I am capturing frames from a video source and want to output them as files in, for example, BMP format.
What I would like to know is a) is there built in functionality for this, and b) if not, how do I get the RGB values for each of the pixels in each frame represented by a Mat object?
Thanks in advance for your help.
a) See imwrite (on the same page of the documentation as the video capture stuff)
b) If you do want to get RGB values from a Mat object, see the Mat documentation, which discusses element access in detail - in summary, M.at<datatype>(i,j) or M.data[ M.step[0]*i + M.step[1]*j ] -- the latter may differ depending on your version of OpenCV, consult the corresponding documentation page.
Related
I wrote a code for watershed segmentation in C API. Now I am converting all those into C++. so, cvsaveimage becomes imwrite. But when I use imwrite ,all i get is a black image.
this is the code:-
Mat img8bit;
Mat img0;
img0 = imread("source.png", 1);
Mat wshed(img0.size(), CV_32S);
wshed.setTo(cv::Scalar::all(0));
////after performing watershed segmentation and
// displaying the watershed image from wshed//
wshed.convertTo(img8bit, CV_32FC3, 255.0);
imwrite("Watershed.png", img8bit);
The original image that I want to save is in wshed. I saw suggestions from the net that we need to convert it to 16 bit or higher so that the imwrite saves it right. Like you see,I tried that. But the wshed image is being displayed correctly when using imshow.The img0 is grey image/black and white while the wshed image is coloured. any help on this?
Edit- I changed the 4th line to
Mat wshed(img0.size(), CV_32FC3);
When calling Mat::convertTo() with a scalar (255 in your case), the values of every matrix item will be multiplied by this scalar value. This will cause all most every result pixel values exceed 255 (i.e. white pixels) except those of 0s where they remain 0 (i.e. black pixels). This is why you will get the black-white pixel in the end.
To make it work, simply change it to:
wshed.convertTo(img8bit, CV_32FC3);
You said:
The original image that I want to save is in wshed. I saw suggestions
from the net that we need to convert it to 16 bit or higher so that
the imwrite saves it right.
If saving the image does not work you should keep in mind that the image data has to be either 8-Bits or 16-Bit unsigned when using the imwrite Function, not 16-Bits or higher.
This is stated in the documentation:
The function imwrite saves the image to the specified file. The image
format is chosen based on the filename extension (see imread() for the
list of extensions). Only 8-bit (or 16-bit unsigned (CV_16U) in case
of PNG, JPEG 2000, and TIFF) single-channel or 3-channel (with ‘BGR’
channel order) images can be saved using this function. If the format,
depth or channel order is different, use Mat::convertTo() , and
cvtColor() to convert it before saving. Or, use the universal
FileStorage I/O functions to save the image to XML or YAML format.
Looking for a way for taking a screenshot of a particular area on the screen in C++. (So not the whole screen) Then it should save it as .png .jpg whatever to use it with another function afterwards.
Also, I am going to use it, somehow, with openCV. Thought i'd mention that, maybe it's a helpful detail.
OpenCV cannot take screenshots from your computer directly. You will need a different framework/method to do this. #Ben is correct, this link would be worth investigating.
Once you have read this image in, you will need to store it into a cv:Mat so that you are able to perform OpenCV operations on it.
In order to crop an image in OpenCV the following code snippet would help.
CVMat * imagesource;
// Transform it into the C++ cv::Mat format
cv::Mat image(imagesource);
// Setup a rectangle to define your region of interest
cv::Rect myROI(10, 10, 100, 100);
// Crop the full image to that image contained by the rectangle myROI
// Note that this doesn't copy the data
cv::Mat croppedImage = image(myROI);
i was capturing live video from my web camera to Mat objects.
is their any efficient way to convert a MAT object in to gray scaled image frame without using any API such as openCV...
I have tried it using openCV.
but i like to implement in to c++...is their any way to do it?
I would recommend you use OpenCV. OpenCV already contains optimized implementations for converting between various color spaces (i.e. even between RGB (actually BGR for OpenCV) to greyscale).
See for more details: http://docs.opencv.org/modules/imgproc/doc/miscellaneous_transformations.html.
OpenCV is allready implemented in C++.
If you really want to implement you own for didactical purposes (I don't see any reason why you would do it otherwise) then the simple way to do it would be to iterate the R G B values in the Mat and apply the formula:
resultingVlue = 0.299 * R + 0.587 * G + 0.114 * B
(See also Stack overflow Question Converting RGB to grayscale/intensity for a more detailed discussion on why the R G B components typically get weighted differently)
Assuming here you want to convert RGB to gray. For other color space conversions, please look at the OpenCv documentation that also details how the transformations are done (see link provided above).
More so, OpenCV is open source. This means if you want to see how a optimal implementation might look like, you can download the source code and take a look.
Google tells me that you have to average the values of the R,G and B values of each pixel. Some algorithms are discussed here
http://www.johndcook.com/blog/2009/08/24/algorithms-convert-color-grayscale/
The simplest is to convert each color R, G and B values by the average (R+G+B)/3. Check the above links for the results of a few different averages.
I'm trying to merge/stitch 2 images together but found that the default stitcher class in OpenCV could not handle my images.
So I started to write my own..
Unfortunately the images are too large to attach to this message (they are both 12600x9000 pixels in size).. so I'll try to explain as good as possible.
The 2 images are not pictures takes by a camera but are tiff files extracted from a PDF file.
The images themselves were actually CAD drawings, so not much gradients in there and therefore I think the default stitcher class could not handle them.
So far, I managed to extract the features and match them.
Also I used the following well known example to stitch them together:
Mat WarpedImage;
cv::warpPerspective(img_2,WarpedImage,homography,cv::Size(2*img_2.cols,2*img_2.rows));
Mat half(WarpedImage,Rect(0,0,img_1.cols,img_1.rows));
img_1.copyTo(half);
I sort of made it fit.. because my problem is that in my case the 2 images could be aligned vertically or horizontally.
By default, all stitch examples on the internet assume the first image is the left image and the 2nd image is the right image.
So my first question would be:
How can I detect if the image is to the left, right, above or below the first image and create a proper sized new image?
Secondly..
Currently I'm getting the proper image.. however, because I'm not having some decent code to check the ideal width and height of the new image, I have a lot of black/empty space in the new image.
What would be the best C++ code to remove those black area's?
(I'm seeing a lot of Python scripts on the net.. but no C++ examples of this.. and I have 0 Python skills....)
Thank you very much in advance for your help.
Greetings,
Floris.
You can reproject the corners of the second image with perspectiveTransform. With the transformed points you can find the relative position of your image and calculate the new image size that will fit both images. This will also let you deal with the black areas, since you have the boundaries of the two images.
I have got a big problem. After searching through the internet, I didn't find a good solution. I read in images from files via opencv (2.3) and manipulate them. Afterwards I want to present the result in my application written in wxwidgets (2.9.3). The main problem is, that my images are grayscale and so I just have got a single data pointer, but wxwidgets just use RGB. just a small example:
cv::imread(filename,CV_LOAD_IMAGE_GRAYSCALE).convertTo(pictureMatrix,CV_32F,(float)(1/2.0f),0);
// here are some more floating point calculations
cv::Mat output;
pictureMatrix.convertTo(output,CV_8U);
wxImage test(output.rows, output.cols, output.data, true);
wxInitAllImageHandlers();
// saving the picture is just for testing, if it works
test.SaveFile("test.png", wxBITMAP_TYPE_PNG);
All you need to to do is to convert from grayscale to RGB (actually, to BGR, if you're on Windows).
cv::Mat grayOutput, rgbOutput;
pictureMatrix.convertTo(grayOutput,CV_8U);
cvtColor(grayOutput, rgbOutput, CV_GRAY2BGR); // note the BGR here.
//If on Linux, set as RGB
wxImage test(rgbOutput.cols, rgbOutput.rows, rgbOutput.data, true);
...
You can always set R=G=B=<your grayscale value> for every pixel. If the format of the image pixels doesn't match up, you can allocate a new array in the format expected by wxImage and fill it with those RGB values.
You can also take a look at this link. It looks similar to what you need to do.