the getsnapshot function takes a lot of time executing since (I guess) initializes the webcam every time is called. This is a problem if you want to acquire images with a high framerate.
I trick I casually discovered is to call the preview function, which keeps the webcam handler open making getsnapshot almost instantaneous, but it keeps a small preview window open:
% dummy example
cam = videoinput(...);
preview(cam);
while(1)
img = getsnapshot(cam);
% do stuff
end
Is there a "cleaner" way to speedup getsnapshot? (without preview window opened)
You could use the new "machine vision" toolbox which is specially built for vision applications. See code below:
vid = videoinput('winvideo', 1, 'RGB24_320x240'); %select input device
hvpc = vision.VideoPlayer; %create video player object
src = getselectedsource(vid);
vid.FramesPerTrigger =1;
vid.TriggerRepeat = Inf;
vid.ReturnedColorspace = 'rgb';
src.FrameRate = '30';
start(vid)
%start main loop for image acquisition
for t=1:500
imgO=getdata(vid,1,'uint8'); %get image from camera
hvpc.step(imgO); %see current image in player
end
As you can see, you can acquire the image with getdata. The bottleneck in video applications in Matlab was the preview window, which delayed to code substantially. The new vision.VideoPlayer is a lot faster (i have used this code in real time vision applications in Matlab. When i had written the first version without the vision toolbox, achieving frame rates at about 18 fps and using the new toolbox got to around 70!).
Note: I you need speed in image apps using Matlab, you should really consider using OpenCV libs through mex to get a decent performance in image manipulation.
Related
The Logitech C910 webcam spec indicates image and video capture. Because image and video capture are listed separately, I am assuming that they are encoded and sent differently: effectively forming two different 'channels' to select from. If this understanding is incorrect, please respond with an explanation of the true nature.
This reference indicates a maximum frame-rate of 15 because of windows.
My search returned webcam video acquisition that comprises a series of time lapsed images 'stitched' together
% Connect to the webcam.
cam = webcam
% Open Video File
vidWriter = VideoWriter('frames.avi');
open(vidWriter);
% Write images file
for index = 1:20
% Acquire frame for processing
img = snapshot(cam);
% Write frame to video
writeVideo(vidWriter,img);
end
%Close file and cam
close(vidWriter);
clear cam
MATLAB has captured successfully images with the C910.
Question
If possible within MATLAB, how does one configure the webcam's **video* frame rate and save the video stream to .avi or the like? (not writing still images to video file as depicted above).
Perhaps someone with experience or sharper Google skills can provide an example of bridging the webcam's video (vs the image) stream into MATLAB. Any example that can be tested is greatly appreciated.
A good way to 'jumpstart' or accelerate newbie familiarity is with the image acquisition tool:
imaqtool
The tool seems to be GUI wrapper that reduces the command-line syntax to a GUI. Note that the bottom right panel shows the command line equivalent of a GUI interaction.
Configurable video capture paramaters comprise:
Resolution & ROI (Region of Interest)
Frame Rate
Video type (.avi .mp4) etc.
I would like to know how I can record a video and how to change the recording framerate in MATLAB with my webcam using Support Package for USB Webcams (not using toolboxes, including Image Acquisition tool box). I am thinking about using it because that package is free.
The code I made is like the below. I tried to set 2 frames/sec (fps) by v.FrameRate=2 but that code doesn't work (the recorded framerate was about 15 fps and the recording ended on about 10 sec), and I was not able to change fps in this way. Does my webcam work in Matlab Support Package for USB Webcams? How do I solve it?
function webcam_recordingvideo()
cam = webcam;
preview(cam)
v = VideoWriter('frames.avi');
get(v);
v.FrameRate=2;
open(v);
frames = 150; % frame number to get
for i = 1:frames
img = snapshot(cam); % Acquire frame for processing
writeVideo(v, img); % Write frame to video
end
close(v);
clear cam
I'm using Matlab 2016a and Logitech HD Webcam C615.
I am trying to employ a chroma key algorithm on a live video. I need to take a live webcam input, process it in real time and display it. I already have the chroma key algorithm working on images.
How do I process the webcam input and display it immediately. I have tried using snapshot() and passing the image to the chroma key algorithm but it is too slow even if I increase the rate of snapshots. I want a smooth output.
[ Also, if there is a better alternative than Matlab please let me know. ]
Instead of using getsnapshot() which connects to the camera and disconnects on EVERY single frame (thus slow framerates), try to use videoinput and then preview the connection: http://www.mathworks.de/de/help/imaq/preview.html
This example is made for you:
http://www.mathworks.de/products/imaq/code-examples.html?file=/products/demos/shipping/imaq/demoimaq_LiveHistogram.html
As shown you can even define a callback-handler-function which is called on every newly received frame.
You must set TriggerType to manual, or else getsnapshot() will create (and destroy) a conection to the camera everytime you need a frame. By setting it to manual, you can start the camera once, get the frames and stop the camera when you're done.
Here is an example:
vidobj = videoinput('winvideo', 1, 'RGB24_640x480');
triggerconfig(vidobj, 'manual');
start(vidobj);
while true % Or any stop condition
img = getsnapshot(vidobj);
% Process the frame.
...
imshow(img);
drawnow;
end
I am currently trying to record footage of a camera and represent it with matlab in a graphic window using the "image" command. The problem I'm facing is the slow redraw of the image and this of course effects my whole script. Here's some quick pseudo code to explain my program:
figure
while(true)
Frame = AcquireImageFromCamera(); % Mex, returns current frame
image(I);
end
AcquireImageFromCamera() is a mex coming from an API for the camera.
Now without displaying the acquired image the script easily grabbs all frames coming from the camera (it records with a limited framerate). But as soon as I display every image for a real-time video stream, it slows down terribly and therefore frames are lost as they are not captured.
Does anyone have an idea how I could split the process of acquiring images and displaying them in order to use multiple cores of the CPU for example? Parallel computing is the first thing that pops into my mind, but the parallel toolbox works entirely different form what I want here...
edit: I'm a student and in my faculty's matlab version all toolboxes are included :)
Running two threads or workers is going to be a bit tricky. Instead of that, can you simply update the screen less often? Something like this:
figure
count = 0;
while(true)
Frame = AcquireImageFromCamera(); % Mex, returns current frame
count = count + 1;
if count == 5
count = 0;
image(I);
end
end
Another thing to try is to call image() just once to set up the plot, then update pixels directly. This should be much faster than calling image() every frame. You do this by getting the image handle and changing the CData property.
h = image(I); % first frame only
set(h, 'CData', newPixels); % other frames update pixels like this
Note that updating pixels like this may then require a call to drawnow to show the change on screen.
I'm not sure how precise your pseudo code is, but creating the image object takes quite a bit of overhead. It is much faster to create it once and then just set the image data.
figure
himg = image(I)
while(true)
Frame = AcquireImageFromCamera(); % Mex, returns current frame
set(himg,'cdata',Frame);
drawnow; %Also make sure the screen is actually updated.
end
Matlab has a video player in Computer vision toolbox, which would be faster than using image().
player = vision.VideoPlayer
while(true)
Frame = AcquireImageFromCamera(); % Mex, returns current frame
step(player, Frame);
end
I am plotting a randomly walking points on the figure and trying to capture the motion at each step by getframe. After I collect all the frames, I output the result as avi with movie2avi, but the output file was so big to fit into my presentation. I am looking for a way to export the movie to mp4, anyone have any idea? I also try to use the 3rd party movie2gif, it save the size to huge extent but when I play the gif, it looks so unsmooth
In later versions of Matlab (e.g. 2012) it is done by creating and writing a video object. For example, the following code generates movie of a randomly moving circle. You can adjust the speed of the movie with the FrameRate and the size with the Quality properties. For more details see the Matlab documentation.
vobj=VideoWriter('MyMovieFile', 'Motion JPEG AVI');
vobj.FrameRate=4;
vobj.Quality=75
open(vobj);
for i=1:100
plot(rand,rand,'o')
F=getframe(gcf);
writeVideo(vobj, F);
cla(gca)
end
close(vobj)