Count no of pixels inside a CRgn object

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How to count number of pixels inside a CRgn object?
One method is to get the bounding rect of the region and then checking for each pixel in the bounding rect whether it lies inside the CRgn or not.
This method takes more time and it depends on the size of CRgn object.
Is there any other method to calculate the number of pixel inside CRgn object which takes less time and which is not dependent on the size of CRgn object?
Thanks in advance.

Posted 21-Jun-11 20:55pm

Roger Gonsalves

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5 solutions

Solution 5

I did ask this question because errors of one pixel on width or height have a great impact.

For instance, in the case of a polygon with oblique sides, all returned rectangles can have a height of 1 row. If you compute 0 instead of 1, the total area will be 0; if you compute 2 instead of 1, you'll get the double area !

Posted 26-Jun-11 20:18pm

YDaoust

Updated 30-Jun-11 6:16am

v2

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Roger Gonsalves 28-Jun-11 0:44am

Your concern about impact of one pixel is right.
I have tried to print the rect information in the region and found that there is not a single case where top and bottom (or left and right) will be same.
If you came across any such scenario, can you please upload the code?

YDaoust 29-Jun-11 11:44am

This is a good sign. If you use a shape with non-vertical sides (ellipse), you should see a number rectangles with top and bottom differing by one, which confirms your formula.

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Roger Gonsalves · Accepted Answer · 2011-06-22T19:27:00

Below is the code for calculating the number of pixel inside CRgn

C#

int nNumberOfPixel = 0;
int nSize = m_pROIRgn->GetRegionData(NULL, NULL);
if(nSize)
{
    RGNDATA * pRegion = (RGNDATA *) new char[nSize];
    m_pROIRgn->GetRegionData(pRegion, nSize);
    const RECT* pRect = (const RECT *) & pRegion->Buffer;
    int rectcount = pRegion->rdh.nCount;
    for(int i = 0; i < rectcount; i++)
    {
        nNumberOfPixel = nNumberOfPixel + ((pRect[ i ].right - pRect[ i ].left) * (pRect[ i ].bottom - pRect[ i ].top));
    }
    delete [] (char *) pRegion;
}

Thanks CPallini and YDaoust for help.

CPallini · Accepted Answer · 2011-06-21T21:20:00

Solution 1

You may call GetRegionData[^] method and then use the RGDNDATAHEADER[^] and RGNDATA[^] info to compute the number of pixel in the rectangles composing the region (just a guess, I didn't test it).

Posted 21-Jun-11 21:20pm

CPallini

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Roger Gonsalves 22-Jun-11 4:50am

Thanks for quick reply.
Could you please explain it with example?

CPallini 22-Jun-11 4:56am

That would be the grunt work (and it is up to you). You should extract info of all the rectangles from the struct.

YDaoust · Accepted Answer · 2011-06-22T10:04:00

Solution 2

I support what CPallini says. You can retrieve a region as a decomposition into rectangles. The total area is the sum of width x height for all these rectangles.

This will be very fast compared to querying every pixel. (Time proportional to the number of rows, or even less if the regions are simple.)

Posted 22-Jun-11 10:04am

YDaoust

Updated 22-Jun-11 10:23am

v2

YDaoust · Accepted Answer · 2011-06-23T03:21:00

Solution 4

Should it be right - left or right - left + 1 ? I mean, is right the last or afterlast abscissa ? (same about bottom)

Posted 23-Jun-11 3:21am

YDaoust

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Roger Gonsalves 27-Jun-11 1:37am

Your concern is right.
In my situation I want to ignore the bottom and right side (by 1 pixel) of the region.

Count no of pixels inside a CRgn object

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