# What in Mathematica is equivalent to GetFrame in Matlab?

I am going to develop an unsteady diffusion calculation in Mathematica. Because it is unsteady, I would like to visualize the solution each time step (or in some time steps). My MATLAB skills are much better than Mathematica, so I am going to give the example from there. I looked at some of the posts about that, but they were for small number of time steps like 10, or saving each plot and creating a movie. But in MATLAB I can do

surf(x,z,U_sol)
F=getframe;

x and z are the position vectors and U_sol is the solution matrix. GetFrame simply preserves frame but not the plot, so it looks like a continuous plotting, like a movie.

In Matlab, you can do something like this to save the movie directly. This is before time steps

fid=figure;
pos=get(fid,'position');
writerObj=VideoWriter('sol.avi');
writerObj.FrameRate=10;
open(writerObj);

and inside the time loop

PLOT...
frame=getframe(gcf);
writeVideo(writerObj,frame)

and at the end of the time loop.

close (writerObj)

and you will have the movie file without dealing with each frame. There should be some equivalent version in Mathematica.

• I guess you want ListPlot3D[] (or Plot3D[] if you're using NDSolve[] for the PDE), Animate[], and/or Export[]... – J. M. will be back soon Dec 27 '16 at 13:16
• mathematica can not Export on a frame by frame basis, you need to compile all frames as a list and Export all at once. – george2079 Dec 27 '16 at 13:48
• I am not using NDSolve. I use my own code and every time step I create a solution which I want continuously plot without recreating the frame and ideally save them while plotting them. But it seems I need to save each frame but that is really inefficient if you have 10000 or more steps. – Erdem Dec 27 '16 at 17:13
• if you want to watch the frames as the solution proceeds have a look at Monitor – george2079 Dec 27 '16 at 20:25
• If you are on Windows you can use my MathMF package to write frames one by one to a video stream. – Simon Woods Dec 27 '16 at 22:24

Mathematica can't build a writer object (in fact it doesn't have proper objects at all, excluding the front-end) but you can always build your snapshots in a temporary directory. Give this a go:

movieDir = CreateDirectory[];
getFrame[plot_, tag_] :=
Export[FileNameJoin@{movieDir, ToString[tag] <> ".png"}, plot];
getImages[] :=
Import /@
SortBy[FileNames["*.png", movieDir], ToExpression@FileBaseName@# &];
createMovie[file_] :=
With[{images = getImages[]},
With[{dims = ImageDimensions@First@images},
Export[file,
ImageResize[ColorConvert[#, "RGB"], dims] & /@ images
]
]
];

Note the manipulations done to the images in createMovie. That's to get around the issues mentioned here.

Then we can build our plots and images:

In[458]:= (Do[
getFrame[Plot[Sin[s*x], {x, 0, 2 \[Pi]}, PlotRange -> {-1, 1}],
IntegerPart[10*s]],
{s, 0, 2, .1}
];
createMovie["~/Desktop/sol.gif"]) // AbsoluteTiming

Out[458]= {3.2397, "~/Desktop/sol.gif"}

And what we get is this (note that .avi works too, I just used .gif so I could embed it here):

• Thank you ..this is what I was trying to do. – Erdem Dec 28 '16 at 17:08