I want to find the length of a video capture in OpenCV;
int frameNumbers = (int) cvGetCaptureProperty(video2, CV_CAP_PROP_FRAME_COUNT);
int fps = (int) cvGetCaptureProperty(video2, CV_CAP_PROP_FPS);
int videoLength = frameNumbers / fps;
but this give me a result which is less than the real answer. What do I have to do?
Actually, I am not sure if there is any issue with the functions that you tried as of today. However, There is an issue with this snippet. Here, it is being assumed that Frames Per Second is an integer value which is not always the case. For example, many videos are encoded at 29.97 FPS, and this code would assume int(29.97) = 29 which obviously results in a larger value in seconds for video length.
The calculation seems to work fine for me if I use floating point values (float) without truncating them.
See this similar post. OpenCV cannt (yet) capture correctly the number of frames
OpenCV captures only a fraction of the frames from a video file
Related
In working on a project I came across the need to generate various waves, accurately. I thought that a simple sine wave would be the easiest to begin with, but it appears that I am mistaken. I made a simple program that generates a vector of samples and then plays those samples back so that the user hears the wave, as a test. Here is the relevant code:
vector<short> genSineWaveSample(int nsamples, float freq, float amp) {
vector<short> samples;
for(float i = 0; i <= nsamples; i++) {
samples.push_back(amp * sinx15(freq*i));
}
return samples;
}
I'm not sure what the issue with this is. I understand that there could be some issue with the vector being made of shorts, but that's what my audio framework wants, and I am inexperienced with that kind of library and so do not know what to expect.
The symptoms are as follows:
frequency not correct
ie: given freq=440, A4 is not the note played back
strange distortion
Most frequencies do not generate a clean wave. 220, 440, 880 are all clean, most others are distorted
Most frequencies are shifted upwards considerably
Can anyone give advice as to what I may be doing wrong?
Here's what I've tried so far:
Making my own sine function, for greater accuracy.
I used a 15th degree Taylor Series expansion for sin(x)
Changed the sample rate, anything from 256 to 44100, no change can be heard given the above errors, the waves are simply more distorted.
Thank you. If there is any information that can help you, I'd be obliged to provide it.
I suspect that you are passing incorrect values to your sin15x function. If you are familiar with the basics of signal processing the Nyquist frequency is the minimum frequency at which you can faithful reconstruct (or in your case construct) a sampled signal. The is defined as 2x the highest frequency component present in the signal.
What this means for your program is that you need at last 2 values per cycle of the highest frequency you want to reproduce. At 20Khz you'd need 40,000 samples per second. It looks like you are just packing a vector with values and letting the playback program sort out the timing.
We will assume you use 44.1Khz as your playback sampling frequency. This means that a snipet of code producing one second of a 1kHz wave would look like
DataStructure wave = new DataStructure(44100) // creates some data structure of 44100 in length
for(int i = 0; i < 44100; i++)
{
wave[i] = sin(2*pi * i * (frequency / 44100) + pi / 2) // sin is in radians, frequency in Hz
}
You need to divide by the frequency, not multiply. To see this, take the case of a 22,050 Hz frequency value is passed. For i = 0, you get sin(0) = 1. For i = 1, sin(3pi/2) = -1 and so on are so forth. This gives you a repeating sequence of 1, -1, 1, -1... which is the correct representation of a 22,050Hz wave sampled at 44.1Khz. This works as you go down in frequency but you get more and more samples per cycle. Interestingly though this does not make a difference. A sinewave sampled at 2 samples per cycle is just as accurately recreated as one that is sampled 1000 times per second. This doesn't take into account noise but for most purposes works well enough.
I would suggest looking into the basics of digital signal processing as it a very interesting field and very useful to understand.
Edit: This assumes all of those parameters are evaluated as floating point numbers.
Fundamentally, you're missing a piece of information. You don't specify the amount of time over which you want your samples taken. This could also be thought of as the rate at which the samples will be played by your system. Something roughly in this direction will get you closer, for now, though.
samples.push_back(amp * std::sin(M_PI / freq *i));
I want to get the property FPS from a video that is recorded by a camera.
I use:
CvCapture* flujo_video = cvCreateFileCapture(argv[1]);
double parametro= cvGetCaptureProperty( flujo_video, CV_CAP_PROP_FPS);
The result of this is -nan and if I use an int format the result is -2147483648.
Try it without using the deprecated C api:
VideoCapture cap(0); // open the video file for reading
double fps = cap.get(CV_CAP_PROP_FPS); //get the frames per seconds of the video
If you look around in the web, you can see lots of people having problem with this parameter. It turns out that being thousands of cameras/codecs/formats openCV cant handle them all, so often you get 0, NaN (not a number) or other illogical parameter. This generally means that you can not get the FPS for your camera.
I have a very basic question about frame capturing using OpenCV. My code look like below:
VideoCapture cap(0);
cv::Mat mat;
int i = 0;
while(cap.read(mat)==true) {
//some code here
i = i + 1;
}
It works well. However, when I look at logcat logs by OpenCV, it says
FRAMES Received 225, grabbed 123.
and this grabbed (123) usually matches with the variable 'i' (number of loops) in my code.
Ideally my code should be able to read all received frames, isn't it? Can someone explain this behavior?
Calling cap.read(mat) takes a certain amount of time as it has to obtain and decode the image's video feed and convert it to the cv::Mat format. This amount of time appears to be greater than the video's capture rate. You can determine the frames per second of the video capture with the following:
double frames_per_second = cap.get(CV_CAP_PROP_FPS);
Try timing the amount of time your cap.read(mat) call takes and see if you can see a relationship between the ratio of frames received to frames grabbed and the ratio of the capture time (1/frames_per_second) and the time cap.read(mat) takes to execute.
Source:
http://opencv-srf.blogspot.ca/2011/09/capturing-images-videos.html
Please, this is not duplicate of similar posts!
I want to find and to decode Nth frame, for example 7th frame.
As I understood, using time_base I can calculate how many ticks is each frame and by multiplying it with 7 we will get position of 7th frame. To calculate the ticks I do
AVStream inStream = getStreamFromAVFormatContext();
int fps = inStream->r_frame_rate.num;
AVRational timeBase = inStream->time_base;
int ticks_per_frame = (1/fps) / timeBase;
int _7thFramePos = ticks_per_frame * 7;
Did I calculated correctly position of 7th frame? If I did, so to go to that frame I just do av_seek_frame(pFormatCtx, -1, _7thFramePos, AVSEEK_FLAG_ANY), right?
What happens if the 7th frame was P-Frame or B-Frame, how I decode it?
I noticed that the calculated value differs from inStream->codec->ticks_per_frame, why? Shouldn't they be the same? What is the difference?
This post explains the issue nicely.
http://www.hackerfactor.com/blog/index.php?/archives/307-Picture-Go-Back.html
[1] comment for AVStream structure clearly mentions that "r_frame_rate" is a guess and may not be accurate, because even if I have frame-rate of (say) 25fps, in term of base_time I may have 24 or 26 frames in a second.
[2] To find the exact frame number you need to decode frame from the start and keep a counter, but that is very in-efficient, this can be optimized for some file-formats like MP4 where information about every frame is present in file-header.
How can I retrieve the current frame number of a video using OpenCV? Does OpenCV have any built-in function for getting the current frame or I have to do it manually?
You can use the "get" method of your capture object like below :
capture.get(CV_CAP_PROP_POS_FRAMES); // retrieves the current frame number
and also :
capture.get(CV_CAP_PROP_FRAME_COUNT); // returns the number of total frames
Btw, these methods return a double value.
You can also use cvGetCaptureProperty method (if you use old C interface).
cvGetCaptureProperty(CvCapture* capture,int property_id);
property_id options are below with definitions:
CV_CAP_PROP_POS_MSEC 0
CV_CAP_PROP_POS_FRAME 1
CV_CAP_PROP_POS_AVI_RATIO 2
CV_CAP_PROP_FRAME_WIDTH 3
CV_CAP_PROP_FRAME_HEIGHT 4
CV_CAP_PROP_FPS 5
CV_CAP_PROP_FOURCC 6
CV_CAP_PROP_FRAME_COUNT 7
POS_MSEC is the current position in a video file, measured in
milliseconds.
POS_FRAME is the position of current frame in video (like 55th frame of video).
POS_AVI_RATIO is the current position given as a number between 0 and 1
(this is actually quite useful when you want to position a trackbar
to allow folks to navigate around your video).
FRAME_WIDTH and FRAME_HEIGHT are the dimensions of the individual
frames of the video to be read (or to be captured at the camera’s
current settings).
FPS is specific to video files and indicates the number of frames
per second at which the video was captured. You will need to know
this if you want to play back your video and have it come out at the
right speed.
FOURCC is the four-character code for the compression codec to be
used for the video you are currently reading.
FRAME_COUNT should be the total number of frames in video, but
this figure is not entirely reliable.
(from Learning OpenCV book )
In openCV version 3.4, the correct flag is:
cap.get(cv2.CAP_PROP_POS_FRAMES)
The way of doing it in OpenCV python is like this:
import cv2
cam = cv2.VideoCapture(<filename>);
print cam.get(cv2.cv.CV_CAP_PROP_POS_FRAMES)