Y-Label upside down on on videoreader

Hello!

So, I have a video I'm reading frame by frame. I need to acquire some data from it using ginput, so I need the axis to be right.

My code is:

mov = VideoReader('pendulo.mp4'); % estrutura com video
nFrames = mov.NumFrames; % numero de frames
framerate = mov.FrameRate; % numero de frames por segundo
dtframes=1/framerate; %tempo entre frames

i=1; t=0; tf=nFrames*dtframes; dt=10*dtframes; 
if ~exist('dados.mat','file')

 while (t <= tf)

 mov.CurrentTime=t; frame=readFrame(mov); image(frame); drawnow
 tv(i)=t; t=t+dt; title(strcat('Frame ', num2str(i)));
if i>1
    hold on;
    plot(x,y,'ro');
end

 [x(i), y(i)]=ginput(1);
 i=i+1;
 
 end
 

I don't understand why the label is upside down... thanks!

Answer 1

[Image Analyst]

It's not upside down. Since you first displayed the image, and then put hold on (so you could overlay graphics on top of it), the graphics from plot will use the image origin as the plot origin, instead of the lower left like plot() uses if there is not an image already in the axes with hold on. This means the origin for plot() is NOW in the upper left, as is usual/normal for images. If the x and y in your plot do not correspond to actual pixel (x,y) coordinates, but to some other coordinate system not based on pixels, then you will need to use rescale() to scale/flip/translate your plotting coordinates to match your image coordinates.

Attach dados.mat with the paperclip icon if you still have doubts, though I have no idea how you're using that mat file since you never seem to load it. And a zipped up 'pendulo.mp4' if it's less than 5 MB.

By the way, with recent versions of MATLAB you can use isfile() and isfolder(), so replace

if ~exist('dados.mat','file')

with

    if ~isfile('dados.mat')

Answer 2

[Sulaymon Eshkabilov]

The cause can be explained with data import and record them into MATLAB workspace. You may consider using flip() to correct the orientation then.