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<h2><font face="Arial, Helvetica">GDI Mapping Modes Explained</font></h2>
<h2><!-- Date and author information --></h2>
<p align="left"><font size="2" face="Arial, Helvetica"><b>Date:</b>
8 May 1997</font><font face="Times New Roman"><br>
</font><font size="2" face="Arial, Helvetica"><b>Author:</b> Rich
Goldstein, MD</font> (<a
href="mailto:goldstei@interport.net">goldstei@interport.net</a>)</p>
<h3 align="left"><font face="Arial, Helvetica">Overview</font></h3>
<p align="left"><font face="Arial, Helvetica">Each device context
(DC, represented by TDC and derivatives in OWL) has the ability
to maintain a coordinate system separate and distinct from the
device it represents. So while the display may be a 640x480 or
800x600 or whatever pixels in dimension, we can tell the DC that
the coordinate space uses some other units.</font></p>
<p align="left"><font face="Arial, Helvetica">The result is the
existence of 2 coordinate systems: the <strong>DEVICE</strong>
coordinate system, and the <strong>LOGICAL</strong> coordinate
system.</font></p>
<p align="left"><font face="Arial, Helvetica">In general, <strong>DEVICE</strong>
coordinates are established by the device or it's related
drivers. So for the monitor, the display driver determines the <strong>DEVICE</strong>
coordinates (0,0 in the top left, width,height in pixels in the
lower right). These are rarely if ever changed programmatically
(the exceptions, of course, include changing screen or printer
resolution, or paper orientation, etc.).</font></p>
<p align="left"><font face="Arial, Helvetica">The <strong>LOGICAL</strong>
coordinates relate to the device context (DC) and are established
by the mapping mode, viewport origin and extents, and window
origin and extents. All DC related functions accept <strong>LOGICAL</strong>
coordinates, unless explicitly stated (e.g. DPtoLP, which
converts <strong>DEVICE</strong> coordinates to <strong>LOGICAL</strong>
coordinates).</font></p>
<p align="left"><font face="Arial, Helvetica">The system maps
your <strong>LOGICAL</strong> coordinates to the <strong>DEVICE</strong>
coordinates using the viewport/window origins and extents. </font></p>
<p align="left"><font face="Arial, Helvetica">So WHAT ARE THEY,
ALREADY???</font></p>
<h3 align="left"><font face="Arial, Helvetica">Origins</font></h3>
<p align="left"><font face="Arial, Helvetica">OK, let's start
with the window, which is expressed in <strong>LOGICAL</strong>
coordinates.</font></p>
<p align="left"><font face="Arial, Helvetica"><strong>SetWindowOrg()</strong>
tells the DC the <strong>LOGICAL</strong> point that maps to the <strong>DEVICE</strong>
point (0,0). So if you call <strong>SetWindowOrg(100,100)</strong>
for a window, the <strong>LOGICAL</strong> point (100,100) occurs
in the top left corner. (Hold on, this was the easy one...)</font></p>
<p align="left"><font face="Arial, Helvetica"><strong>SetViewportOrg()</strong>
tells the DC which <strong>DEVICE</strong> coordinate maps to <strong>LOGICAL</strong>
point (0,0). So calling <strong>SetViewportOrg()</strong>
with half the width and height of your window (in pixels), for
example, sets the <strong>LOGICAL</strong> point (0,0) to the
center of the window.</font></p>
<p align="left"><font face="Arial, Helvetica"><strong>SetWindowOrg()</strong>
and <strong>SetViewportOrg()</strong> can be called on
any Mapping Mode. They serve to offset the origin of one system
within the other. They have no effect on the relative distances
specified by the two coordinate systems.</font></p>
<table border="0" cellpadding="2" width="100%">
<tr>
<td colspan="3"><font size="3" face="Arial, Helvetica"><strong>Advanced
Note:</strong></font></td>
</tr>
<tr>
<td width="10%"><font face="Arial, Helvetica"></font> </td>
<td width="80%"><font size="2" face="Arial, Helvetica">Believe
it or not, there are some (though few) reasons to use
BOTH SetWindowOrg() and SetViewportOrg(). OWL's TScroller
provides one such opportunity. In AutoOrg mode, the
TScroller calls SetViewportOrg in it's BeginView member
function, which is called before Paint is called. If your
<strong>LOGICAL </strong>coordinate system uses an origin
other that the top-left of the window, you can call
SetWindowOrg in the Paint method to choose some other
origin. Because both functions can be used safely
together, you can let TScroller adjust the Viewport
origin to facilitate scrolling, and you can adjust the
Window origin to facilitate alternative coordinate
systems.</font></td>
<td width="10%"> </td>
</tr>
</table>
<h3 align="left"><font face="Arial, Helvetica">Mapping Modes</font></h3>
<p align="left"><font face="Arial, Helvetica">There are several
mapping modes, some of which are constrained to a fixed
relationship between <strong>DEVICE</strong> and <strong>LOGICAL</strong>
coordinate systems. Here is the list:</font></p>
<div align="center"><center>
<table border="1" cellspacing="1" width="63%">
<tr>
<th width="30%" nowrap><font size="2"
face="Arial,Helvetica">Name/Constant </font> </th>
<th width="20%"><font size="2" face="Arial,Helvetica">Constrained? </font>
</th>
<th width="40%"><font size="2" face="Arial,Helvetica">Logical
Unit </font> </th>
</tr>
<tr>
<td><font size="2" face="Arial,Helvetica">MM_TEXT
(default) </font> </td>
<td align="center"><font size="2" face="Arial,Helvetica">Yes </font>
</td>
<td align="center"><font size="2" face="Arial,Helvetica">Pixel </font>
</td>
</tr>
<tr>
<td><font size="2" face="Arial,Helvetica">MM_LOENGLISH </font>
</td>
<td align="center"><font size="2" face="Arial,Helvetica">Yes </font>
</td>
<td align="center"><font size="2" face="Arial,Helvetica">0.01
inch </font> </td>
</tr>
<tr>
<td width="25%"><font size="2" face="Arial,Helvetica">MM_HIENGLISH </font>
</td>
<td align="center"><font size="2" face="Arial,Helvetica">Yes </font>
</td>
<td align="center"><font size="2" face="Arial,Helvetica">0.001
inch </font> </td>
</tr>
<tr>
<td><font size="2" face="Arial,Helvetica">MM_LOMETRIC </font>
</td>
<td align="center"><font size="2" face="Arial,Helvetica">Yes </font>
</td>
<td align="center"><font size="2" face="Arial,Helvetica">0.1
mm </font> </td>
</tr>
<tr>
<td><font size="2" face="Arial,Helvetica">MM_HIMETRIC </font>
</td>
<td align="center"><font size="2" face="Arial,Helvetica">Yes </font>
</td>
<td align="center"><font size="2" face="Arial,Helvetica">0.01
mm </font> </td>
</tr>
<tr>
<td><font size="2" face="Arial,Helvetica">MM_TWIPS </font>
</td>
<td align="center"><font size="2" face="Arial,Helvetica">Yes </font>
</td>
<td align="center"><font size="2" face="Arial,Helvetica">1/20
of a point, or 1/1440 of an inch </font> </td>
</tr>
<tr>
<td><font size="2" face="Arial,Helvetica">MM_ISOTROPIC </font>
</td>
<td align="center"><font size="2" face="Arial,Helvetica">No </font>
</td>
<td align="center"><font size="2" face="Arial,Helvetica">User
Defined </font> </td>
</tr>
<tr>
<td><font size="2" face="Arial,Helvetica">MM_ANISOTROPIC </font>
</td>
<td align="center"><font size="2" face="Arial,Helvetica">No </font>
</td>
<td align="center"><font size="2" face="Arial,Helvetica">User
Defined </font> </td>
</tr>
</table>
</center></div>
<p><font face="Arial,Helvetica">For the 'constrained' modes, all
you are allowed is to change the origin of the logical system
using either <strong>SetWindowOrg</strong> or <strong>SetViewportOrg</strong>.</font></p>
<p><font face="Arial,Helvetica">This means that if I set the
mapping mode to <strong>MM_LOENGLISH</strong>, the point (0,0)
and (0,100) are 1 inch apart (1 Logical inch, defined by the
device capabilities, see <strong>GetDeviceCaps()</strong>)</font></p>
<h3><font face="Arial,Helvetica">Extents</font></h3>
<p><font face="Arial,Helvetica">Extents are a little trickier,
because how they are interpreted depends on the mapping mode.
They are only appropriate for the non-constrained modes, <strong>MM_ISOTROPIC</strong>
and <strong>MM_ANISOTROPIC</strong>.</font></p>
<p><font face="Arial,Helvetica">The difference between these two
modes is that <strong>MM_ISOTROPIC</strong> takes the parameters
you pass to <strong>SetViewportExt</strong> and <strong>SetWindowExt</strong>
as 'suggestions' (</font><a href="#suggestion"><font
face="Arial,Helvetica">see below</font></a><font
face="Arial,Helvetica">) and adjusts the extents so the the x and
y axis coordinates represent the same distance on the device.
This way, a <strong>LOGICAL</strong> unit in the x direction is
the same length (in terms of the output) as a <strong>LOGICAL</strong>
unit in the y direction (this is not intuitive... some printers
may have different resolutions in the two axes... this mode
ensures that the <strong>LOGICAL</strong> units are equivalent in
space).</font></p>
<p><font face="Arial,Helvetica"><strong>MM_ANISOTROPIC</strong>
differs in that the parameters passed to <strong>SetViewportExt</strong>
and <strong>SetWindowExt</strong> are taken literally.
Windows make no adjustment. Therefore, you can have very
different coordinate systems in the two axes.</font></p>
<p><font face="Arial,Helvetica"><strong>SetWindowExt</strong>
and <strong>SetViewportExt</strong> are used as a team.
These two functions set some internal members in the DC, which
are used to map points between coordinate systems. As such, each
is essentially meaningless taken alone.</font></p>
<p><font face="Arial,Helvetica"><strong>SetWindowExt</strong>
tells the DC that a rectangle with the <strong>LOGICAL</strong>
width and height passed in, has the <strong>DEVICE</strong> width
and height passed in via <strong>SetViewportExt</strong>.
Confused yet?</font></p>
<p><font face="Arial,Helvetica">Let's say that I call <strong>SetViewportExt</strong>
for a display device with the parameters 100,50. Taken alone,
that's rather meaningless. Now I call <strong>SetWindowExt</strong>
with parameters 100,100. This means that for each <strong>LOGICAL</strong>
unit in the x direction, I will move 1 <strong>DEVICE</strong>
unit. On the other hand, for each <strong>LOGICAL</strong> unit I
move in the y direction, I move 1/2 a unit in the <strong>DEVICE</strong>
coordinate.</font></p>
<p><font face="Arial,Helvetica">These functions can also be
called with negative numbers. When the sign of the parameters to <strong>SetWindowExt</strong>
and <strong>SetViewportExt</strong> a different, the
direction of the axes changes. So that the positive y direction
can be up,</font> <font face="Arial,Helvetica">instead of the
usual default of down, if I call:</font></p>
<pre><font face="Arial,Helvetica">SetWindowExt(1,-1);</font>
<font face="Arial,Helvetica">SetViewportExt(1,1);</font></pre>
<p><font face="Arial,Helvetica">One LOGICAL x unit translates to
1 <strong>DEVICE</strong> x unit, but 1 <strong>LOGICAL</strong>
y unit translates to 1 <strong>DEVICE</strong> y unit, in the
opposite direction.</font></p>
<p><font face="Arial,Helvetica">Basically, here is the formula
used (by Windows) to convert <strong>LOGICAL</strong> points to <strong>DEVICE</strong>
points:</font></p>
<p><font face="Arial,Helvetica">where x<sub>D</sub> = the <strong>DEVICE</strong>
coordinate</font><br>
<font face="Arial,Helvetica"> and x<sub>L</sub> = the <strong>LOGICAL</strong>
coordinate</font></p>
<p><font face="Arial,Helvetica"> x<sub>D</sub>
= (x<sub>L</sub> - xWindowOrg)*(xViewportExt/xWindowExt) +
xViewportOrg</font> </p>
<p><font face="Arial,Helvetica">If that makes anything clearer.</font></p>
<h3><a name="suggestion"><font face="Arial,Helvetica"></font></a><font
face="Arial,Helvetica">How Windows handles extents for
MM_ISOTROPIC</font></h3>
<p><font face="Arial,Helvetica">Assuming that <strong>SetWindowExt</strong>
is called BEFORE <strong>SetViewportExt</strong>
(recommended), how the adjustments are made depends on the actual
physical dimensions of the extents passed to <strong>SetViewportExt</strong>.
Since device coordinates are not necessarily in equal units in
the two axes, they are probably adjusted internally according to
LOGPIXELSX/LOGPIXELSY.</font></p>
<p><font face="Arial,Helvetica">If the physical dimensions of the
Viewport Extents are wider than they are tall, the x extent is
adjusted so that it's <strong>LOGICAL</strong> units are equal to
the <strong>LOGICAL</strong> units for the y axis (as defined by
the y parameters passed to <strong>SetWindowExt</strong> /
<strong>SetViewportExt</strong>).</font></p>
<p><font face="Arial,Helvetica">If the physical dimensions of the
Viewport Extents are taller than they are wide, the y extent is
adjusted.</font></p>
<hr>
<p><font size="2" face="Arial,Helvetica"><em>Contents of this
page Copyright © 1997, Rich Goldstein, MD. All Rights Reserved.</em></font></p>
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