how to convert coloured image to grayscale in C or C++

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how to convert coloured image to grayscale in C or C++ ?
All the threads present here give solution in C#

Posted 9-Feb-12 1:14am

Amritanshu Pandia

Updated 20-Apr-20 16:56pm

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3 solutions

Solution 3

The "exact" colour weightings are:
0.299083 0.585841 0.114076

Posted 20-Apr-20 16:57pm

Member 13820371

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Eugen Podsypalnikov · Accepted Answer · 2012-02-09T01:25:00

:) :

0. Any color can be presented as a 3D vector with coordinates (r, g, b)
1. For any w/b color is valid (r == g == b)
2. Any vector has a length
3. Any converted w/b vector has the same length to the original
4. Any pixel of an image has a color

See also[^]

C++

void convert(CImage* pcImage)
{
  ASSERT(pcImage);

  int iWidth(pcImage->GetWidth());
  int iHeight(pcImage->GetHeight());
  
  if (iWidth && iHeight) {
    for (int i = 0; i < iWidth; i++) {
      for (int j = 0; j < iHeight; j++) {
        COLORREF clrOriginal(pcImage->GetPixel(i, j));
        float fR(GET_R(clrOriginal));
        float fG(GET_G(clrOriginal));
        float fB(GET_B(clrOriginal));

        float fWB = sqrt((fR * fR + fG * fG + fB * fB) /3);
        pcImage->SetPixel(i, j, RGB(fWB, fWB, fWB));
      }
    }
  }
}

thatraja · Accepted Answer · 2012-02-09T01:36:00

Solution 2

Converting Colors to Gray Shades[^]

Convert RGB to Gray Scale without using Pointers[^](Couple of Tip/Tricks in C#)

Posted 9-Feb-12 1:36am

thatraja

Comments

Espen Harlinn 9-Feb-12 8:38am

5'ed!

JackDingler 9-Feb-12 11:29am

Note that every color element has a factor to determine how much input it should have in the final pixel value.

The following code though, begs for optimization. There is no need to use floating point math here.
byColor = ( GetRValue(cr) * 0.30 ) +
( GetGValue(cr) * 0.59 ) +
( GetBValue(cr) * 0.11 );

We can do this with all integers. We can multiply the factors by 1024, then do the math and shift by 10 to fake a divide by 1024...
byColor = (( (int) GetRValue(cr) * 307 ) +
( (int) GetGValue(cr) * 604 ) +
( (int) GetBValue(cr) * 113 )) >> 10;