How do I change line thickness and color on a video

This code provides a video output of two videos for comparison purposes side by side. From the second line of code, I get a black line across the specified pixel (517). I would like to be able to change the thickness and the color of the line. I know how to do this for plots but not sure how to do that for this example.

%% Create Movie
    img=cat(2,video,uint8(video_translated)); %videos put side to side for comparison purposes 
    img(517,:,:,:)=0; %change thickness of line and color
    vidname= ['stabilize' fname '.mp4']
    vidObj = VideoWriter(vidname,'MPEG-4'); %any name can be given to this 
    vidObj.FrameRate = 15; 
    open(vidObj);
    for i=1:size(img,3) %Number of frames 
        writeVideo(vidObj,img);
    end 
    close (vidObj)

Answer 1

[Walter Roberson]

img(517:517+LineWidth-1, :, :, :) = 0;

to change the width of the line (while still leaving it black)

You are doing 4 dimensional indexing there, but it is difficult for us to figure out what the four indices represent. You have the line

    for i=1:size(img,3) %Number of frames 

implying that the third dimension is frames. But videowriter expects one of:

• a 2D array, for grayscale or indexed images
• a rows x columns x 1 x frames array for multiple grayscale or indexed images
• a rows x columns x 3 x frames array for multiple rgb images

Number of frames is never the third dimension for writeVideo

Furthermore, you loop 1 : size(img,3) but you send all of img to writeVideo(), repeating the same information multiple times.

Can we get a hint from the way you cat() the two videos? Unfortunately, no. That cat() does restrict the situation to either being one single frame consisting of a single merged image, or else to two multi-frame videos that happen to have the same number of frames... but we cannot tell whether they are rows x columns x 3 x frames or rows x columns x 1 x frames

If they happen to be rows x columns x 3 x frames then for colour you would do

img(517:517+LineWidth-1, :, 1, :) = RedComponent;
img(517:517+LineWidth-1, :, 2, :) = GreenComponent;
img(517:517+LineWidth-1, :, 3, :) = BlueComponent;

If they happen to be rows x columns x 1 x frames then you would first do

img = repmat(img, 1, 1, 3, 1);

and then do the R G B assignment.

[Diego]

Hi Walter. I am sorry for not including the entire code. I did not think it would be neccesary and thought it would be an easy solution fix. The indices represent Y:X:RGB:number of frames. Earlier in the code, I converted the original RGB move to grayscale which left me with three indices for the movie which is probably why I did "img(:,:,3)". Could number of frames be the third dimension when the video is in grayscale? Are you suggesting, it should technically not be written that way because we would need 4 indices for video writer to work?

"Furthermore, you loop 1 : size(img,3) but you send all of img to writeVideo(), repeating the same information multiple times." - This would probably explain why I have the video repeated multiple times and length of video is longer.

They do happen to be rows x columns x 3 x frames. Cat allows me to show two multi-frame videos.

[Walter Roberson]

writeVideo cannot handle rows x columns x frames for grayscale, only rows x columns x 1 x frames for grayscale.

When you have multiple frames, you have two choices:

• a single call, writeVideo(vidObj,img) with no looping, which will write all of the frames at the same time; OR
• loop over the number of rames and writeVideo(vidObj,img(:,:,:,FRAMENUMBER)))

I would expect the single call to be more efficient and to have more opportunities for compressing the video well; I would recommend that approach whenever all of the data is already available. (Writing individual frames is fine for the case where you are generating the frames as you go.)

[Diego]

img(517:517+'LineWidth'-1,:,:,:)=0;

I am not sure if I did this correctly. The output gives me an extremely large line. How can I modify the thickness of it? I initially thought it was due to the "-1" but that does not happen to do anything to the line.

[Diego]

    img=cat(2,video,uint8(video_translated));
    %img(517,:,:,:)=0; %Original thin black line 
    img(517:517+'LineWidth'-1,:,1,:)=0; %change thickness of line and color. Line too large
    vidname= ['stabilize' fname '.mp4'];
    vidObj = VideoWriter(vidname,'MPEG-4'); %any name can be given to this 
    vidObj.FrameRate = 15; 
    open(vidObj);
    
    for i=1:size(img,3) %Number of frames 
        writeVideo(vidObj,img(:,:,:,i)); % <<<-------
    end 

    close (vidObj)
    

I attempted to do the loop format but the output only yields a one frame video. I think I am bit lost on your instructuctions on how to improve the code. How would you do the single call? Would this be how? I am also not sure how to prevent the video writing to repeat the same information multiple times.

    img=uint8(video_translated);
    img(517,:,:,:)=0;
    vidname= ['stabilize' fname '.mp4'];
    vidObj = VideoWriter(vidname,'MPEG-4'); 
    vidObj.FrameRate = 15;
    
    open(vidObj);
    writeVideo(vidObj,img);
    close (vidObj)

[Walter Roberson]

You asked for a wider line, but you did not specify how wide. If I had put in any particular numeric value, that might have been the wrong value for your purpose. So I put in a variable named LineWidth. Assign LineWidth the width of the line you want, in pixels.

LineWidth = 7;   %for example

%linecolor... You didn't say that either. So why not hotpink?
LineColorRGB = [255,105,180] ./ 255;

RedComponent = LineColorRGB(1);
GreenComponent = LineColorRGB(2);
BlueComponent = LineColorRGB(3);

img(517:517+LineWidth-1,:,1,:) = RedComponent;
img(517:517+LineWidth-1,:,2,:) = GreenComponent;
img(517:517+LineWidth-1,:,3,:) = BlueComponent;

Then

    open(vidObj);
    writeVideo(vidObj,img);
    close (vidObj)

[Diego]

Thank you so much Walter. My last question. What does "-1" do following LineWidth? Is this necessary?

[Steven Lord]

If I want a vector starting with 0 containing 7 elements (each element 1 greater than the previous), what's the last element of the vector? Is it 7 or 6? See the Wikipedia page for off-by-one error.