C / C++ / MFC

Re: Facial Recognition in C++, from scratch

3-Apr-20 1:59

See Viola-Jones - Google Search[^]

_Flaviu5-Apr-20 0:42

_Flaviu

5-Apr-20 0:42

A way to accomplish this is use the source code from a library that do that, which is huge work Smile | :)

... even is a school work, I guess it can be used OpenCV directly, most of them are free.

inverse sinus, floats

Calin Negru2-Apr-20 21:25

2-Apr-20 21:25

I have a tough time finding the inverse sinus math function in c++. There is asinf() but I`m not sure about what it does. [fixed] Displaying a float in StringCchPrintfA

modified 3-Apr-20 4:00am.

Richard MacCutchan2-Apr-20 21:35

2-Apr-20 21:35

It's all in the documentation: Format Specification Syntax: printf and wprintf Functions | Microsoft Docs[^].

Calin Negru2-Apr-20 21:53

2-Apr-20 21:53

found it thanks

CPallini2-Apr-20 21:57

2-Apr-20 21:57

The documentation of asin should be just enoough:

'(it) Computes the principal value of the arc sine of arg'

That is you get the first quadrant angle correnspondig to the sin value passed as argument.
The following program

C++

#include <cmath>
#include <iostream>
using namespace std;

int main()
{
  const size_t N = 18;
  const double delta = M_PI/N;

  for ( double phi = 0; phi < 2*M_PI; phi += delta)
  {
    double s = sin(phi);
    double a = asin(s);
    cout << "phi = " << phi << ", sin(phi) = " << s << ", asin(sin(phi)) = " << a << "\n";
  }
  cout << endl;
}

outputs

phi = 0, sin(phi) = 0, asin(sin(phi)) = 0
phi = 0.174533, sin(phi) = 0.173648, asin(sin(phi)) = 0.174533
phi = 0.349066, sin(phi) = 0.34202, asin(sin(phi)) = 0.349066
phi = 0.523599, sin(phi) = 0.5, asin(sin(phi)) = 0.523599
phi = 0.698132, sin(phi) = 0.642788, asin(sin(phi)) = 0.698132
phi = 0.872665, sin(phi) = 0.766044, asin(sin(phi)) = 0.872665
phi = 1.0472, sin(phi) = 0.866025, asin(sin(phi)) = 1.0472
phi = 1.22173, sin(phi) = 0.939693, asin(sin(phi)) = 1.22173
phi = 1.39626, sin(phi) = 0.984808, asin(sin(phi)) = 1.39626
phi = 1.5708, sin(phi) = 1, asin(sin(phi)) = 1.5708
phi = 1.74533, sin(phi) = 0.984808, asin(sin(phi)) = 1.39626
phi = 1.91986, sin(phi) = 0.939693, asin(sin(phi)) = 1.22173
phi = 2.0944, sin(phi) = 0.866025, asin(sin(phi)) = 1.0472
phi = 2.26893, sin(phi) = 0.766044, asin(sin(phi)) = 0.872665
phi = 2.44346, sin(phi) = 0.642788, asin(sin(phi)) = 0.698132
phi = 2.61799, sin(phi) = 0.5, asin(sin(phi)) = 0.523599
phi = 2.79253, sin(phi) = 0.34202, asin(sin(phi)) = 0.349066
phi = 2.96706, sin(phi) = 0.173648, asin(sin(phi)) = 0.174533
phi = 3.14159, sin(phi) = 1.45473e-15, asin(sin(phi)) = 1.45473e-15
phi = 3.31613, sin(phi) = -0.173648, asin(sin(phi)) = -0.174533
phi = 3.49066, sin(phi) = -0.34202, asin(sin(phi)) = -0.349066
phi = 3.66519, sin(phi) = -0.5, asin(sin(phi)) = -0.523599
phi = 3.83972, sin(phi) = -0.642788, asin(sin(phi)) = -0.698132
phi = 4.01426, sin(phi) = -0.766044, asin(sin(phi)) = -0.872665
phi = 4.18879, sin(phi) = -0.866025, asin(sin(phi)) = -1.0472
phi = 4.36332, sin(phi) = -0.939693, asin(sin(phi)) = -1.22173
phi = 4.53786, sin(phi) = -0.984808, asin(sin(phi)) = -1.39626
phi = 4.71239, sin(phi) = -1, asin(sin(phi)) = -1.5708
phi = 4.88692, sin(phi) = -0.984808, asin(sin(phi)) = -1.39626
phi = 5.06145, sin(phi) = -0.939693, asin(sin(phi)) = -1.22173
phi = 5.23599, sin(phi) = -0.866025, asin(sin(phi)) = -1.0472
phi = 5.41052, sin(phi) = -0.766044, asin(sin(phi)) = -0.872665
phi = 5.58505, sin(phi) = -0.642788, asin(sin(phi)) = -0.698132
phi = 5.75959, sin(phi) = -0.5, asin(sin(phi)) = -0.523599
phi = 5.93412, sin(phi) = -0.34202, asin(sin(phi)) = -0.349066
phi = 6.10865, sin(phi) = -0.173648, asin(sin(phi)) = -0.174533
phi = 6.28319, sin(phi) = -4.68582e-15, asin(sin(phi)) = -4.68582e-15

Calin Negru2-Apr-20 23:01

2-Apr-20 23:01

I`m doing something wrong. In a right triangle the hypotenuse length is 80.62 the triangle side length opposing the angle is 40. The windows calculator result for inverse sin 40/80.6225 is 29.74 When I do the math with asinf() I`m getting 0.519

CPallini2-Apr-20 23:18

2-Apr-20 23:18

Post your code, then.

The following program

C++

#include <cmath>
#include <iostream>
using namespace std;

int main()
{
  double angle = asin( 40/80.6225 );
  cout << "angle " << angle << " (" << (angle * 180 / M_PI) << " degrees)" << endl;
}

Outputs

angle 0.519147 (29.7449 degrees)

Calin Negru2-Apr-20 23:36

2-Apr-20 23:36

after conversion the result is as it should be , thanks

CPallini3-Apr-20 2:17

3-Apr-20 2:17

You are welcome.

Victor Nijegorodov2-Apr-20 23:39

Victor Nijegorodov

2-Apr-20 23:39

You probably mix degrees with radians.

Calin Negru3-Apr-20 0:02

3-Apr-20 0:02

Math is tricky

Calin Negru6-Apr-20 5:53

6-Apr-20 5:53

I`m working on several things at the same time. Don`t go wandering if I`m active simultaneously in two threads.

Quote:
That is you get the first quadrant angle correnspondig to the sin value passed as argument.

What`s the formula when you want work with remaining quadrants? This formula doesn`t work always.

CPallini6-Apr-20 11:12

6-Apr-20 11:12

You have to use the atan2[^] function.

Calin Negru6-Apr-20 20:55

6-Apr-20 20:55

Thanks CPallini

CPallini6-Apr-20 22:29

6-Apr-20 22:29

You are welcome.

Calin Negru12-Apr-20 6:38

Not able to set DPI for PNG files - libpng

12-Apr-20 6:38

I have my object rotation algorithm working though it will only rotate 180 degrees atm. It`s an algorithm that handles object rotations without matrices.

modified 12-Apr-20 12:58pm.

Gopi Nath2-Apr-20 18:23

Gopi Nath

2-Apr-20 18:23

Hello Everyone,

I am using libpng library for generating PNG files which is working fine. But it always generating with DPI 96 by default.

I tried using png_set_pHYs for setting the resolution before calling png_write_png. But its not generating with the specified DPI.

Before and after calling png_set_pHYs, i am checking the DPI using png_get_pHYs, it was showing the correct values whatever i set.

What may be the issue? And is that possible to set DPI for PNG file?

Please suggest.

Regards,
Gopinath.

Re: Not able to set DPI for PNG files - libpng

Richard MacCutchan2-Apr-20 21:33

Re: Not able to set DPI for PNG files - libpng

2-Apr-20 21:33

Gopi Nath wrote:
What may be the issue?

Without the source code of the library it is impossible to guess.

Gopi Nath2-Apr-20 22:08

Gopi Nath

2-Apr-20 22:08

That's the problem. they didn't provide source code nowhere.

Re: Not able to set DPI for PNG files - libpng

Richard MacCutchan2-Apr-20 22:11