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Hi Experts,

I would like to build the following sum (gradient calculation) of a 8 bit monochromatic image (bitmap) with a resolution of 640x480 in an effective way:

j=rows-1 i=cols-1

Sum Sum ((A[i][j]-A[i+1][j])*(A[i][j]-A[i+1][j]))+((A[i][j]-A[i][j+1])*(A[i][j]-A[i][j+1]))

j=0 i=0

where the first sum runs from j=0 to j=number of rows and the second sum from i=0 to i= number of columns and A[i][j] ist the pixel intensity at the position (i,j). I've created a mfc dialog and here is coding:

void CImageAppDlg::OnBnClickedButton()

{

HBITMAP hBmp= (HBITMAP)LoadImage(NULL,L"C:\\Notebook.bmp", IMAGE_BITMAP, 0, 0, LR_LOADFROMFILE);

// m_MyBmp ist der member variable associated to the picture control

m_MyBmp.SetBitmap( hBmp );

CBitmap* pcBmp= CBitmap::FromHandle( hBmp );

BITMAP rBmp;

int n= pcBmp->GetBitmap( &rBmp );

const int nWide= rBmp.bmWidth;

const int nHigh= rBmp.bmHeight;

BYTE abPixels[640][480];

CDC* pCDC= m_MyBmp.GetWindowDC();

int x,y;

int Summe;

Summe=0;

for ( y=0; y<(nHigh-1); y++ ){

for ( x=0; x<(nWide-1); x++ ){

COLORREF clrRGB= pCDC->GetPixel(x,y);

abPixels[x][y]= GetRValue( clrRGB );

Summe+=((abPixels[x][y]-abPixels[x+1][y])*(abPixels[x][y]-abPixels[x+1][y]))+

((abPixels[x][y]-abPixels[x][y+1])*(abPixels[x][y]-abPixels[x][y+1]));

}

}

CString s;

s.Format( L"Summe=%d", Summe );

MessageBox( s );

}

I tested the coding above with the attached file below and I'm not sure if the implementation is correct...

Thank you

Notebook.bmp

I would like to build the following sum (gradient calculation) of a 8 bit monochromatic image (bitmap) with a resolution of 640x480 in an effective way:

j=rows-1 i=cols-1

Sum Sum ((A[i][j]-A[i+1][j])*(A[i]

j=0 i=0

where the first sum runs from j=0 to j=number of rows and the second sum from i=0 to i= number of columns and A[i][j] ist the pixel intensity at the position (i,j). I've created a mfc dialog and here is coding:

void CImageAppDlg::OnBnClickedB

{

HBITMAP hBmp= (HBITMAP)LoadImage(NULL,L"

// m_MyBmp ist der member variable associated to the picture control

m_MyBmp.SetBitmap( hBmp );

CBitmap* pcBmp= CBitmap::FromHandle( hBmp );

BITMAP rBmp;

int n= pcBmp->GetBitmap( &rBmp );

const int nWide= rBmp.bmWidth;

const int nHigh= rBmp.bmHeight;

BYTE abPixels[640][480];

CDC* pCDC= m_MyBmp.GetWindowDC();

int x,y;

int Summe;

Summe=0;

for ( y=0; y<(nHigh-1); y++ ){

for ( x=0; x<(nWide-1); x++ ){

COLORREF clrRGB= pCDC->GetPixel(x,y);

abPixels[x][y]= GetRValue( clrRGB );

Summe+=((abPixels[x][y]-ab

((abPixels[x][y]-abPixels[

}

}

CString s;

s.Format( L"Summe=%d", Summe );

MessageBox( s );

}

I tested the coding above with the attached file below and I'm not sure if the implementation is correct...

Thank you

Notebook.bmp

abPixels[x][y]= GetRValue( clrRGB );

Summe+=((abPixels[x][y]-ab

((abPixels[x][y]-abPixels[

You seem to be looking at abPixels[x+1][y] and abPixels[x][y+1] before setting it

As this is a grayscale, you can work with any one of the RGB values (they are all the same). But when you set them back into the bitmap or into another bitmap, be sure to set R,G, and B to the same value.

So, if I've understood you mean I have to add something like the following right:

COLORREF clrRGB1= pCDC->GetPixel(x+1,y);

COLORREF clrRGB2= pCDC->GetPixel(x,y+1);

abPixels[x+1][y]= GetRValue(clrRGB1);

abPixels[x][y+1]= GetRValue(clrRGB2);

But be careful of boundary conditions.

as written you never take abPixels[x][nHigh-1]-abPix

or abPixels[nWide-1][y]-abPix

I don't know if that matters to anything

The image is 8 bits per pixel in grey scale. This is a byte lenght.

By calling GetRValue( clrRGB ), the program gets the red component of a grey image...

Also, clrRGB is supposed to be a DWORD, say 32 bits long.

The result is very close to the real one because most of the pixels are sampled anyway.

I'll look deeply later.

Jose

+

(abPixels[x][y]-abPixels[x

In words, the gradient calculation is:

The difference between this pixel and the one to the right, squared

The difference between this pixel and the one below, squared

Let's look at some examples. The focus pixel is pure white (255) and the neighbors are pure black (0)

[ ] [#]

[#]

((255-0)*(255-0)) + ((255-0)*(255-0))

255*255 + 255*255= 130050

The focus pixel is black (0) and the neighbors are white (255)

[#] [ ]

[ ]

((0-255)*(0-255)) + ((0-255)*(0-255))

-255*-255 + -255*-255= 130050

The focus pixel is grey (128) and the neighbors are white (255)

[::] [ ]

[ ]

((128-255)*(128-255)) + ((128-255)*(128-255))

-127*-127 + -127*-127= 32258

The focus pixel is grey (128) and the neighbors are grey (128)

[::] [::]

[::]

((128-128)*(128-128)) + ((128-128)*(128-128))

0*0 + 0*0= 0

So you will be summing up about 300,000 values ranging from 0 to 130,050. Most of the time, the slope will be much closer to 0, but in a worst case scenario (checkerboard), your Summe variable will be

For clarity, I'd use this sort of code in the inner loop:

int nThis= pCDC->GetRValue( GetPixel(x,y) );

int nRight= pCDC->GetRValue( GetPixel(x+1,y) );

int nBelow= pCDC->GetRValue( GetPixel(x,y+1) );

int nDeltaRight= nThis-nRight;

int nDeltaBelow= nThis-nBelow;

Summe += (nDeltaRight*nDeltaRight)+

It contains intermediate steps, but if you are worrying about efficiency, that is the least of your worries. The real time killer will be the three calls to GetPixel for each pixel. There are several ways to speed that up, including one complicated one, but we can worry about efficiency once the main algorithm is working.

-- Dan

Thank you about the hint. I didn't even think about that possibility...

I've checked the limits header (#include ) and found the following:

#define INT_MIN (-2147483647 - 1) /* minimum (signed) int value */

#define INT_MAX 2147483647 /* maximum (signed) int value */

So I could declare my Summe variable as double right?

>>The real time killer will be the three calls to GetPixel for each pixel. There are

how could that look like?

Andy

so a simple way do it could separate your function into two loops, one to fill abPixels , and one that access that array .

A different approach, since the sums of vertical and horizontal differences are independent.

could be to run one loop to sum all the vertical differences and one loop to sum all the horizontal differences.

each of which would only need to save a single GetRValue(), so you wouldn't need a 2-D array

But I agree that you should get the code working first.

Until you understand how to do that, you probably won't understand how to do it without the array

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As to the alternative, faster ways to do this, they are (in brief):

1) create a 2-D array with the GetRValue() responses for every pixel, then access that array rather than doing GetPixel. That way, you would need to use GetPixel() only once per pixel. It would still be slow, but it will take about 1/3rd of the time.

2) Use CDC.GetBitMap, then CBitmap.GetBitmapBits that gives you a buffer full of pixel data. You can now write specialized code to access individual values -- in effect providing your own GetPixel functions that will probably be much faster than making hundreds of thousands of call to the GetPixel() API.

I'd rather not move in that direction until you have working code using the simpler GetPixel code. I see that you are rather new ro programming and I know that adding such complications will only make your task more difficult, and if GetPixel is fast enough, then there is no need to go there. There is a well-know developer's maxim: Don't start optimizing until you actully need to.