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Width of text in italic font

, 23 Aug 2006
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Shows an alternative to GetCharABCWidths and GetCharABCWidthsFloat.

Introduction

Our purpose is to measure accurately the width of text written in italic font.

Using the code

The GetTextExtentPoint32 Win32 API function or the DrawText Win32 API function with the DT_CALCRECT flag, will -not- tell us the correct width of text in italic font, only the calculated height is correct. For most of the italic fonts, using these two functions, the calculated width is too narrow and the displayed text looks right-trimmed.

We could try using the GetCharABCWidths or GetCharABCWidthsFloat Win32 API functions. These functions will give us information about the underhang and the overhang of each character of the text we're interested in.

Please check these MSDN articles for more details about using the underhang and overhang values:

This updated version of the EXE includes a new checkbox, to see how adding the 'overhang of the last character' will help getting a more accurate result.

However, in this article, I am suggesting another approach:

  1. Paint the text in black colour, in a memory device context filled with white colour, like this:
    SIZE sizeText;
    //Calculate the width of the text, by using the classic method
    GetTextExtentPoint32(hDCMem,szText,lstrlen(szText),&sizeText);
    
    //Calculate the width of the last character, as suggested by 'oupoi'
    SIZE sizeLastCharacter;
    GetTextExtentPoint32(hDCMem,&szText[-1+lstrlen(szText)],1,&sizeLastCharacter);
    
    //Set a bounding rectangle wide enough to fit the painted text
    RECT rect={0,0,sizeText.cx+sizeLastCharacter.cx,sizeText.cy};
    
    //Fill the background with white colour then paint the text in black colour
    FillRect(hDCMem,&rect,(HBRUSH)GetStockObject(WHITE_BRUSH));
    DrawText(hDCMem,szText,-1,&rect, 
             DT_LEFT|DT_TOP|DT_SINGLELINE|DT_NOPREFIX);
  2. Then, scan the colour of the pixels in the memory device context, from right-to-left, like this:
    int iXmax=0;
    BOOL bFound=FALSE;
    for(int x=rect.right-1; x>=0 && !bFound; x--)
    {
       for(int y=0; y<=rect.bottom-1 && !bFound; y++)
       {
          COLORREF rgbColor=GetPixel(hDCMem,x,y);
          if(rgbColor!=RGB(255,255,255))
          {
              //found a non-white pixel, save the horizontal position 
              //and exit the loops. Job finished.
              iXmax=x;
              bFound=TRUE;
          }
       }
    }
    
    //this is the width of the text painted in italic font!
    LONG lWidthOfText=iXmax+1;//+1 because we use 0-based indexes

A few comments:

  • Painted text can be narrower than text calculated with GetTextExtentPoint32. See, for example, Verdana 12 Italic.
  • In a dialog box, the edit controls have left/right and up/down margins to take into account. I have not used edit controls in this sample.
  • In a dialog box, the label controls seem to be +1 pixel wider, unless the SS_SIMPLE style is used. See the image below.

History

  • Version 1.0 [July 23, 2006] - Created.
  • Version 1.1 [August 6, 2006] - New checkbox to see how adding the 'overhang of the last character' to the classic method of calculating the text width will help getting a more accurate result.

    Like this:

    SIZE sizeText; 
    GetTextExtentPoint32(hDCMem, szText, lstrlen(szText), &sizeText);
    LONG lWidthOfText= sizeText.cx;
                        
    ABCFLOAT WidthsABC[256];
    GetCharABCWidthsFloat(hDCMem, 0, 255, WidthsABC); 
    
    // overhang of the last character
    double dOverhangTrailing = WidthsABC[szText[lstrlen(szText)-1]].abcfC;
    
    if(dOverhangTrailing<0)
    {
       //if the overhang is negative then adjust 
       //the calculated width of the text
       lWidthOfText-=dOverhangTrailing;
    }

    But as already mentioned above, this article is suggesting another method, which doesn't require using any of these underhang and overhang values.

  • Version 1.2 [August 17, 2006] - Adjustment to the right-to-left pixel colour scanning algorithm.

    Set the right-limit of the bounding rectangle to be scanned to just 'sizeText.cx+widthOfTheLastCharacter', as suggested by 'oupoi'.

    We now also have a new checkbox to test if using the Mihai Nita's trick will help getting a faster calculation and at the same time, of course, an accurate result.

    Unless I've made a mistake somewhere, I'm not satisfied by the precision of the result by using the Mihai Nita's trick.

    Like this:

    //fill with white
    FillRect(hDCMem,&rect,(HBRUSH)GetStockObject(WHITE_BRUSH));
    
    // Added this by Mihai Nita - August 17, 2006
    // trick, set text background colour to black, before painting!
    SetBkColor(hDCMem,RGB(0,0,0)); 
    
    //reality shows that DrawText fails to properly 
    //paint the surrounding rectangle (??)
    DrawText(hDCMem,szText,-1, &rect, 
             DT_LEFT|DT_TOP|DT_SINGLELINE|DT_NOPREFIX);
    
    int iXmax=0;
    BOOL bFound=FALSE;
    int iYmed=(rect.bottom+rect.top)/2; // middle
    
    for(int x=rect.right-1;x>=0 && !bFound ;x--)
    {
      COLORREF rgbColor=GetPixel(hDCMem,x,iYmed);
      if(rgbColor!=RGB(255,255,255))
      {
        iXmax=x;
        bFound=TRUE;
      }
    }

License

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About the Author

Alexandru Matei
Web Developer
Romania Romania
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Comments and Discussions

 
GeneralRe: It seems quite inefficient PinmemberMihai Nita20-Aug-06 11:40 
GeneralRe: It seems quite inefficient [modified] PinmemberAlexandru Matei20-Aug-06 19:58 
GeneralDT_CALCRECT and GetCharABCWidths() [modified] Pinmemberbob1697230-Jul-06 19:16 
GeneralRe: DT_CALCRECT and GetCharABCWidths() [modified] PinmemberAlexandru Matei30-Jul-06 22:38 
Yes, for Tahoma, your code certainly works very good.
For this font, the values of the ABC structure are:
 Tahoma 16, Italic
                     abcA abcB abcC  
for character 'J':    0   13    -4     
for character '6':    2   12    -3 
 
Given the text 'July 2006', the overhang of the last character('6'), consists of 3 pixels. By adding this amount in absolute value, you get a more accurate result.

However I think that care must be taken with the arithmetic of these values of the 'underhang of the first character' and 'overhang of the last character'.
 
I feel that if someone goes your way, it will be safer using:
rect.left-=abs(pWidthsABC[sOutput[0]].abcA); // always increase width (First character)
rect.right+=abs(pWidthsABC[sOutput[nCount-1]].abcC); // always increase width(Last character)
 
Why that? I've noticed that some fonts may have unexpected values or I don't know how to interpret them correctly.
 
Example 1: Verdana 16, Italic
Example 2: Times New Roman 16, Italic
Example 3: Microsoft Sans Serif 16, Italic
Example 4: Courier New 16, Italic
 
Take the same 'July 2006' text:
 
The values of the ABC structure data members are:
Example 1:Verdana 16, Italic
                    abcA abcB abcC  
for character 'J':   -1   11    0     
for character '6':    1   11    1 
Example 2: Times New Roman 16, Italic
                    abcA abcB abcC  
for character 'J':   -1   12    -2
for character '6':    1   10    0 
Example 3: Microsoft Sans Serif 16, Italic
                     abcA abcB abcC  
for character 'J':    2   12    -3
for character '6':    2   13    -3 
Example 4: Courier New 16, Italic
                     abcA abcB abcC  
for character 'J':    2   13    -2
for character '6':    3   10    0
 
We can easily see that the underhang of '6' or 'J' can be a positive number for italic font and for this reason the statement:
 
rect.left+=pWidthsABC[sOutput[0]].abcA; // First character
 
is making the width smaller and that *may* cause the text to not fit anymore...
 
Also we can easily see that the overhang of '6' can be a positive number for italic font and for this reason the statement:
 
rect.right-=pWidthsABC[sOutput[nCount-1]].abcC; // Last character
 
is making the width smaller and that *may* cause the text to not fit anymore...
 
If you agree I will update the article's EXE and add this 3rd method too.
 

 

-- modified at 6:03 Monday 31st July, 2006
GeneralRe: DT_CALCRECT and GetCharABCWidths() [modified] Pinmemberbob1697231-Jul-06 4:14 
GeneralRe: DT_CALCRECT and GetCharABCWidths() [modified] PinmemberAlexandru Matei31-Jul-06 9:01 
GeneralRe: DT_CALCRECT and GetCharABCWidths() Pinmemberbob1697231-Jul-06 10:19 
GeneralRe: DT_CALCRECT and GetCharABCWidths() PinmemberAlexandru Matei31-Jul-06 10:20 
GeneralUsing DT_CALCRECT PinmemberJan Rodriguez25-Jul-06 15:10 
GeneralRe: Using DT_CALCRECT [modified] PinmemberAlexandru Matei25-Jul-06 20:59 

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