I'm trying to print an image using OpenCV defining a 400x400 Mat:
plot2 = cv::Mat(400,400, CV_8U, 255);
But when I try print the points, something strange happens. The y coordinate only prints to the first 100 values. That is, if I print the point (50,100), it does not print it in the 100/400th part of the columns, but at the end. Somehow, 400 columns have turned into 100.
For example, when running this:
for (int j = 0; j < 95; ++j){
plot2.at<int>(20, j) = 0;
}
cv::imshow("segunda pared", plot2);
Shows this (the underlined part is the part corresponding to the code above):
A line that goes to 95 almost occupies all of the 400 points when it should only occupy 95/400th of the screen.
What am I doing wrong?
When you defined your cv::Mat, you told clearly that it is from the type CV_8U:
plot2 = cv::Mat(400,400, CV_8U, 255);
But when you are trying to print it, you are telling that its type is int which is usually a signed 32 bit not unsigned 8 bit. So the solution is:
for (int j = 0; j < 95; ++j){
plot2.at<uchar>(20, j) = 0;
}
Important note: Be aware that OpenCV uses the standard C++ types not the fixed ones. So, there is no need to use fixed size types like uint16_t or similar. because when compiling OpenCV & your code on another platform both of them will change together.
BTW, one of the good way to iterate through your cv::Mat is:
for (size_t row = 0; j < my_mat.rows; ++row){
auto row_ptr=my_mat.ptr<uchar>(row);
for(size_t col=0;col<my_mat.cols;++col){
//do whatever you want with row_ptr[col] (read/write)
}
}
Related
My title may not be clear enough, but please look carefully on the following description.Thanks in advance.
I have a RGB image and a binary mask image:
Mat img = imread("test.jpg")
Mat mask = Mat::zeros(img.rows, img.cols, CV_8U);
Give some ones to the mask, assume the number of ones is N. Now the nonzero coordinates are known, based on these coordinates, we can surely obtain the corresponding pixel RGB value of the origin image.I know this can be accomplished by the following code:
Mat colors = Mat::zeros(N, 3, CV_8U);
int counter = 0;
for (int i = 0; i < mask.rows; i++)
{
for (int j = 0; j < mask.cols; j++)
{
if (mask.at<uchar>(i, j) == 1)
{
colors.at<uchar>(counter, 0) = img.at<Vec3b>(i, j)[0];
colors.at<uchar>(counter, 1) = img.at<Vec3b>(i, j)[1];
colors.at<uchar>(counter, 2) = img.at<Vec3b>(i, j)[2];
counter++;
}
}
}
And the coords will be as follows:
enter image description here
However, this two layer of for loop costs too much time. I was wondering if there is a faster method to obatin colors, hope you guys can understand what I was trying to convey.
PS:If I can use python, this can be done in only one sentence:
colors = img[mask == 1]
The .at() method is the slowest way to access Mat values in C++. Fastest is to use pointers, but best practice is an iterator. See the OpenCV tutorial on scanning images.
Just a note, even though Python's syntax is nice for something like this, it still has to loop through all of the elements at the end of the day---and since it has some overhead before this, it's de-facto slower than C++ loops with pointers. You necessarily need to loop through all the elements regardless of your library, you're doing comparisons with the mask for every element.
If you are flexible with using any other open source library using C++, try Armadillo. You can do all linear algebra operations with it and also, you can reduce above code to one line(similar to your Python code snippet).
Or
Try findNonZero()function and find all coordinates in image containing non-zero values. Check this: https://stackoverflow.com/a/19244484/7514664
Compile with optimization enabled, try profiling this version and tell us if it is faster:
vector<Vec3b> colors;
if (img.isContinuous() && mask.isContinuous()) {
auto pimg = img.ptr<Vec3b>();
for (auto pmask = mask.datastart; pmask < mask.dataend; ++pmask, ++pimg) {
if (*pmask)
colors.emplace_back(*pimg);
}
}
else {
for (int r = 0; r < img.rows; ++r) {
auto prowimg = img.ptr<Vec3b>(r);
auto prowmask = img.ptr(r);
for (int c = 0; c < img.cols; ++c) {
if (prowmask[c])
colors.emplace_back(prowimg[c]);
}
}
}
If you know the size of colors, reserve the space for it beforehand.
I try to modify a BGRA mat using a pointer like this:
//Bound the value between 0 to 255
uchar boundPixelValue(double c) {
c = int(c);
if (c > 255)
c = 255;
if (c < 0)
c = 0;
return (uchar) c;
}
for (int i = 0; i < rows; i++)
for (int j = 0; j < cols; j++)
for (int k = 0; k < 3; k++){
//This loop is accessing the first three channels
mat.ptr<Vec4b>(i)[j][k] = boundPixelValue(
1.0 * mat.ptr<Vec4b>(i)[j][k] * max / avg[k]);
}
But this gives different outputs every time, sometimes work and sometimes give a white blank image. I am suspecting if this is due to the noncontinuous data, can anyone help?
One extra question, usually we access the columns of a 2D array first before accessing the rows because it is usually faster. However, I have to access the pixel using mat.ptr<Vec4b>(row)[col]. So, should I loop through the rows first then column?
Easier, less intensive way of doing is:
std::vector<cv::Mat> matArray;
cv::split(toBoundMat, matArray);
matArray[0].setTo(0, matArray[0] < 0);
matArray[0].setTo(255, matArray[0] > 255);
matArray[1].setTo(0, matArray[1] < 0);
matArray[1].setTo(255, matArray[1] > 255);
matArray[2].setTo(0, matArray[2] < 0);
matArray[2].setTo(255, matArray[2] > 255);
cv::Mat boundedMat;
cv::merge(matArray, boundedMat);
But I really don't understand what you are trying to do. Your double data may have values between 1.7E +/- 308. You either are expecting a very specific kind of data, or you are going to mess it up. If you want to make a Mat visualizable, just normalize it like this:
cv::normalize(inMat, destMat, 0, 255, CV_MINMAX);
cv::cvtColor(destMat, destMat, CV_8UC1) //--(8 bit visualizable mat)
This will check the min and max of your current Mat and will set the minimum to 0, the maximum to 255, and all the in between values proportionally :)
I guess it's such an easy question (I'm coming from Java), but I can't figure out how it works.
I simply want to increment an vector element by one. The reason for this is, that I want to compute a histogram out of image values. But whatever I try I just can accomplish to assign a value to the vector. But not to increment it by one!
This is my histogram function:
void histogram(unsigned char** image, int height,
int width, vector<unsigned char>& histogramArray) {
for (int i = 0; i < width; i++) {
for (int j = 0; j < height; j++) {
// histogramArray[1] = (int)histogramArray[1] + (int)1;
// add histogram position by one if greylevel occured
histogramArray[(int)image[i][j]]++;
}
}
// display output
for (int i = 0; i < 256; i++) {
cout << "Position: " << i << endl;
cout << "Histogram Value: " << (int)histogramArray[i] << endl;
}
}
But whatever I try to add one to the histogramArray position, it leads to just 0 in the output. I'm only allowed to assign concrete values like:
histogramArray[1] = 2;
Is there any simple and easy way? I though iterators are hopefully not necesarry at this point, because I know the exakt index position where I want to increment something.
EDIT:
I'm so sorry, I should have been more precise with my question, thank you for your help so far! The code above is working, but it shows a different mean value out of the histogram (difference of around 90) than it should. Also the histogram values are way different than in a graphic program - even though the image values are exactly the same! Thats why I investigated the function and found out if I set the histogram to zeros and then just try to increase one element, nothing happens! This is the commented code above:
for (int i = 0; i < width; i++) {
for (int j = 0; j < height; j++) {
histogramArray[1]++;
// add histogram position by one if greylevel occured
// histogramArray[(int)image[i][j]]++;
}
}
So the position 1 remains 0, instead of having the value height*width. Because of this, I think the correct calculation histogramArray[image[i][j]]++; is also not working properly.
Do you have any explanation for this? This was my main question, I'm sorry.
Just for completeness, this is my mean function for the histogram:
unsigned char meanHistogram(vector<unsigned char>& histogram) {
int allOccurences = 0;
int allValues = 0;
for (int i = 0; i < 256; i++) {
allOccurences += histogram[i] * i;
allValues += histogram[i];
}
return (allOccurences / (float) allValues) + 0.5f;
}
And I initialize the image like this:
unsigned char** image= new unsigned char*[width];
for (int i = 0; i < width; i++) {
image[i] = new unsigned char[height];
}
But there shouldn't be any problem with the initialization code, since all other computations work perfectly and I am able to manipulate and safe the original image. But it's true, that I should change width and height - since I had only square images it didn't matter so far.
The Histogram is created like this and then the function is called like that:
vector<unsigned char> histogramArray(256);
histogram(array, adaptedHeight, adaptedWidth, histogramArray);
So do you have any clue why this part histogramArray[1]++; don't increases my histogram? histogramArray[1] remains 0 all the time! histogramArray[1] = 2; is working perfectly. Also histogramArray[(int)image[i][j]]++; seems to calculate something, but as I said, I think it's wrongly calculating.
I appreciate any help very much! The reason why I used a 2D Array is simply because it is asked for. I like the 1D version also much more, because it's way simpler!
You see, the current problem in your code is not incrementing a value versus assigning to it; it's the way you index your image. The way you've written your histogram function and the image access part puts very fine restrictions on how you need to allocate your images for this code to work.
For example, assuming your histogram function is as you've written it above, none of these image allocation strategies will work: (I've used char instead of unsigned char for brevity.)
char image [width * height]; // Obvious; "char[]" != "char **"
char * image = new char [width * height]; // "char*" != "char **"
char image [height][width]; // Most surprisingly, this won't work either.
The reason why the third case won't work is tough to explain simply. Suffice it to say that a 2D array like this will not implicitly decay into a pointer to pointer, and if it did, it would be meaningless. Contrary to what you might read in some books or hear from some people, in C/C++, arrays and pointers are not the same thing!
Anyway, for your histogram function to work correctly, you have to allocate your image like this:
char** image = new char* [height];
for (int i = 0; i < height; ++i)
image[i] = new char [width];
Now you can fill the image, for example:
for (int i = 0; i < height; ++i)
for (int j = 0; j < width; ++j)
image[i][j] = rand() % 256; // Or whatever...
On an image allocated like this, you can call your histogram function and it will work. After you're done with this image, you have to free it like this:
for (int i = 0; i < height; ++i)
delete[] image[i];
delete[] image;
For now, that's enough about allocation. I'll come back to it later.
In addition to the above, it is vital to note the order of iteration over your image. The way you've written it, you iterate over your columns on the outside, and your inner loop walks over the rows. Most (all?) image file formats and many (most?) image processing applications I've seen do it the other way around. The memory allocations I've shown above also assume that the first index is for the row, and the second is for the column. I suggest you do this too, unless you've very good reasons not to.
No matter which layout you choose for your images (the recommended row-major, or your current column-major,) it is in issue that you should always keep in your mind and take notice of.
Now, on to my recommended way of allocating and accessing images and calculating histograms.
I suggest that you allocate and free images like this:
// Allocate:
char * image = new char [height * width];
// Free:
delete[] image;
That's it; no nasty (de)allocation loops, and every image is one contiguous block of memory. When you want to access row i and column j (note which is which) you do it like this:
image[i * width + j] = 42;
char x = image[i * width + j];
And you'd calculate the histogram like this:
void histogram (
unsigned char * image, int height, int width,
// Note that the elements here are pixel-counts, not colors!
vector<unsigned> & histogram
) {
// Make sure histogram has enough room; you can do this outside as well.
if (histogram.size() < 256)
histogram.resize (256, 0);
int pixels = height * width;
for (int i = 0; i < pixels; ++i)
histogram[image[i]]++;
}
I've eliminated the printing code, which should not be there anyway. Note that I've used a single loop to go through the whole image; this is another advantage of allocating a 1D array. Also, for this particular function, it doesn't matter whether your images are row-major or column major, since it doesn't matter in what order we go through the pixels; it only matters that we go through all the pixels and nothing more.
UPDATE: After the question update, I think all of the above discussion is moot and notwithstanding! I believe the problem could be in the declaration of the histogram vector. It should be a vector of unsigned ints, not single bytes. Your problem seems to be that the value of the vector elements seem to stay at zero when your simplify the code and increment just one element, and are off from the values they need to be when you run the actual code. Well, this could be a symptom of numeric wrap-around. If the number of pixels in your image are a a multiple of 256 (e.g. 32x32 or 1024x1024 image) then it is natural that the sum of their number would be 0 mod 256.
I've already alluded to this point in my original answer. If you read my implementation of the histogram function, you see in the signature that I've declared my vector as vector<unsigned> and have put a comment above it that says this victor counts pixels, so its data type should be suitable.
I guess I should have made it bolder and clearer! I hope this solves your problem.
I'm new to C++ and getting a bit frustrated with it. Below, in pixelsVector, I am storing each pixel RGB float-value in Pixel and want to dump
all the values to a byte array with pixelsArray so I can output to an image file. HEIGHT and WIDTH refer to the image dimensions. The code below works fine, but I need to specify
the sizes of pixelsArray at run-time, because it may not always be a 500x500 image.
// WIDTH and HEIGHT specified at run-time
vector<vector<Pixel>> pixelsVector (WIDTH, vector<Pixel> (HEIGHT));
...
unsigned char pixelsArray[500][500][3];
for (int i = 0; i < 500; i++)
{
for (int j = 0; j < 500; j++)
{
// Returns RGB components
vector<float> pixelColors = pixelArray[i][j].getColor();
for (int k = 0; k < 3; k++)
{
pixels[i][j][k] = pixelColors.at(k);
}
}
}
// write to image file
fwrite(pixelsArray, 1, 500*500*3, file);
If I put HEIGHT and WIDTH instead of 500 and 500 above, I get an error since they are not constant values. Now using a 3D vector does seem to work, but fwrite won't take a vector for its first argument. I tried using a triple-pointer array but
it doesn't seem to work at all - maybe I was using it wrong. Here it is using a 3D vector for pixelsArray:
vector<vector<Pixel>> pixelsVector (WIDTH, vector<Pixel> (HEIGHT));
...
vector< vector< vector<unsigned char> > > pixelsArray;
for (int i = 0; i < HEIGHT; i++)
{
pixels.push_back(vector< vector<unsigned char> >());
for (int j = 0; j < WIDTH; j++)
{
pixels[i].push_back(vector<unsigned char>());
vector<float> pixelColors;
pixelColors = pixelArray[i][j].getColor();
for (int k = 0; k < 3; k++)
{
pixels[i][j][k] = pixelColors.at(k);
}
}
}
// Error
fwrite(pixelsArray, 1, 500*500*3, file);
Suggestions?
You could use Boost.MultiArray insead of vectors of vectors, which lets you access he underlying memory with the .data() method.
It looks like you are trying to manipulate images, so you might want to consider using Boost.Gil.
From the last code snippet:
vector<vector<Pixel>> pixelsVector (WIDTH, vector<Pixel> (HEIGHT));
Using uppercase names for variables you risk name collisions with macros. In C++ all uppercase names are conventionally reserved for macros.
...
vector< vector< vector<unsigned char> > > pixelsArray;
Presumably this vector is the same as is called pixels below?
If so, then the standard advice is that it helps to post real code.
Anyway, in order to output those bytes in one efficient operation you need the bytes to be contiguously stored in memory. So a vector of vectors of vectors is out. Use a single vector (C++ guarantees contiguous storage for the buffer of a std::vector).
for (int i = 0; i < HEIGHT; i++)
{
pixels.push_back(vector< vector<unsigned char> >());
for (int j = 0; j < WIDTH; j++)
{
pixels[i].push_back(vector<unsigned char>());
At this point you have an inner vector but it's empty, size 0.
vector<float> pixelColors;
pixelColors = pixelArray[i][j].getColor();
Presumably pixelArray is an instance of a class you have defined?
for (int k = 0; k < 3; k++)
{
pixels[i][j][k] = pixelColors.at(k);
}
Here you're trying to assign to non-existent elements of the empty innermost vector. You can either size it properly in advance, or use the push_back method for each value.
In addition, are you sure that the float values are integers in range 0 through 255 (or more generally, 0 through UCHAR_MAX) and not, say, in the range 0 through 1?
Perhaps you need to scale those values.
}
}
// Error
fwrite(pixelsArray, 1, 500*500*3, file);
If pixelsArray had been a (non-empty) vector of bytes, then you could use &pixelsArray[0] to obtain a pointer to the first byte.
Now, I know, the above only dissects some of what's wrong, and doesn't tell you directly what's right. :-)
But some more information would be needed to give example code for doing this, like (1) what are your float values, and (2) what do you want in your file?
Anyway, hope this helps,
– Alf
I have an assignment about fftw and I was trying to write a small program to create an fft of an image. I am using CImg to read and write images. But all I get is a dark image with a single white dot :(
I'm most likely doing this the wrong way and I would appreciate if someone could explain how this should be done. I don't need the code, I just need to know what is the right way to do this.
Here is my code:
CImg<double> input("test3.bmp");
CImg<double> image_fft(input, false);
unsigned int nx = input.dimx(), ny = input.dimy();
size_t align = sizeof(Complex);
array2<Complex> in (nx, ny, align);
fft2d Forward(-1, in);
for (int i = 0; i < input.dimx(); ++i) {
for (int j = 0; j < input.dimy(); ++j) {
in(i,j) = input(i,j);
}
}
Forward.fft(in);
for (int i = 0; i < input.dimx(); ++i) {
for (int j = 0; j < input.dimy(); ++j) {
image_fft(i,j,0) = image_fft(i,j,1) = image_fft(i,j,2) = std::abs(in(i,j));
}
}
image_fft.normalize(0, 255);
image_fft.save("test.bmp");
You need to take the log of the magnitude. The single white dot is the base value (0 Hz, DC, whatever you want to call it), so it will almost ALWAYS be by far the largest component of any image you take (Since pixel values cannot be negative, the DC value will always be positive and large).
What you need to do is calculate the log (ln, whatever, some type of logarithmic calculation) of the magnitude (so after you've converted from complex to magnitude/phase form (phasor notation iirc?)) on each point before you normalize it.
Please note that the values are there, they are just REALLY small compared to the DC value, taking the log (Which makes smaller values bigger by a lot, and bigger values only slightly larger) will make the other frequencies visible.