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Course IP_MFC: Image processing with C++/MFC
Chapter C2: Comments to the Histogram Project


Copyright © by V. Miszalok, last update: 2011-03-19
In histo1Doc.h in front of class CHisto1Doc : public CDocument
#include < vector > //declares the dynamic arrays of the Standard Template Library STL. The blanks inside the < > clause around the word vector may be suppressed. They are just necessary in a HTML-Document otherwise HTML treats the word vector as HTML-Tag.
BITMAPFILEHEADER FH; //structure of overall length = 14 bytes containing 5 variables (see MSDN and C6_Bitmap_FAQs)
BYTE IBytes[1200]; //untyped space for 1200 bytes for BitmapInfoHeader+Palette. You don't know the types of BitmapInfoHeaders you will read and you don't know if there will be palettes or not. You reserve 1200 bytes for the worst case: BITMAPV5HEADER (=136 byte) plus 256 palette entries ( =1024 bytes). 1160 bytes are wasted in case of traditional 24-Bit-Bitmaps but you can probably afford that.
BITMAPINFOHEADER* pIH; //typed pointer allows the access to biSize, biWidth etc.
BITMAPINFO* pI; //This typed pointer is needed by function StretchDIBits. Structure BITMAPINFO (see MSDN and C6_Bitmap_FAQs) is longer than BITMAPINFOHEADER but it contains in its first part the structure BITMAPINFOHEADER completely . It allows access to both the BitmapInfoHeader and (if biClrUsed > 0) the palette. If biClrUsed > 0 then StretchDIBits automatically accesses and uses the palette with the help of this pointer.
std::vector< BYTE > Pixel; //name of a dyn. array of bytes aimed to store the original pixels
std::vector< BYTE > PixelBinary; //name of a dyn. array of bytes for pixels after thresholding and binarisation
int Histogram[256]; //space for 256 integers
In histo1Doc.cpp inside the constructor CBitmap1Doc() you have to initialize 3 variables:
memset( IBytes, 0, sizeof(IBytes) ); // clear the 1200 untyped bytes in order to mark the absence of an image. Function CHisto1View::OnDraw() reads from here if you already loaded an image or not.
pIH = (BITMAPINFOHEADER*) IBytes; //The typed pointer points now to the head of IBytes[1200]
pI = (BITMAPINFO*) IBytes; //The typed pointer points now to the head of IBytes[1200] as pIH does.
In histo1View.h you have to declare the following private variables of class CHisto1View : public CView
CRect histo_r; //rectangle (256x100) to draw the histogram in the right lower corner of the image
int threshold; //interactive threshold separating the black background from the white foreground
BOOL MouseFlag; //used to remember if a threshold has been choosen via the left mouse button
In histo1View.cpp inside the constructor CHisto1View() you must intialize a variable
MouseFlag = false; Indicates, that there is no threshold yet.
In histo1Doc.cpp inside Serialize(CArchive& ar) inside the else clause: Code for reading an image and for computing its histogram
ar.Read( & FH, sizeof(BITMAPFILEHEADER) ); //Read 14 bytes from the harddisk.
if ( FH.bfType != 'MB') { forget_it(); return; //The first 2 bytes form the reversed string of BM.
if ( FH.bfSize <= 54 ) { forget_it(); return; //A bitmap file contains at least 55 bytes.
if ( FH.bfOffBits < 54 ) { forget_it(); return; //The shortest possible headers need 54 bytes.
int nBytesInfo = FH.bfOffBits - sizeof(BITMAPFILEHEADER); //That is the space left for the BitmapInfoHeader and palette.
int nBytesPixel = FH.bfSize - FH.bfOffBits; //That is the space for all the pixels.
ar.Read( IBytes, nBytesInfo ); //Read BitmapInfoHeader+palette from harddisk
if ( !(pIH→biBitCount == 8 || pIH→biBitCount ==24) ) { forget_it(); return; } //Ignore all 1, 4, 16 and 32 bit bitmaps. ! = logical NOT and || = logical OR.
Pixel .resize( nBytesPixel ); //Keep free the necessary space for all the original pixels in main memory.
PixelBinary.resize( nBytesPixel ); //Double the space for storing the black and white pixels.
ar.Read( &Pixel.front(), nBytesPixel ); //Read all original pixels from the harddisk into the main memory starting at the first adress of the dynamic byte array named Pixel.
memset( Histogram, 0, sizeof(Histogram) ); //Clear the 256 histogram integers to zero values.
int sum, i, hmax = 0; //some local variables, one initialized by zero
std::vector< BYTE >::iterator pointer; //declaration of a pointer pointing arbitrarily into a dyn. byte array. Such special pointers are called iterators.
switch ( pIH→biBitCount ) //jump depending on biBitCount, which can be 8 or 24.
case 8: for ( pointer=Pixel.begin(); pointer < Pixel.end(); pointer++ ) //Loop through all pixels of a 8-bit-bitmap.
Histogram[ *pointer ]++; //Take the gray value (or the color index) and increment one of the counters of the array Histogram.
break; //This is a jump out of and beyond the end of the switch clause. If you forget this statement, the program continues with case 24 and computes a strange Histogram.
case 24: for ( pointer=Pixel.begin(); pointer < Pixel.end(); pointer+=3 ) //Loop through all pixels of a 24-bit-bitmap.
sum = *pointer + *(pointer+1) + *(pointer+2); //Add the 3 values for blue, green and red.
Histogram[ sum / 3 ]++; //Divide the sum by 3 and increment one of the counters of the array Histogram.
for ( i = 0; i < 256; i++ ) if ( Histogram[i] > hmax ) hmax = Histogram[i]; //Find out hmax = the most common gray value of the image = highest column of Histogram.
for ( i = 0; i < 256; i++ ) Histogram[i] = (100*Histogram[i])/hmax; //Scale hmax to 100 and all other columns of Histogram linearily.
In histo1Doc.cpp inside void CHisto1Doc::forget_it()() //Small private member function of CHisto1Doc
memset( IBytes, 0, sizeof(IBytes) ); //If this was not a reasonable bitmap, then clear all 1200 bytes of IBytes to zero.
for ( int i=0; i < 10; i++ ) MessageBeep(-1); //This is a loud protest against misuse.
In histo1View.cpp inside void CHisto1View::OnDraw(CDC* pDC) //
CHisto1Doc* pDoc = GetDocument(); //This line was prepared by Visual Studio. Keep it. It give us a pointer to reach the variables which have been declared in class CHisto1Doc
if ( !pDoc→pIH→biSize ) { pDC→TextOut(0,0,"Open a *.BMP file !"); return; } //Advice to users who do not know what they should do after they started the program.
BYTE * pointer; //Declares a local pointer.
if ( !MouseFlag ) pointer = &(pDoc→Pixel.front()); //If nobody presses the left mouse button upon the rectangle showing the histogram in the right lower orner of the image , set the pointer to the first pixel of the original image.
else pointer = &(pDoc→PixelBinary.front()); //If we have a threshold, set the pointer to first pixel of the binary image.
CRect R; GetClientRect( R ); //Find out the current dimensions of the client area that could be covered with the image.
StretchDIBits( pDC→GetSafeHdc(), //Draw the image into the graphics board and on the screen. The first parameter has to be a handle to the Device Context.
0, 0, R.Width(), R.Height(), //Parameters 2,3,4,5 of StretchDIBits indicate the rectangular destination dimensions.
0, 0, pDoc→pIH→biWidth, pDoc→pIH→biHeight, //Parameters 6,7,8,9 of StretchDIBits indicate the rectangular original source dimensions.
pointer, pDoc→pI, //Parameter 10 has to point to the first pixel and parameter 11 must be a typed pointer to a BitmapInfo-structure (BitmapInfoheader plus palette).
DIB_RGB_COLORS, SRCCOPY ); //Parameters 12 and 13 are constants which influence the use of colors and transparency (see MSDN).
histo_r.right = R.Width() - 10; //Right border of the histogram box near the right side of the image.
histo_r.left = histo_r.right - 256; //Left border is 256 pixels left of the right border.
histo_r.bottom = R.Height() - 10; //Bottom of the box near the bottom of the image.
histo_r.top = histo_r.bottom - 100; //Top of the box is 100 pixel above the bottom.
pDC→Rectangle( histo_r ); //Draw a white rectangle surrounded by a black border.
pDC→TextOut( histo_r.left+1, histo_r.top+1, "click and move here!" ); //Display this text inside the rectangle.
for ( int i = 0; i < 256; i++ ) //Draw one perpendicular black line for each of the 256 entries of Histogram
pDC→MoveTo( histo_r.left + i, histo_r.bottom ); //from bottom
pDC→LineTo( histo_r.left + i, histo_r.bottom - pDoc→Histogram[i] ); //in upward direction.
if ( MouseFlag ) //If somebody had pressed the left mouse button over the histogram box, you have to visualize the position of the interactive threshold.
pDC→MoveTo( histo_r.left + threshold, histo_r.top ); //Show the current threshold as black line from bottom
pDC→LineTo( histo_r.left + threshold, histo_r.bottom ); //to top
In void CHisto1View::OnLButtonDown(UINT nFlags, CPoint point) //The user presses the left mouse button.
if ( !histo_r.PtInRect( point ) ) return; //There is nothing to do, if the mouse is not over the histogram.
MouseFlag = true; //Inform OnDraw to display the binary image instead of the original one.
In void CHisto1View::OnMouseMove(UINT nFlags, CPoint point) //This function computes the current threshold and the current binary image and calls OnDraw via Invalidate()
if ( !nFlags ) return; //Do nothing if the user moves the mouse without pressing a button.
if ( !histo_r.PtInRect( point ) ) return; //Do nothing if the mouse is not over the Histogram.
CHisto1Doc* pDoc = GetDocument(); //You need a pointer to class CHisto1Doc in order to access the BitmapInfoHeader and the pixels.
if ( !pDoc→pIH→biSize ) return; //There is no image at all.
threshold = point.x - histo_r.left; //The threshold depends on the horizontal position relative to the left border of the histogram box.
std::vector< BYTE >::iterator pointer1 = pDoc→Pixel .begin(); //Pointer to the first pixel of the original image
std::vector< BYTE >::iterator pointer2 = pDoc→PixelBinary.begin(); //Pointer to the first pixel of the binary image
switch ( pDoc→pIH→biBitCount ) //Jump depending on biBitCount which may be 8 or 24.
case 8: for ( ; pointer1 < pDoc→Pixel.end(); pointer1++, pointer2++ ) //Go through all pixels of the 8-bit-image
if ( *pointer1 > threshold ) *pointer2 = 255; //If the grey value is above the threshold, make it completly white.
else *pointer2 = 0; //Otherwise set it black.
break; //Jump out of and beyond the end of the switch-clause.
case 24: for ( ; pointer1 < pDoc→Pixel.end(); pointer1+=3, pointer2+=3 ) //Go through all RGBTriples of the original image
int sum = *pointer1 + *(pointer1+1) + *(pointer1+2); //Add the red, green and blue values together.
if ( sum > 3*threshold ) *pointer2=*(pointer2+1)=*(pointer2+2)=255; //If the sum is greater than 3 thresholds, set the RGBTRIPLE to white,
else *pointer2=*(pointer2+1)=*(pointer2+2)= 0; //otherwise set it to black.
Invalidate( false ); //Ask the operating system to redraw the client area. It is not necassary to erase the former content.
In void CHisto1View::OnLButtonUp(UINT nFlags, CPoint point) //The user releases the mouse button.
MouseFlag = false; //No more thresholding and binary images.
Invalidate( false ); //Redraw the original image.
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