How to adjust my video to only save 30 fps while in my for loop that goes upto 20000 iterations?

My video is created to about 5.5 mins and I am trying to make the video to be about just 20 seconds with 30 fps.

% Video Making:
        VidFile = VideoWriter('Part1movie.mp4','MPEG-4');
        VidFile.FrameRate = 30;
        open(VidFile);
        % 5 pt stencil:
        stencil = 5;
        % change in time:
        dt = 1e-3;
        % time vector and time iteration size.
        t = t0:dt:tf;
        mt = length(t);
        % Plotting initial:
        figure (1)
        imagesc(phi)
        % Material A is 1 and Material B is -1
        colorbar
        caxis([-1 1])
        title(['1st Order Euler Field Distribution at stencil = ' num2str(stencil) ', dt = ' num2str(dt) 's, and t = ' num2str(t0) 's.'],'FontSize',18)
        set(gca,'LineWidth',1,'FontSize',8)
        axis equal
        for k = 1:mt-1
            % Eqn 4:
            v1 = Laplacian_2D(phi,h,stencil);
            v2 = (b^4 * phi.^3 - a * b^2 * phi) - gamma * v1;
            v3 = D * Laplacian_2D(v2,h,stencil);
            newphi = phi + dt * v3;
            phi = newphi;
            if t(k) == 5
                % Plotting at 5 seconds:
                figure (2)
                imagesc(phi)
                colorbar
                caxis([-1 1])
                title(['1st Order Euler Field Distribution at stencil = ' num2str(stencil) ', dt = ' num2str(dt) 's, and t = ' num2str(t(k)) 's.'],'FontSize',18)
                set(gca,'LineWidth',1,'FontSize',8)
                axis equal
            elseif t(k) == 10
                % Plotting at 10 seconds:
                figure (3)
                imagesc(phi)
                colorbar
                caxis([-1 1])
                title(['1st Order Euler Field Distribution at stencil = ' num2str(stencil) ', dt = ' num2str(dt) 's, and t = ' num2str(t(k)) 's.'],'FontSize',18)
                set(gca,'LineWidth',1,'FontSize',8)
                axis equal
            elseif t(k) == 15
                % Plotting at 15 seconds:
                figure (4)
                imagesc(phi)
                colorbar
                caxis([-1 1])
                title(['1st Order Euler Field Distribution at stencil = ' num2str(stencil) ', dt = ' num2str(dt) 's, and t = ' num2str(t(k)) 's.'],'FontSize',18)
                set(gca,'LineWidth',1,'FontSize',8)
                axis equal
            elseif t(k) == 19.999
                % Plotting at 20 seconds:
                figure (5)
                imagesc(phi)
                colorbar
                caxis([-1 1])
                title(['1st Order Euler Field Distribution at stencil = ' num2str(stencil) ', dt = ' num2str(dt) 's, and t = ' num2str(t(k)) 's.'],'FontSize',18)
                set(gca,'LineWidth',1,'FontSize',8)
                axis equal
            elseif t(k) <= 10
                % Plotting the first 10 seconds to make the video:
                figure (6)
                imagesc(phi)
                colorbar
                caxis([-1 1])
                title(['1st Order Euler Field Distribution at stencil = ' num2str(stencil) ', dt = ' num2str(dt) 's, and t = ' num2str(t(k)) 's.'],'FontSize',18)
                set(gca,'LineWidth',1,'FontSize',8)
                axis equal
                Fr(k + 1) = getframe(gcf);
                writeVideo(VidFile,Fr(k + 1))
            end
        end
        close(VidFile);

Answer 1

[darova]

If you want 20 sec video with 30 fps you just need to write 600 frames

For dt=0.001 it is 10000 frames for 10 sec simulation so: 10000/600 = 16.66

Write each 16.66th frame

k1 = 1;
for k = 1:mt-1
    % Eqn 4:
    % ...
    % get 17, 33, 50, 67 ... frames
    elseif t(k) <= 10 && k == round(k1*100/6)
        k1 = k1 + 1;
        % actions
    end
end

By the way, your code can be shorter

    % create 6 figures with the same properties
for i = 1:6
    figure(i)
    colorbar
    caxis([-1 1])
    title(['1st Order Euler Field Distribution at stencil = ' num2str(stencil) ', dt = ' num2str(dt) 's, and t = ' num2str(t(k)) 's.'],'FontSize',18)
    set(gca,'LineWidth',1,'FontSize',8)
    axis equal
end
k1 = 1;
for k = 1:mt-1
    % Eqn 4:
    % ...
    if t(k) == 5
        % Plotting at 5 seconds:
        figure (2)
        imagesc(phi)
    elseif t(k) == 10
        % Plotting at 10 seconds:
        figure (3)
        imagesc(phi)
    elseif t(k) == 15
        % Plotting at 15 seconds:
        figure (4)
        imagesc(phi)
    elseif t(k) == 19.999
        % Plotting at 20 seconds:
        figure (5)
        imagesc(phi)
    elseif t(k) <= 10 && k == round(k1*100/6)            % get 17, 33, 50, 67 ... frames
        % Plotting the first 10 seconds to make the video:
        k1 = k1 + 1;
        figure (6)
        imagesc(phi)
        Fr = getframe(gcf);
        writeVideo(VidFile,Fr)
    end
end

[darova]

It is not a good idea to use equal sign for double numbers

if t(k) == 5

Use tolerance instead

tol = 0.0001;                % tolerance
if abs(t(k) - 5) < tol

[darova]

Even shorter

    % create 6 figures with the same properties
ch = [5 10 15 20];                    % seconds to display
for i = 1:6
    figure(i)
    colorbar
    caxis([-1 1])
    title(['1st Order Euler Field Distribution at stencil = ' num2str(stencil) ', dt = ' num2str(dt) 's, and t = ' num2str(t(k)) 's.'],'FontSize',18)
    set(gca,'LineWidth',1,'FontSize',8)
    axis equal
end
k1 = 1;
for k = 1:mt-1
    % Eqn 4:
    % ...
    if any(abs(t(k)-ch) < 0.001)    % if t(k) equals to any of [5 10 15 20]
        figure(round(t(k)/5)+1)
        imagesc(phi)
    elseif t(k) <= 10 && k == round(k1*100/6)            % get 17, 33, 50, 67 ... frames
        % Plotting the first 10 seconds to make the video:
        k1 = k1 + 1;
        figure (6)
        imagesc(phi)
        Fr = getframe(gcf);
        writeVideo(VidFile,Fr)
    end
end

[Mina Mansour]

The abbreviation does not work like I want. it doesn't show color bar, axis equal, or title!

[Walter Roberson]

Add

 hold on

after the axis equal call.

You might also need to set the CData property of the image object instead of calling imagesc each time.