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VC++ Article Draw Rotate / slant text strings SetWorldTransform

This article shows how to draw rotate / slant text string 





The function provided here can be used to draw a text string with an oblique or slant angle. Such text outputs are useful in isometric or perspective 3D views to make the text strings look like in their 3D space. An example is shown in the following picture:

Screenshot - obluqie1.jpg


Windows GDI function TextOut() does not allow a text slant angle. To draw such slanted strings, we need to set a transformation using SetWorldTransform(). Windows drawing function will then take care of the shearing and rotation of the output. This procedure is incorporated into a new function similar to Windows TextOut() function:

void ObliqueTextOut( CDC *dc, int oblique, int x,int y,const CString &Text )

This function has the same arguments as Windows TextOut() function with an additional argument, oblique, to specify the text slant angle. The function can be placed where TextOut() is normally used.

Using the code

Insert the function source code into your source code file. Call the function at places where you would normally call Windows TextOut() function. Remember to select the font, set the text background mode, color and background color etc, as you would normally do before calling TextOut().

Angle oblique is positive if the text slants forward(to the right) and negative if it slants backwards(to the left). The oblique angle, s, in the figure below is positive. The angle is in 1/10th degrees. Therefore, if the text slants forward 15 degrees, oblique=150.

Points of Interest

The key to the question is to set up the transformation in DC. Function SetWorldTransform() needs an XFORM structure for the transformation. Therefore, we need to prepare the XFORM structure before calling SetWorldTransform( ). XFORM has 6 member data. They are eM11, eM21, eM12, eM22, eDx, eDx. They are defined as:

X = eM11 * x + eM21 * y + eDx
Y = eM12 * x + 2M22 * y + eDy

where (x,y) are the World coordinates and (X,Y) are the Paper space coordinates.

In the figure below, x,y are the World space axes. The string will always be drawing at (0,0) and horizontally in the world space. xs,ys are the Sheared space axes. The transformation from World to the Sheared space is:

xs = x - y * tan(s)
ys = y

where s is the slant or oblique angle.

Screenshot - oblique3.jpg

The Paper space is noted as X,Y. From the Sheared space to Paper space, the transformation is a rotation(angle r) and translation(Xo,Yo).

X = Xo + xs * cos(r) + ys * sin(r)
Y = Yo + ys * cos(r) - xs * sin(r)

Where (Xo,Yo) are simply the text insertion point in Paper space. Substitute (xs,ys) into the above, we get:

X = cos(r) * x + (sin(r)-tan(s)*cos(r)) * y + Xo
Y = -sin(r) * x + (cos(r)+tan(s)*sin(r)) * y + Yo

Compare this to the XFORM structure, it is obvious that:

eM11 = cos(r)
eM21 = sin(r) - tan(s) * cos(r)
eM12 = -sin(r)
eM22 = cos(r) + tan(s) * sin(r)
eDx = Xo
eDy = Yo

The above is translated into function code(dc is the input device context):

XFORM xForm;
xForm.eDx = (float) x;
xForm.eDy = (float) y;
xForm.eM11 = (float) cos(txtRotate);
xForm.eM21 = (float) (sin(txtRotate) - tan(txtOblique)*cos(txtRotate));
xForm.eM12 = (float) -sin(txtRotate);
xForm.eM22 = (float) (cos(txtRotate) + tan(txtOblique)*sin(txtRotate));
SetGraphicsMode( dc->m_hDC, GM_ADVANCED );
SetWorldTransform( dc->m_hDC, &xForm );

The call to SetGraphicsMode() is needed. Otherwise, function SetWorldTranform() will have no effect. Since now we are drawing in World space, we need to adjust the font's rotation(lfEscapement) to be horizontal and the character orientation(lfOrintation) to be from the World X-axis.

dc->GetCurrentFont()->GetLogFont( &lgf );
lgf.lfOrientation -= lgf.lfEscapement;
lgf.lfEscapement = 0;
CFont horFont;
horFont.CreateFontIndirect( &lgf );
CFont *OldFont = dc->SelectObject( &horFont );

Now, we can call:

dc->TextOut( 0,0, Text );

The work is done. But before returning, we need to restore the graphics mode and font:

ModifyWorldTransform( dc->m_hDC, &xForm, MWT_IDENTITY );
SetGraphicsMode( dc->m_hDC, GM_COMPATIBLE );
dc->SelectObject( OldFont );


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