Find brightest frame in video file

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Bill White
Bill White 2024-1-9
编辑： Bill White 2024-1-19
I need to import a video file (which contains 1000s or 10,000s of frames) and find the brightest frame; and isolate and save that frame.
The image can br happily converted to greyscale if that makes things easier.

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Image Analyst
Image Analyst 2024-1-9
See my attached demo that runs through a video getting the mean R, G, and B, and gray scale brightness. Once you've run though the video you can look at the vector of brightnesses to determine which frame was the brightest and then copy that particular frame somewhere, like to disk with imwrite.
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Image Analyst
Image Analyst 2024-1-12
Try this:
hFig = figure;
hFig.Name = 'Brightest Frame';
hFig.WindowState = 'maximized';
% Show brightest frame in the middle.
subplot(1, 3, 2);
thisFrame = read(videoObject, max_brightness_image_frame);
% IMPORTANT NOTE: convert to gray level if needed - if that's how you decided upong the brightest frame..
meanOfThisFrame = mean(thisFrame);
imshow(thisFrame);
impixelinfo; % Let user mouse around and see (x, y, RGB)
axis('on', 'image');
caption = sprintf('The brightest frame is #%d and the mean is %.3f', max_brightness_image_frame, meanOfThisFrame);
title(caption);
% Show frame before the brightest frame on the left.
subplot(1, 3, 1);
thisFrame = read(videoObject, max_brightness_image_frame - 1);
% IMPORTANT NOTE: convert to gray level if needed - if that's how you decided upong the brightest frame..
meanOfThisFrame = mean(thisFrame);
imshow(thisFrame);
impixelinfo; % Let user mouse around and see (x, y, RGB)
axis('on', 'image');
caption = sprintf('The frame before is #%d and the mean is %.3f', max_brightness_image_frame - 1, meanOfThisFrame);
title(caption);
% Show frame after the brightest frame on the right.
subplot(1, 3, 3);
thisFrame = read(videoObject, max_brightness_image_frame + 1);
% IMPORTANT NOTE: convert to gray level if needed - if that's how you decided upong the brightest frame..
meanOfThisFrame = mean(thisFrame);
imshow(thisFrame);
impixelinfo; % Let user mouse around and see (x, y, RGB)
axis('on', 'image');
caption = sprintf('The frame after is #%d and the mean is %.3f', max_brightness_image_frame + 1, meanOfThisFrame);
title(caption);
Bill White
Bill White 2024-1-15
thanks for this; I'll give it a go. Image manipualtion is completely new to me, so this is very intersting. Thanks again. (then onto the GUI!)

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Bill White
Bill White 2024-1-19
编辑：Bill White 2024-1-19
I have got this working to find the single brightest frame, and the single frames either side; and then composite (merely add) these three frames to a single image that is useful.
However, how would I find an arbitrary number of frames (in videos of varying lengths) above an arbitrary brightness threshold? ( the brightness is mereley the mean gray value of the entire frame)
For example, say there are "x" number of frames in a video that are above a brightness threshold of "y" : "frame_a", "frame_b", "frame_c"...."frame_x"
I would want to find, make and save composite images so that
image a is
(frame_a-1) + (frame_a) + (frame_a+1)
image b is
(frame_b-1) + (frame_b) + (frame_b+1)
image x is
(frame_x1) + (frame_x) + (frame_x+1)
I was getting a bit lost making a loop with changing variable names depending on the loop iteration, and the documentation recommends cells?
I also am aware of an issue if "frame_a" is a bright frame but it is the first frame in the video; then there is no (frame_a-1)
so the composite image a would just be
(frame_a) + (frame_a+1)
or if frame_x is the last frame then there is no (frame_x+1) so image x would just be
(frame_x-1) + (frame_x)
even though this would be rare, it can happen and cause an error if looking for a frame that does not exist.
The images will be saved with a file name that is (for example) "bright frame_a of filename" where "filename" is the name of the video file imported
I have attached what (seems to) work for a single bright composite image.
thanks again
clc;
close all; % Close all figures (except those of imtool.)
imtool close all; % Close all imtool figures.
clear;
workspace; % show workspace
% fontSize = 8;
%%% open video file
[baseFileName, folderName, FilterIndex] = uigetfile('*.*', 'Select Video File');
if ~isequal(baseFileName, 0)
movieFullFileName = fullfile(folderName, baseFileName);
else
return;
end
try
videoObject = VideoReader(movieFullFileName)
% % Determine how many frames there are.
numberOfFrames = videoObject.NumberOfFrames;
% vidHeight = videoObject.Height;
% vidWidth = videoObject.Width;
%
numberOfFramesWritten = 0;
%
% % Loop through the movie, writing all frames out.
% % Each frame will be in a separate file with unique name.
meanGrayLevels = zeros(numberOfFrames, 1);
for frame = 1 : numberOfFrames
% Extract the frame from the movie structure.
thisFrame = read(videoObject, frame);
% Calculate the mean gray level.
grayImage = rgb2gray(thisFrame);
meanGrayLevels(frame) = mean(grayImage(:));
% Update user with the progress. Display in the command window.
progressIndication = sprintf('Processed frame %4d of %d.', frame, numberOfFrames);
disp(progressIndication);
% Increment frame count (should eventually = numberOfFrames
% unless an error happens).
numberOfFramesWritten = numberOfFramesWritten + 1;
end
% Alert user that we're done. finishedMessage = sprintf('Done! It processed %d frames of\n"%s"', numberOfFramesWritten, movieFullFileName);
finishedMessage = sprintf('Done! It processed %d frames of\n"%s"', numberOfFramesWritten, movieFullFileName);
disp(finishedMessage); % Write to command window.
uiwait(msgbox(finishedMessage)); % Also pop up a message box.
catch ME
% Some error happened if you get here.
strErrorMessage = sprintf('Error extracting movie frames from:\n\n%s\n\nError: %s\n\n)', movieFullFileName, ME.message);
uiwait(msgbox(strErrorMessage));
end
% determine value and index (frame number) of brightest frame
[max_brightness_value,max_brightness_image_frame] = max(meanGrayLevels)
%%%% show composite of three frames
composite = ...
(read(videoObject, max_brightness_image_frame))+...
(read(videoObject, max_brightness_image_frame+1))+...
(read(videoObject, max_brightness_image_frame-1));
hFig = figure;
hFig.Name = 'Composite Frames';
hFig.WindowState = 'maximized';
imshow(composite);
% display title properly without underscores being coverted to subscript
file_name_title = strrep((num2str(baseFileName)), '_', '\_') % ensure underscores show properly in title
title(['composite of frames ',num2str(max_brightness_image_frame-1),...
' + ', num2str(max_brightness_image_frame) ,...
' + ', num2str(max_brightness_image_frame+1),...
' from file ',...
num2str(file_name_title) ]);

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