plot with reversed y-axis in a normal y-axis

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Suzuki 2023-3-30

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编辑： Suzuki 2023-4-5

I have a video plotted (or played frame by fame) and I want to plot a shape representing the corresponding eye position.

The video has its y-axis reversed (can't control it) (0 up). The eye position is with coordinates in which the y-axis is normal (0 at bottom).

I need to plot this eye position somehow with reversed y-axis on the video.

I'm using insertShape but I can't change the position of the shape to be plotted in a reversed y-axis on the video. I tried flip, axis handel, and many things, nothing is working.

Any suggestion please?!

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Answer 1

Jack 2023-3-30

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https://ww2.mathworks.cn/matlabcentral/answers/1937959-plot-with-reversed-y-axis-in-a-normal-y-axis#answer_1205119

在 MATLAB Online 中打开

You can use the image function in MATLAB to display the video frames and then use insertShape to plot the eye position on top of the frame. To reverse the y-axis, you can set the YDir property of the axes object to 'reverse'. Here's some sample code to get you started:

% Load the video file
v = VideoReader('my_video.mp4');
% Create a figure and axes to display the frames
fig = figure;
ax = axes('Parent', fig);
% Set the y-axis direction to 'reverse'
set(ax, 'YDir', 'reverse');
% Loop through the frames and display them
while hasFrame(v)
    frame = readFrame(v);
    image(ax, frame);
    
    % Get the eye position for the current frame
    eye_pos = [x, y, width, height]; % Replace with your own code
    
    % Plot the eye position on top of the frame
    insertShape(ax, 'Rectangle', eye_pos, 'Color', 'red');
    
    % Update the figure
    drawnow;
end

In this code, x, y, width, and height are the coordinates of the eye position in a normal y-axis coordinate system. You may need to adjust these values to match the dimensions of your video frames. The insertShape function plots a red rectangle at the specified eye position, and the drawnow function updates the figure to display the current frame with the eye position.

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Jack 2023-3-31

在 MATLAB Online 中打开

If the yyaxis command is slowing down the video playback when you plot it together with the eye position, you can try a different approach to adjust the y-axis limits of the video plot to match the reversed y-axis of the eye position plot. Here's an example code snippet that might help:

% create a sample video and eye position data
video = rand(100, 100, 50);
eye_pos = rand(50, 2);
% plot the video and eye position separately to adjust the y-axis limits
subplot(2, 1, 1);
imshow(video(:,:,1));
xlim([1 size(video, 2)]);
ylim([1 size(video, 1)]);
subplot(2, 1, 2);
plot(eye_pos(:,1), size(video, 1) - eye_pos(:,2) + 1, 'b'); % adjust the y-coordinates
xlim([1 size(video, 2)]);
ylim([1 size(video, 1)]);
% plot the video and eye position together
figure;
imshow(video(:,:,1));
hold on;
plot(eye_pos(:,1), size(video, 1) - eye_pos(:,2) + 1, 'b'); % adjust the y-coordinates
hold off;
xlim([1 size(video, 2)]);
ylim([1 size(video, 1)]);

In this code, we plot the video and eye position separately to adjust the y-axis limits of both plots to match the reversed y-axis of the eye position data. Then we plot them together using imshow for the video and plot for the eye position, with the hold on and hold off commands to combine the plots. We also adjust the y-coordinates of the eye position data by subtracting them from the height of the video and adding 1 to shift them to the correct y-axis position.

I hope this helps! Let me know if you have any further questions.

Suzuki 2023-4-3

编辑：Suzuki 2023-4-3

Thanks.

When I tried that it plots frame 1 then the eye trace on what seems to be a different figure (because the background is white, not the video frame 1 (for ex)), but if i remove the hold off it' ok. However, the eye traces are not in the correct position. Furthermore,it plots the video images in black and white, don't know why.

And I also tried your example, but the eye traces are also plotted in the lower corner of the video, I guess this is not the right position.

Suzuki 2023-4-5