How do I capture images in OpenCV and saving in pgm format? - opencv

Question

I am brand new to programming in general, and am working on a project for which I need to capture images from my webcam (possibly using OpenCV), and save the images as pgm files.
What's the simplest way to do this? Willow Garage provides this code for image capturing:
http://opencv.willowgarage.com/wiki/CameraCapture
Using this code as a base, how might I modify it to:
capture an image from the live cam every 2 seconds
save the images to a folder in pgm format
Thanks so much for any help you can provide!

Answer 1

First of all, please use newer site - opencv.org. Using outdated references leads to chain effect, when new users see old references, read old docs and post old links again.
There's actually no reason to use old C API. Instead, you can use newer C++ interface, which, among other things, handles capturing video gracefully. Here's shortened version of example from docs on VideoCapture:
#include "opencv2/opencv.hpp"
using namespace cv;
int main(int, char**)
{
VideoCapture cap(0); // open the default camera
if(!cap.isOpened()) // check if we succeeded
return -1;
for(;;)
{
Mat frame;
cap >> frame; // get a new frame from camera
// do any processing
imwrite("path/to/image.png", frame);
if(waitKey(30) >= 0) break; // you can increase delay to 2 seconds here
}
// the camera will be deinitialized automatically in VideoCapture destructor
return 0;
}
Also, if you are new to programming, consider using Python interface to OpenCV - cv2 module. Python is often considered simpler than C++, and using it you can play around with OpenCV functions right in an interactive console. Capturing video with cv2 looks something like this (adopted code from here):
import numpy as np
import cv2
cap = cv2.VideoCapture(0)
while(True):
# Capture frame-by-frame
ret, frame = cap.read()
# do what you want with frame
# and then save to file
cv2.imwrite('path/to/image.png', frame)
if cv2.waitKey(30) & 0xFF == ord('q'): # you can increase delay to 2 seconds here
break
# When everything done, release the capture
cap.release()
cv2.destroyAllWindows()

Answer 2

Since ffriend's answer is only partially true, I'll add some more to it (in C++). The author of this question asks explicitly for exporting to PGM (PXM file format that stores each pixel in 8 bits) and not PNG (as ffriend describes in his/her reply). The main issue here is that the official documentation for imwrite is omho not clear about this matter at all:
For PPM, PGM, or PBM, it can be a binary format flag ( CV_IMWRITE_PXM_BINARY ), 0 or 1. Default value is 1.
If we read the sentence in normal English, we have a list of options: CV_IMWRITE_PXM_BINARY, 0 or 1. There is no mention that those can and actually are supposed to be combined! I had to experiment a little bit (I also needed to store 8-bit images for my project) and finally got to the desired solution:
std::vector<int> compression_params; // Stores the compression parameters
compression_params.push_back(CV_IMWRITE_PXM_BINARY); // Set to PXM compression
compression_params.push_back(0); // Set type of PXM in our case PGM
const std::string imageFilename = "myPGM.pgm"; // Some file name - C++ requires an std::string
cv::imwrite(imageFilename, myImageMatrix, compression_params); // Write matrix to file
My investigation was also fueled by this question where the author was (maybe still is) struggling with the very same issue and also by some basic information on the PXM format, which you can find here.
The result (only part of the image) is displayed below:
P2
32 100
255
121 237 26 102 88 143 67 224 160 164 238 8 119 195 138 16 176 244 72 106 72 211 168 45 250 161 37 1 96 130 74 8
126 122 227 86 106 120 102 150 185 218 164 232 111 230 207 191 39 222 236 78 137 71 174 96 146 122 117 175 34 245 6 125
124 121 241 67 225 203 118 209 227 168 175 40 90 19 197 190 40 254 68 90 88 242 136 32 123 201 37 35 99 179 198 163
97 161 129 35 48 140 234 237 98 73 105 77 211 234 88 176 152 12 68 93 159 184 238 5 172 252 38 68 200 130 194 216
160 188 21 53 16 79 71 54 124 234 34 92 246 49 0 17 145 102 72 42 105 252 81 63 161 146 81 16 72 104 66 41
189 100 252 37 13 91 71 40 123 246 33 157 67 96 71 59 17 196 96 110 109 116 253 30 42 203 69 53 97 188 90 68
101 36 84 5 41 59 80 8 107 160 168 9 194 8 71 55 152 132 232 102 12 96 213 24 134 208 1 55 64 43 74 22
92 77 30 44 139 96 70 152 160 146 142 8 87 243 11 91 49 196 104 250 72 67 159 44 240 225 69 29 34 115 42 2
109 176 145 90 137 172 65 25 162 57 169 92 214 211 72 94 149 20 104 56 27 67 218 17 203 182 5 124 138 2 130 48
121 225 25 106 89 76 69 189 34 25 173 8 114 83 72 52 145 154 64 40 91 2 251 53 251 237 20 124 82 2 194 42 ...
Which is exactly what is required in this case. You can see the "P2" marking at the top and also the values are clearly from 0 to 255, which is exactly 8 bits per pixel.

Answer 3

Read most of the answers but none of them could satisfy my requirement. Here's how I implemented it.
This program will use webcam as a camera and clicks picture when you press 'c' - we can change the condition, then make it to click pictures automatically after certain interval
# Created on Sun Aug 12 12:29:05 2018
# #author: CodersMine
import cv2
video_path = 0 # 0 internal cam, 1 external webcam
cap = cv2.VideoCapture(video_path)
img_ctr = 0 # To Generate File Names
while(True):
ret, frame = cap.read()
cv2.imshow("imshow",frame)
key = cv2.waitKey(1)
if key==ord('q'): # Quit
break
if key==ord('c'): # Capture
cv2.imshow("Captured",frame)
flag = cv2.imwrite(f"image{img_ctr}.png", frame)
print(f"Image Written {flag}")
img_ctr += 1
# Release the Camera
cap.release()
cv2.destroyAllWindows()

Answer 4

If you don't need superaccurate 2seconds then simply put a sleep(2) or Sleep(2000) in the while(1) loop to wait fro 2seconds before each grab,
Write images with cvSaveImage() just put the extention .pgm on the filename and it will use pgm.

Answer 5

I believe that the format is chosen from the extension of the filename - so assuming your opencv lib's are linked against the appropriate libs you can do something like: (this is from memory, might not be correct.)
CvCapture* capture = cvCaptureFromCam(0);
IplImage* frame = cvQueryFrame(capture);
while (1) {
frame = cvQueryFrame(capture);
cvSaveImage("foo.pgm", &frame);
sleep(2);
}
cvReleaseImage(&frame);
cvReleaseCapture(&capture);