images in the upper half of the screen.
figure;
screenSize = get(0, 'ScreenSize');
newWindowPosition = [1 screenSize(4)/2 - 70 screenSize(3) screenSize(4)/2];
set(gcf, 'Position', newWindowPosition); % Maximize figure.
% set(gcf, 'Position', get(0,'Screensize')); % Maximize figure.
% Ask user if they want to write the individual frames out to disk.
promptMessage = sprintf('Do you want to save the individual frames out to individual disk files?');
button = questdlg(promptMessage, 'Save individual frames?', 'Yes', 'No', 'Yes');
if strcmp(button, 'Yes')
writeToDisk = true;
% Extract out the various parts of the filename.
[folder, baseFileName, extentions] = fileparts(movieFullFileName);
% Make up a special new output subfolder for all the separate
% movie frames that we're going to extract and save to disk.
% (Don't worry - windows can handle forward slashes in the folder name.)
folder = pwd; % Make it a subfolder of the folder where this m-file lives.
outputFolder = sprintf('%s/Movie Frames from %s', folder, baseFileName);
% Create the folder if it doesn't exist already.
if ~exist(outputFolder, 'dir')
mkdir(outputFolder);
end
else
writeToDisk = false;
end
% Loop through the movie, writing all frames out.
% Each frame will be in a separate file with unique name.
meanGrayLevels = zeros(numberOfFrames, 1);
meanRedLevels = zeros(numberOfFrames, 1);
meanGreenLevels = zeros(numberOfFrames, 1);
meanBlueLevels = zeros(numberOfFrames, 1);
for frame = 1 : numberOfFrames
% Extract the frame from the movie structure.
thisFrame = mov(frame).cdata;
% Display it
hImage = subplot(1,2,1);
image(thisFrame);
axis square;
caption = sprintf('Frame %4d of %d.', frame, numberOfFrames);
title(caption, 'FontSize', fontSize);
drawnow; % Force it to refresh the window.
% Write the image array to the output file, if requested.
if writeToDisk
% Construct an output image file name.
outputBaseFileName = sprintf('Frame %4.4d.png', frame);
outputFullFileName = fullfile(outputFolder, outputBaseFileName);
% Stamp the name and frame number onto the image.
% At this point it's just going into the overlay,
% not actually getting written into the pixel values.
text(5, 15, outputBaseFileName, 'FontSize', 20);
% Extract the image with the text "burned into" it.
frameWithText = getframe(gca);
% frameWithText.cdata is the image with the text
% actually written into the pixel values.
% Write it out to disk.
imwrite(frameWithText.cdata, outputFullFileName, 'png');
end
% Calculate the mean gray level.
grayImage = rgb2gray(thisFrame);
meanGrayLevels(frame) = mean(grayImage(:));
% Calculate the mean R, G, and B levels.
meanRedLevels(frame) = mean(mean(thisFrame(:, :, 1)));
meanGreenLevels(frame) = mean(mean(thisFrame(:, :, 2)));
meanBlueLevels(frame) = mean(mean(thisFrame(:, :, 3)));
% Plot the mean gray levels.
hPlot = subplot(1,2,2);
hold off;
plot(meanGrayLevels, 'k-', 'LineWidth', 2);
hold on;
plot(meanRedLevels, 'r-');
plot(meanGreenLevels, 'g-');
plot(meanBlueLevels, 'b-');
% Put title back because plot() erases the existing title.
title('Mean Gray Levels', 'FontSize', fontSize);
if frame == 1
xlabel('Frame Number');
yLabel('Gray Level');
% Get size data later for preallocation if we read
% the movie back in from disk.
[rows columns numberOfColorChannels] = size(thisFrame);
end
% Update user with the progress. Display in the command window.
if writeToDisk
progressIndication = sprintf('Wrote frame %4d of %d.', frame, numberOfFrames);
else
progressIndication = sprintf('Processed frame %4d of %d.', frame, numberOfFrames);
end
disp(progressIndication);
% Increment frame count (should eventually = numberOfFrames
% unless an error happens).
numberOfFramesWritten = numberOfFramesWritten + 1;
end
% Alert user that we're done.
if writeToDisk
finishedMessage = sprintf('Done! It wrote %d frames to folder\n"%s"', numberOfFramesWritten, outputFolder);
else
finishedMessage = sprintf('Done! It processed %d frames of\n"%s"', numberOfFramesWritten, movieFullFileName);
end
disp(finishedMessage); % Write to command window.
uiwait(msgbox(finishedMessage)); % Also pop up a message box.
% Exit if they didn't write any individual frames out to disk.
if ~writeToDisk
return;
end
% Ask user if they want to read the individual frames from the disk,
% that they just wrote out, back into a movie and display it.
promptMessage = sprintf('Do you want to recall the individual frames\nback from disk into a movie?\n(This will take several seconds.)');
button = questdlg(promptMessage, 'Recall Movie?', 'Yes', 'No', 'Yes');
if strcmp(button, 'No')
return;
end
% Read the frames back in, and convert them to a movie.
% I don't know of any way to preallocate recalledMovie.
for frame = 1 : numberOfFrames
% Construct an output image file name.
outputBaseFileName = sprintf('Frame %4.4d.png', frame);
outputFullFileName = fullfile(outputFolder, outputBaseFileName);
% Read the image in from disk.
thisFrame = imread(outputFullFileName);
% Convert the image into a "movie frame" structure.
recalledMovie(frame) = im2frame(thisFrame);
end
% Get rid of old image and plot.
delete(hImage);
delete(hPlot);
% Create new axes for our movie.
subPlot(1, 3, 2);
axis off; % Turn off axes numbers.
title('Movie recalled from disk', 'FontSize', fontSize);
% Play the movie in the axes.
movie(recalledMovie);
% Note: if you want to display graphics or text in the overlay
% as the movie plays back then you need to do it like I did at first
% (at the top of this file where you extract and imshow a frame at a time.)
msgbox('Done with this demo!');
catch ME
% Some error happened if you get here.
stError = lasterror;
strErrorMessage = sprintf('Error extracting movie frames from:\n\n%s\n\nError: %s\n\n)', movieFullFileName, stError.message);
uiwait(msgbox(strErrorMessage));
end
  9 Comments
John
John on 14 Aug 2014
My mistake... The drop down that shows up when searching for the file was not set to "all files" but was set to "avi" and I didn't realize it.
But, I did change the code to adapt to the location of the file. I may be a little slow sometimes, but that's one thing I did catch when looking through the code. :)

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dan kin
dan kin on 12 Dec 2019
How can I add a legend like this Redline - meanRed, Blueline - meanBlue, ... to the subplot(2, 2, 2)?
  2 Comments
Image Analyst
Image Analyst on 12 Dec 2019
See my attached demo, ExtractMovieFrames.m.
You can also use text() instead of legend() if you want more precise control over where the legend appears.
00_Screenshot.png

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msp
msp on 28 Apr 2013
how to extract only key frames from the video ??
  2 Comments
Walter Roberson
Walter Roberson on 30 Jan 2016
We need to know the error message. But you already asked this as a Question so the discussion should go there

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Ameni chetouane
Ameni chetouane on 27 Jun 2019
I have read frames with VideoReader,but i don'i get the exact number of frames that contain the video.
the number of frames of the video is 400 but readFrame return only 200 frames.
Does any one have solution to this problem?
clear
clc
% Set up video reader and player
videoFile = 'copie2.avi';
videoReader = VideoReader(videoFile);
iStop = []; % Stop the frame display if this is not empty
fps = get(videoReader, 'FrameRate');
disp(fps); % the fps is correct: it's the same declared in the video file properties
t=1;
while hasFrame(videoReader)& isempty(iStop)
%%for i=0:videoReader.Duration-1
%% videoReader.CurrentTime=i;
fr(:,:,:,t)=readFrame(videoReader);
frGray(:,:,t)=rgb2gray(readFrame(videoReader));
t=t+1;
iStop = input('0 to stop, return to continue');
end

Sushil  Sharma
Sushil Sharma on 15 Oct 2019
Upadte answer
In the lestest veriosn of matlab, we have to use VideoReader instead of mmreader,then you are able to convert any video file into a frames
Here the simple of code to get the frames
%% Change .avi format to images frames
obj = VideoReader('test2.avi');
vid = read(obj);
frames = obj.NumberOfFrames;
for x = 1 : frames
imwrite(vid(:,:,:,x),strcat('frame-',num2str(x),'.png'));
end

Ely Raz
Ely Raz on 16 Dec 2019
How can I add the possibility to select a ROI in Image Analyst script and analyzed it?
  1 Comment
Image Analyst
Image Analyst on 16 Dec 2019
You can call imcrop() if you want a rectangular sub-image. If you want an irregularly-shaped region, see my attached demos.

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