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POAbout GDI/GDI+ coordinate compatibility?
 

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  1. POAbout GDI/GDI+ coordinate compatibility?
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    <p>I have a problem while drawing with both GDI and GDI+ interchangeably. The page transformation&mdash;in particular scaling&mdash;seems to be a little bit off between the two. Which properties of the GDI context affects the scaling of the output other than <code>SetViewportExt</code> and <code>SetWindowExt</code>?</p> <p>The code uses almost exclusively GDI for its drawing, but uses GDI+ in a few cases where its features (semitransparency) are needed. It uses <code>SetViewportExt</code>, <code>SetWindowExt</code> and <code>SetViewportOrg</code> to enable zooming and scrolling.</p> <p>When GDI+ is needed I construct a <code>Gdiplus::Graphics</code> object around the HDC and do the drawing. I assume this makes the graphics context wrap the device context and relay its rendering to the device context. If I extract the transformation matrix of the GDI+ graphics context, I see that it is the identity matrix, so the scaling is done elsewhere (in the device context I guess).</p> <p>I devised a simple test where I draw the same array of rectangles with GDI and GDI+ to be sure that all the transformations are the same in both cases. The code snippet follows:</p> <pre><code>CRect rect = ...; // Draw the rectangle using GDI CPen cpen(PS_DASH, 0, RGB(0,0,255)); pDC-&amp;gt;SelectObject(&amp;amp;cpen); pDC-&amp;gt;Rectangle(rect); { // Draw the rectangle using GDI+ Gdiplus::Graphics graphics(pDC-&amp;gt;m_hDC); Gdiplus::Pen pen(Gdiplus::Color(180,180,180)); graphics.DrawRectangle( &amp;amp;pen, Gdiplus::Rect(rect.left, rect.top, rect.Width(), rect.Height())); } </code></pre> <p>And the result is here: (the blue dashed is drawn by GDI and the gray is drawn by GDI+) </p> <p><a href="https://i.stack.imgur.com/2DZ6w.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/2DZ6w.png" alt="Result drawn by code"></a> </p> <p>I can clearly see that the two coordinate systems are different. I expected some round-off errors, but not a scaling error as seen here. Also, when I change the zoom factor, the GDI+ jumps around ±4 pixel in both directions depending on the zoom. This is also highlighted in the screenshot as the GDI+ rectangle has a positive offset on the X axis and a negative offset on the Y axis as compared to the GDI rectangle.</p> <ul> <li><p>Does anybody know what's going on here?</p></li> <li><p>How would I go about investigating/debugging this? This happens in the bowels of windows so I'm unfortunately unable to debug it.</p></li> </ul> <p>For reference, this is what my viewport/window org/ext looks like:</p> <pre><code>Window Ext: (134000, 80500) Window Org: (0, 0) Viewport Ext: (1452 872) Viewport Org: (35 35) </code></pre> <h3>Update:</h3> <p>I have fixed the problem, but it's not pretty. The basic approach is:</p> <ol> <li><p>Take two coordinates (origin and a second appropriate point) in screen space and transform them to logical coordinates using GDI (<code>DPtoLP</code> function).</p></li> <li><p>Reset the GDI transformation to <code>MM_TEXT</code>.</p></li> <li><p>Use the transformed points to construct a transformation matrix for GDI+ which represent the same transformation</p></li> <li><p>And finally use this matrix to construct a GDI+ context with the correct transformation.</p></li> </ol> <p>This is a bit of a hack, but it works. I still don't know why there is a difference between the two, though. At least it goes to show that it is <em>possible</em> to have a GDI+ context mimic the GDI transformation.</p>
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