I am going to detect a yellow color object when i open up my System CAM using Open CV programming, i got some help from the tutorial Object Recognition in Open CV but i am not clear about this line of code, what it does, i don't know. please elaborate me on the below line of code, which i am using.
cvInRangeS(imgHSV, cvScalar(20, 100, 100), cvScalar(30, 255, 255), imgThreshed);
other part of program:
CvMoments *moments = (CvMoments*)malloc(sizeof(CvMoments));
cvMoments(imgYellowThresh, moments, 1);
// The actual moment values
double moment10 = cvGetSpatialMoment(moments, 1, 0);
double moment01 = cvGetSpatialMoment(moments, 0, 1);
double area = cvGetCentralMoment(moments, 0, 0);
What about reading documentation?
inRange:
Checks if array elements lie between the elements of two other arrays.
And actually that article contains clear explanation:
And the two cvScalars represent the lower and upper bound of values
that are yellowish in colour.
About second code. From that calculations author finds center of object and its square. Quote from article:
You first allocate memory to the moments structure, and then you
calculate the various moments. And then using the moments structure,
you calculate the two first order moments (moment10 and moment01) and
the zeroth order moment (area).
Dividing moment10 by area gives the X coordinate of the yellow ball,
and similarly, dividing moment01 by area gives the Y coordinate.
Related
So I'm working on a project that involves taking pre-existing skeleton code of an application that simulates "fluid flow and visualization" and applying different visualization techniques on it.
The first step of the project is to apply different color-mapping techniques on three different data-sets which are as follows: fluid density (rho), fluid velocity magnitude (||v||) and the force field magnitude ||f||.
The skeleton code provided already has an example that I can study to be able to determine how best to design and implement different color-maps such as red-to-white or blue-to-yellow or what have you.
The snippet of code I'm trying to understand is the following:
//rainbow: Implements a color palette, mapping the scalar 'value' to a rainbow color RGB
void rainbow(float value,float* R,float* G,float* B)
{
const float dx=0.8;
if (value<0) value=0; if (value>1) value=1;
value = (6-2*dx)*value+dx;
*R = max(0.0,(3-fabs(value-4)-fabs(value-5))/2);
*G = max(0.0,(4-fabs(value-2)-fabs(value-4))/2);
*B = max(0.0,(3-fabs(value-1)-fabs(value-2))/2);
}
The float valuebeing passed by the first parameter is, as far as I can tell, the fluid density. I've determined this by studying these two snippets.
//set_colormap: Sets three different types of colormaps
void set_colormap(float vy)
{
float R,G,B;
if (scalar_col==COLOR_BLACKWHITE)
R = G = B = vy;
else if (scalar_col==COLOR_RAINBOW)
rainbow(vy,&R,&G,&B);
else if (scalar_col==COLOR_BANDS)
{
const int NLEVELS = 7;
vy *= NLEVELS; vy = (int)(vy); vy/= NLEVELS;
rainbow(vy,&R,&G,&B);
}
glColor3f(R,G,B);
}
and
set_colormap(rho[idx0]);
glVertex2f(px0, py0);
set_colormap(rho[idx1]);
glVertex2f(px1, py1);
set_colormap(rho[idx2]);
glVertex2f(px2, py2);
set_colormap(rho[idx0]);
glVertex2f(px0, py0);
set_colormap(rho[idx2]);
glVertex2f(px2, py2);
set_colormap(rho[idx3]);
glVertex2f(px3, py3);
With all of this said, could somebody please explain to me how the first method works?
Here's the output when the method is invoked by the user and matter is injected into the window by means of using the cursor:
Whereas otherwise it would look like this (gray-scale):
I suspect that this is a variation of HSV to RGB.
The idea is that you can map your fluid density (on a linear scale) to the Hue parameter of a color in HSV format. Saturation and Value can just maintain constant value of 1. Normally Hue starts and ends at red, so you also want to shift your Hue values into the [red, blue] range. This will give you a "heatmap" of colors in HSV format depending on the fluid density, which you then have to map to RGB in the shader.
Because some of your values can be kept constant and because you don't care about any of the intermediate results, the algorithm that transforms fluid density to RGB can be simplified to the snippet above.
I'm not sure which part of the function you don't understand, so let me explain this line by line:
void rainbow(float value,float* R,float* G,float* B){
}
This part is probably clear to you - the function takes in a single density/color value and outputs a rainbow color in rgb space.
const float dx=0.8;
Next, the constant dx is initialised. I'm not sure what the name "dx" stands for, but looks like it's later used to determine which part of the color spectrum is used.
if (value<0) value=0; if (value>1) value=1;
This clamps the input to a value between 0 and 1.
value = (6-2*dx)*value+dx;
This maps the input to a value between dx and 6-dx.
*R = max(0.0,(3-fabs(value-4)-fabs(value-5))/2);
This is probably the most complicated part. If value is smaller than 4, this simplifies to max(0.0,(2*value-6)/2) or max(0.0,value-3). This means that if value is less than 3, the red output will be 0, and if it is between 3 and 4, it will be value-3.
If value is between 4 and 5, this line instead simplifies to max(0.0,(3-(value-4)-(5-value))/2) which is equal to 1. So if value is between 4 and 5, the red output will be 1.
Lastly, if value is greater than 5, this line simplifies to max(0.0,(12-2*value)/2) or just 6-value.
So the output R is 1 when value is between 4 and 5, 0 when value is smaller than 3, and something in between otherwise. The calculations for the green and blue output or pretty much the same, just with tweaked value; green is brightest for values between 2 and 4, and blue is brightest for values between 1 and 2. This way the output forms a smooth rainbow color spectrum.
For some reason, I need to know the color of the vertices of an object. The way I can think of is to render the vertex to the screen and then call glReadPixels to fetch the color of the vertices from screen space.
My program is implemented as follows:
Render the ith vertex:
glPointSize(8.0);
glDrawArrays(GL_POINTS, i, 1);
Compute the screen coordinates of this vertex:
oPos[3] = 1.0
// assume the object space coordinates of the vertex is oPos.
multiply oPos by the model-view-projection matrix to get the normalized device coordinates of this vertex, denoted as ndcPos;
ndcPos[1~3] /= ndcPos[3]
finally, multiply ndcPos with the viewport matrix to get the screen coordinates, denoted as screenPos. The viewport matrix is defined as:
GLfloat viewportMat[] = {
screen_width/2, 0, 0, 0,
0, screen_height/2, 0, 0,
0, 0, 1, 0,
(screen_width-1)/2.0, (screen_height-1)/2.0, 0, 1};
Finally, call glReadPixels as:
glReadPixels(int(screenPos[0]+0.5), int(screenPos[1]+0.5),
1, 1, GL_RGB, GL_FLOAT, currentColor);
The resulting color will then be stored at currentColor, which is a vector of length three.
My questions are:
Is there any better ways to get the vertex color rather than query them from screen space?
Any ideas about the correctness of my second and third step?
OpenGL transform feedback allows a shader to write arbitrary values into a buffer object, which can then be read by the host program. You can use this to make your vertex shader pass the computed vertex colors back to the host program.
The nice thing about transform feedback is that you can use it while drawing to the screen, so you can draw your geometry and capture the vertex colors in a single pass. Or, if you prefer, you can draw the geometry with rasterization turned off to capture the feedback data without touching the screen.
Since the data produced by transform feedback is stored in a buffer object, you can use it as input for other drawing operations, too. Depending on what you plan to do with the computed vertex colors, you may be able to avoid transferring them back to the host program at all.
I am a beginner programmer that is working on elementary tests for myself to grasp core values of working with C. I have a test case and don't really know where to begin in structuring it for compiling with GCC. I have a basic theory and pseudocode summary but mainly needing a little help stepping forward.
I have googled the related questions and permutations to this question and have been unable to make heads or tails of the logic for C.
Given the following logic:
Using the C language, have the function OverlappingRectangles(strArr)
read the strArr parameter being passed which will represent two
rectangles on a Cartesian coordinate plane and will contain 8
coordinates with the first 4 making up rectangle 1 and the last 4
making up rectange 2. It will be in the following format:
"(0,0),(2,2),(2,0),(0,2),(1,0),(1,2),(6,0),(6,2)." Your program should
determine the area of the space where the two rectangles overlap, and
then output the number of times this overlapping region can fit into
the first rectangle. For the above example, the overlapping region
makes up a rectangle of area 2, and the first rectangle (the first 4
coordinates) makes up a rectangle of area 4, so your program should
output 2. The coordinates will all be integers. If there's no overlap
between the two rectangles return 0.
I'm lost.
Should have added this at first:
Given a string(n1,n2,n3,n4,m1,m2,m3,m4)
Split string into string1(n1,n2,n3,n4) string2(m1,m2,m3,m4)
If n1+n4 < m1 or n2+n3 < m2 or m1+m4 < n1 or m2+m3 < m1
Calculate area of intersecting rectangle and divide into area of first rectangle.
Else
Print 0
You have a string of the form:
(x1,y1)(x2,y2)(x2,y1)(x1,y2)(x3,y3)(x4,y4)(x4,y3)(x3,y4)
defining 2 rectangles:
r1 = (x1,y1) to (x2,y2)
r2 = (x3,y3) to (x4,y4)
You need to first:
define a representation (structure) for the rectangles
parse (read) the string to extract the numbers for x1-x4 and y1-y4 -- look at e.g. sscanf and its return value for doing this
You can create a helper function, e.g.:
const char *parse_rectangle(const char *str, rectangle *r);
That will read a rectangle r from str in the form (x1,y1)(x2,y2)(x2,y1)(x1,y2)(x3,y3) (including any validation) and return a pointer to the next character.
Now, you will have two rectangles.
You can then compute the intersection of these rectangles as a third rectangle, e.g.:
int intersection(const rectangle *r1, const rectangle *r2, rectangle *result);
which will return 1 if the rectangles intersect, or 0 if they don't and fill result with the intersection. If you are using C99, you can use _Bool instead.
Now, you need a function to compute the area, e.g.:
int area(const rectangle *r);
You can pass this through the intersected rectangle and the first rectangle to get the areas of both.
Now, you simply divide the first rectangle area by the intersected rectangle area and print the result.
In the book learning opencv there's a question in chapter 3 :
Create a two dimensional matrix with three channels of type byte with data size 100-by-100 and initialize all the values to 0.
Use the pointer element to access cvptr2D to point to the middle 'green' channel.Draw the rectangle between 20,5 and 40,20.
I've managed to do the first part, but I can't get my head around second part. Here's what I've done so far :
/*
Create a two dimensional matrix with three channels of type byte with data size 100- by-100 and initialize all the values to 0.
Use the pointer element to access cvptr2D to point to the middle 'green' channel.Draw `enter code here`the rectangle between 20,5 and 40,20.
*/
void ex10_question3(){
CvMat* m = cvCreateMat(100,100,CV_8UC3);
CvSetZero(m); // initialize to 0.
uchar* ptr = cvPtr2D(m,0,1); // if RGB, then start from first RGB pair, Green.
cvAdd(m,r);
cvRect r(20,5,20,15);
//cvptr2d returns a pointer to a particular row element.
}
I was considering adding both the rect and matrix, but obviously that won't work because a rect is just co-ordinates, and width/height. I'm unfamiliar with cvPtr2D(). How can I visualise what the exercise wants me to do and can anyone give me a hint in the right direction? The solution must be in C.
From my understanding with interleaved RGB channels the 2nd channel will always be the channel of interest. (array index : 1,4,6..)
So that's the direction where the winds blow from...
First of all, the problem is the C API. This API is still present for legacy reasons, but will soon become obsolete. If you are serious about OpenCV please refer to C++ API. The official tutorials are great source of information.
To further target your question, this would be implementation of your question in C++.
cv::Mat canvas = cv::Mat::zero(100,100, CV_8UC3); // create matrix of bytes, filled with 0
std::vector<cv::Mat> channels(3); // prepare storage for splitting
split(canvas, channels); // split matrix to single channels
rectangle(channels[1], ...); // draw rectangle [I don't remember exact params]
merge(channels, canvas); // merge the channels together
If you only need to draw green rectangle, it's actually much easier:
cv::Mat canvas = cv::Mat::zero(100,100, CV_8UC3); // create matrix of bytes, filled with 0
rectangle(canvas, ..., Scalar(0,255,0)); // draw green rectangle
Edit:
To find out how to access single pixels in image using C++ API please refer to this answer:
https://stackoverflow.com/a/8139210/892914
Try this code:
cout<<"Chapter 3. Task 3."<<'\n';
CvMat *Mat=cvCreateMat(100, 100, CV_8UC3);
cvZero(Mat);
for(int J=5; J<=20; J++)
for(int I=20; I<40; I++)
(*(cvPtr2D(Mat, J, I)+1))=(uchar)(255);
cvNamedWindow("Chapter 3. Task 3", CV_WINDOW_FREERATIO);
cvShowImage("Chapter 3. Task 3", Mat);
cvWaitKey(0);
cvReleaseMat(&Mat);
cvDestroyAllWindows;
I have got a four 2d vertices A B C D of rotated rectangle,
I need to rasterize/draw it (efficiently) in pixelbufer
with setpixel(x,y,color)
how to do it?
i was trying with some code like
// convertilg a b c d do up down left right,
// calculating some dx_left dx_right on y--
// etc (frustrating on special cases when there are 2 up_y vertices in same line etc)
for(;;)
{
drawhorizontalline(y, xstart, xend, color);
if(y==downy) break;
y--;
xstart+=dxstart;
xend+=dxend;
if(y==lefty) dxstart = dxright;
if(y==righty) dxend = dxleft;
}
but it is most frustrating (terribly bug prone and most frustrating)
i am really tired of debuging this all day yesterday and i need to find
maybe some working code rather than to try to debug this
To fill your rectangle handle it as closed convex polygon (almost the same as triangle filling)
order your points to match winding rule
so there are lines AB BC CD DA or reverse
create left and right buffer
address is y-coordinate, its an array of x-positions and if needed also array of color,texture coordinates,.... for starters:
int buf_x0[ys],buf_x1[ys];
where ys is screen y-resolution
implement any draw line algorithm
but instead of draw to screen just store x coordinate of pixel to buffer.
instead of: setpixel(x,y,color); do: buf_x?[y]=x;.
Which buffer is the destination depends on the line Y direction
if dy<0 then fill buff_x0
if dy>0 then fill buff_x1
if dy==0 then buf_x0[y]=min(x) and buf_x1[y]=max(x)
Beware you have to sort the line endpoints by x coordinate before rasterizing to avoid seams/holes in meshes caused by different pixels produced for reversed endpoints lines.
apply this line algorithm to all border lines of polygon (AB,BC,CD,DA)
after this the buffers contains start and end x-positions of your horizontal lines
fill the rectangle on screen
for (y=min(Ay,By,Cy,Dy);y<=max(Ay,By,Cy,Dy);y++)
draw_horizontal_line(y,buf_x0[y],buf_x1[y],color);
Image for clarity (taken from my lectures on low level computer graphics)
image description:
vertical rectangles represents the border buffers buf_x0[],buf_x1[]
clockwise winding rule ensures the destination buffer. If its coded properly than buf_x0[y] <= buf_x1[y] so draw of horizontal line colapses to single for loop
Also se these:
simple 2D raster C++ example of mine
simple 3D voxel grid C++ example of mine