I'm using the code suggested in ( how to convert an opencv cv::Mat to qimage ) to display a cv::Mat in my Qt application. However, I'm getting strange results. The black parts are displayed as black, but all other values are inverted.
Conversion code:
QImage ImgConvert::matToQImage(Mat_<double> src)
{
double scale = 255.0;
QImage dest(src.cols, src.rows, QImage::Format_ARGB32);
for (int y = 0; y < src.rows; ++y) {
const double *srcrow = src[y];
QRgb *destrow = (QRgb*)dest.scanLine(y);
for (int x = 0; x < src.cols; ++x) {
unsigned int color = srcrow[x] * scale;
destrow[x] = qRgba(color, color, color, 255);
}
}
return dest;
}
Display code:
void MainWindow::redraw()
{
static QImage image = ImgConvert::matToQImage(im);
static QGraphicsPixmapItem item( QPixmap::fromImage(image));
static QGraphicsScene* scene = new QGraphicsScene;
scene->addItem(&item);
ui->graphicsView->setScene(scene);
ui->graphicsView->repaint();
}
Right now I'm using if(color>0) color = 255-color; to correct for this effect, but I'd much rather understand where it's coming from.
Also, a second mini-question: if I remove the static declarations in redraw(), the image gets removed from memory immediately when the method exits. Is this the best way to fix this, and am I going to have any unintended side effects if I display multiple frames?
I don't know. Setting an array first for me sounds like a cleaner way, see https://stackoverflow.com/a/3387400/1705967 , that could give you ideas.
Although I also use Ypnos's solution with a great success on color images. :)
Ah, and as for the second question, don't worry about the QPixmap. It makes the image data private (clones when necessary) as I have experienced so you won't overwrite it by mistake.
In case anyone is having this problem, I quickly and dirtily fixed it by subtracting the pixel value to 256:
QImage ImgConvert::matToQImage(Mat_<double> src)
{
double scale = 255.0;
QImage dest(src.cols, src.rows, QImage::Format_ARGB32);
for (int y = 0; y < src.rows; ++y) {
const double *srcrow = src[y];
QRgb *destrow = (QRgb*)dest.scanLine(y);
for (int x = 0; x < src.cols; ++x) {
unsigned int color = 256 - (srcrow[x] * scale);
destrow[x] = qRgba(color, color, color, 255);
}
}
return dest;
}
This will slightly corrupt the image, though, modifying by 1 its bright. My purpose was visualizing so the difference was negligible to the eye, however for certain applications in image processing this corruption might be critical. I could not find why was this happening and as I was in a hurry I did not look any further.
Related
I have this code:
QImage grayImage = image.convertToFormat(QImage::Format_Grayscale8);
int size = grayImage.width() * grayImage.height();
QRgb *data = new QRgb[size];
memmove(data, grayImage.constBits(), size * sizeof(QRgb));
QRgb *ptr = data;
QRgb *end = ptr + size;
for (; ptr < end; ++ptr) {
int gray = qGray(*ptr);
}
delete[] data;
It is based on this: https://stackoverflow.com/a/40740985/8257882
How can I set the color of a pixel using that pointer?
In addition, using qGray() and loading a "bigger" image seem to crash this.
This works:
int width = image.width();
int height = image.height();
for (int y = 0; y < height; ++y) {
for (int x = 0; x < width; ++x) {
image.setPixel(x, y, qRgba(0, 0, 0, 255));
}
}
But it is slow when compared to explicitly manipulating the image data.
Edit
Ok, I have this code now:
for (int y = 0; y < height; ++y) {
uchar *line = grayImage.scanLine(y);
for (int x = 0; x < width; ++x) {
int gray = qGray(line[x]);
*(line + x) = uchar(gray);
qInfo() << gray;
}
}
And it seems to work. However, when I use an image that has only black and white colors and print the gray value, black color gives me 0 and white gives 39. How can I get the gray value in a range of 0-255?
First of all you are copying too much data in this line:
memmove(data, grayImage.constBits(), size * sizeof(QRgb));
The size ob Qrgb is 4 bytes, but according to the documentation, the size of a Format_Grayscale8 pixel is only 8 bits or 1 byte. If you remove sizeof(QRgb) you should be copying the correct amount of bytes, assuming all the lines in the bitmap are consecutive (which, according to the documentation, they are not -- they are aligned to at minimum 32-bits, so you would have to account for that in size). The array data should not be of type Qrgb[size] but ucahr[size]. You can then modify data as you like. Finally, you will probably have to create a new QImage with one of the constructors that accept image bits as uchar and assign the new image to the old image:
auto newImage = QImage( data, image.width(), image.height(), QImage::Format_Grayscale8, ...);
grayImage = std::move( newImage );
But instead of copying image data, you could probably just modify grayImage directly by accessing its data through bits(), or even better, through scanLine(), maybe something like this:
int line, column;
auto pLine = grayImage.scanLine(line);
*(pLine + column) = uchar(grayValue);
EDIT:
According to scanLine documentation, the image is at least 32-bit aligned. So if your 8-bit grayScale image is 3 pixels wide, a new scan line will start every 4 bytes. If you have a 3x3 image, the total size of the memory required to hold the image pixels will be 12. The following code shows the required memory size:
int main() {
auto image = QImage(3, 3, QImage::Format_Grayscale8);
std::cout << image.bytesPerLine() * image.height() << "\n";
return 0;
}
The fill method (setting all gray values to 0xC0) could be implemented like this:
auto image = QImage(3, 3, QImage::Format_Grayscale8);
uchar gray = 0xc0;
for ( int i = 0; i < image.height(); ++i ) {
auto pLine = image.scanLine( i );
for ( int j = 0; j < image.width(); ++j )
*pLine++ = gray;
}
I need to compare two color RBG images and get a result images of difference pixel by pixel. Is there any idea how can I do that in qt ?
I would appreciate for any help or advice.
Here is an alternative based on this QtForum question:
void substract(const QImage &left, const QImage &rigth, QImage &result)
{
int w=min(left.width(), rigth.width());
int h=min(left.height(),rigth.height();
w=min(w, result.width());
h=min(h, result.height();
//<-This ensures that you work only at the intersection of images areas
for(int i=0;i<h;i++){
QRgb *rgbLeft=(QRgb*)left.constScanLine(i);
QRgb *rgbRigth=(QRgb*)rigth.constScanLine(i);
QRgb *rgbResult=(QRgb*)result.constScanLine(i);
for(int j=0;j<w;j++){
rgbResult[j] = rgbLeft[j]-rgbRigth[j];
}
}
}
First, an RGB image is a 3-dimensional matrix containing the 3 channels (R,G,B)
To achieve the difference you can simply subtract the matrixes.
If you're using OpenCv consider the code below, otherwise you can traverse the matrixes and subtract each position separately.
#include <cv.h>
#include <highgui.h>
using namespace cv;
Mat img = imread("...");
Mat img2 = imread("...");
Mat diff_img = img - img2;
Using QImage you can iterate on pixel level, and simply output the RBG differences into a third image.
QRgb QImage::pixel(int x, int y) const
void QImage::setPixelColor(int x, int y, const QColor &color)
Keep in mind to iterate row sequentially for optimal performance. Meaning that the row should be your inner loop. A lot of the time people instinctively do the opposite, probably because most people prioritize width over height, therefore putting the row as the outer loop.
bool ImagesAreSimilar(QImage *img1,QImage *img2)
{
if (img1->isNull()||img2->isNull())
{
return false ;
}
if (img1->height()!=img2->height()) {
return false ;
}
if (img1->width()!=img2->width()) {
return false ;
}
auto pixel1 = img1->bits();
auto pixel2 = img2->bits();
bool similar=true;
for (int y = 0; y < img1->height(); y++)
{
for (int x = 0; x < img1->width(); x++)
{
if ( (pixel1[0]!=pixel2[0])||
(pixel1[1]!=pixel2[1])||
(pixel1[2]!=pixel2[2])||
(pixel1[3]!=pixel2[3])) {
return false ;
}
pixel1 += 4;
pixel2 += 4;
}
}
return similar;
}
I have a kinect streaming data into a cv::Mat. I am trying to get some example code running that uses OpenNI.
Can I convert my Mat into an OpenNI format image somehow?
I just need the depth image, and after fighting with OpenNI for a long time, have given up on installing it.
I am using OpenCV 3, Visual Studio 2013, Kinect v2 for Windows.
The relevant code is:
void CDifodoCamera::loadFrame()
{
//Read the newest frame
openni::VideoFrameRef framed; //I assume I need to replace this with my Mat...
depth_ch.readFrame(&framed);
const int height = framed.getHeight();
const int width = framed.getWidth();
//Store the depth values
const openni::DepthPixel* pDepthRow = (const openni::DepthPixel*)framed.getData();
int rowSize = framed.getStrideInBytes() / sizeof(openni::DepthPixel);
for (int yc = height-1; yc >= 0; --yc)
{
const openni::DepthPixel* pDepth = pDepthRow;
for (int xc = width-1; xc >= 0; --xc, ++pDepth)
{
if (*pDepth < 4500.f)
depth_wf(yc,xc) = 0.001f*(*pDepth);
else
depth_wf(yc,xc) = 0.f;
}
pDepthRow += rowSize;
}
}
First you need to understand how your data is coming... If it is already in cv::Mat you should be receiving two images, one for the RGB information that usually is a 3 channel uchar cv::Mat and another image for the depth information that usually it is saved in a 16 bit representation in milimeters (you can not save float mat as images, but you can as yml/xml files using opencv).
Assuming you want to read and process the image that contains the depth information, you can change your code to:
void CDifodoCamera::loadFrame()
{
//Read the newest frame
//the depth image should be png since it is the one which supports 16 bits and it must have the ANYDEPTH flag
cv::Mat depth_im = cv::imread("img_name.png",CV_LOAD_IMAGE_ANYDEPTH);
const int height = depth_im.rows;
const int width = depth_im.cols;
for (int y = 0; y < height; y++)
{
for (int x = 0; x < width; x++)
{
if (depth_im<unsigned short>(y,x) < 4500)
depth_wf(y,x) = 0.001f * (float)depth_im<unsigned short>(y,x);
else
depth_wf(y,x) = 0.f;
}
}
}
I hope this helps you. If you have any question just ask :)
I have searched internet and stackoverflow thoroughly, but I haven't found answer to my question:
How can I get/set (both) RGB value of certain (given by x,y coordinates) pixel in OpenCV? What's important-I'm writing in C++, the image is stored in cv::Mat variable. I know there is an IplImage() operator, but IplImage is not very comfortable in use-as far as I know it comes from C API.
Yes, I'm aware that there was already this Pixel access in OpenCV 2.2 thread, but it was only about black and white bitmaps.
EDIT:
Thank you very much for all your answers. I see there are many ways to get/set RGB value of pixel. I got one more idea from my close friend-thanks Benny! It's very simple and effective. I think it's a matter of taste which one you choose.
Mat image;
(...)
Point3_<uchar>* p = image.ptr<Point3_<uchar> >(y,x);
And then you can read/write RGB values with:
p->x //B
p->y //G
p->z //R
Try the following:
cv::Mat image = ...do some stuff...;
image.at<cv::Vec3b>(y,x); gives you the RGB (it might be ordered as BGR) vector of type cv::Vec3b
image.at<cv::Vec3b>(y,x)[0] = newval[0];
image.at<cv::Vec3b>(y,x)[1] = newval[1];
image.at<cv::Vec3b>(y,x)[2] = newval[2];
The low-level way would be to access the matrix data directly. In an RGB image (which I believe OpenCV typically stores as BGR), and assuming your cv::Mat variable is called frame, you could get the blue value at location (x, y) (from the top left) this way:
frame.data[frame.channels()*(frame.cols*y + x)];
Likewise, to get B, G, and R:
uchar b = frame.data[frame.channels()*(frame.cols*y + x) + 0];
uchar g = frame.data[frame.channels()*(frame.cols*y + x) + 1];
uchar r = frame.data[frame.channels()*(frame.cols*y + x) + 2];
Note that this code assumes the stride is equal to the width of the image.
A piece of code is easier for people who have such problem. I share my code and you can use it directly. Please note that OpenCV store pixels as BGR.
cv::Mat vImage_;
if(src_)
{
cv::Vec3f vec_;
for(int i = 0; i < vHeight_; i++)
for(int j = 0; j < vWidth_; j++)
{
vec_ = cv::Vec3f((*src_)[0]/255.0, (*src_)[1]/255.0, (*src_)[2]/255.0);//Please note that OpenCV store pixels as BGR.
vImage_.at<cv::Vec3f>(vHeight_-1-i, j) = vec_;
++src_;
}
}
if(! vImage_.data ) // Check for invalid input
printf("failed to read image by OpenCV.");
else
{
cv::namedWindow( windowName_, CV_WINDOW_AUTOSIZE);
cv::imshow( windowName_, vImage_); // Show the image.
}
The current version allows the cv::Mat::at function to handle 3 dimensions. So for a Mat object m, m.at<uchar>(0,0,0) should work.
uchar * value = img2.data; //Pointer to the first pixel data ,it's return array in all values
int r = 2;
for (size_t i = 0; i < img2.cols* (img2.rows * img2.channels()); i++)
{
if (r > 2) r = 0;
if (r == 0) value[i] = 0;
if (r == 1)value[i] = 0;
if (r == 2)value[i] = 255;
r++;
}
const double pi = boost::math::constants::pi<double>();
cv::Mat distance2ellipse(cv::Mat image, cv::RotatedRect ellipse){
float distance = 2.0f;
float angle = ellipse.angle;
cv::Point ellipse_center = ellipse.center;
float major_axis = ellipse.size.width/2;
float minor_axis = ellipse.size.height/2;
cv::Point pixel;
float a,b,c,d;
for(int x = 0; x < image.cols; x++)
{
for(int y = 0; y < image.rows; y++)
{
auto u = cos(angle*pi/180)*(x-ellipse_center.x) + sin(angle*pi/180)*(y-ellipse_center.y);
auto v = -sin(angle*pi/180)*(x-ellipse_center.x) + cos(angle*pi/180)*(y-ellipse_center.y);
distance = (u/major_axis)*(u/major_axis) + (v/minor_axis)*(v/minor_axis);
if(distance<=1)
{
image.at<cv::Vec3b>(y,x)[1] = 255;
}
}
}
return image;
}
I am trying to take the imageData of image in this where w= width of image and h = height of image
for (int i = x; i < x+h; i++) //height of frame pixels
{
for (int j = y; j < y+w; j++)//width of frame pixels
{
int pos = i * w * Channels + j; //channels is 3 as rgb
// if any data exists
if (data->imageData[pos]>0) //Taking data (here is the problem how to take)
{
xPos += j;
yPos += i;
nPix++;
}
}
}
jeff7 gives you a link to a very old version of OpenCV. OpenCV 2.0 has a new C++ wrapper that is much better than the C++ wrapper mentioned in the link. I recommend that you read the C++ reference of OpenCV for information on how to access individual pixels.
Another thing to note is: you should have the outer loop being the loop in y-direction (vertical) and the inner loop be the loop in x-direction. OpenCV is in C/C++ and it stores the values in row major.
See good explanation here on multiple methods for accessing pixels in an IplImage in OpenCV.
From the code you've posted your problem lies in your position variable, you'd want something like int pos = i*w*Channels + j*Channels, then you can access the RGB pixels at
unsigned char r = data->imageData[pos];
unsigned char g = data->imageData[pos+1];
unsigned char b = data->imageData[pos+2];
(assuming RGB, but on some platforms I think it can be stored BGR).
uchar* colorImgPtr;
for(int i=0; i<colorImg->width; i++){
for(int j=0; j<colorImg->height; j++){
colorImgPtr = (uchar *)(colorImg->imageData) + (j*colorImg->widthStep + i-colorImg->nChannels)
for(int channel = 0; channel < colorImg->nChannels; channel++){
//colorImgPtr[channel] here you have each value for each pixel for each channel
}
}
}
There are quite a few methods to do this (the link provided by jeff7 is very useful).
My preferred method to access image data is the cvPtr2D method. You'll want something like:
for(int x = 0; x < width; ++x)
{
for(int y = 0; y < height; ++y)
{
uchar* ptr = cvPtr2D(img, y, x, NULL);
// blue channel can now be accessed with ptr[0]
// green channel can now be accessed with ptr[1]
// red channel can now be accessed with ptr[2]
}
}
(img is an IplImage* in the above code)
Not sure if this is the most efficient way of doing this etc. but I find it the easiest and simplest way of doing it.
You can find documentation for this method here.