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<p class="MsoNormal">Ravi,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">During the projection matrix phase, OpenGL switches to left-handed system (i.e. Z-axis points positive going into the screen). But I missed the rotation, so it stays left-handed with +x (right), -y (look), and +z(up).
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks for the interpretation of the parameter names. I interpreted “center” to mean a position point – not a vector. And “eye” to mean the direction the eye is pointed to.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:#1F497D">----------------------------------------<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:#1F497D">Marlin Rowley<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:#1F497D">Software Engineer, Staff<o:p></o:p></span></p>
<p class="MsoNormal"><img width="156" height="40" style="width:1.625in;height:.4166in" id="Picture_x0020_1" src="cid:image001.jpg@01D432EC.2AE9C0E0" alt="cid:image002.jpg@01D39374.DEC5A2E0"><span style="font-size:10.0pt;font-family:"Times New Roman",serif;color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><i><span style="font-size:12.0pt;font-family:"Times New Roman",serif;color:#1F497D">Missiles and Fire Control</span></i><span style="color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:#1F497D">972-603-1931 (office)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:#1F497D">214-926-0622 (mobile)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><a href="mailto:marlin.r.rowley@lmco.com"><span style="color:#0563C1">marlin.r.rowley@lmco.com</span></a></span><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><b>From:</b> osg-users <osg-users-bounces@lists.openscenegraph.org>
<b>On Behalf Of </b>Ravi Mathur<br>
<b>Sent:</b> Monday, August 13, 2018 9:32 AM<br>
<b>To:</b> OpenSceneGraph Users <osg-users@lists.openscenegraph.org><br>
<b>Subject:</b> EXTERNAL: Re: [osg-users] LookAt() function parameter meanings..<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Hey Martin,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">You are not interpreting the parameters properly. Look at the header for Matrixd.cpp:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> /** Set the position and orientation to be a view matrix, </span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> * using the same convention as gluLookAt.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> */</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> void makeLookAt(const Vec3d& eye,const Vec3d& center,const Vec3d& up );</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">It follows the same convention and algorithm as gluLookAt(), so you can look at the many online reference pages for that (<a href="https://docs.microsoft.com/en-us/windows/desktop/opengl/glulookat" target="_blank">microsoft</a>,
<a href="https://www.khronos.org/registry/OpenGL-Refpages/gl2.1/xhtml/gluLookAt.xml" target="_blank">
khronos</a>). There is no question about its correctness or compatibility with OpenGL, since OSG has been used with OpenGL in hundreds of projects for like 20 years. :D<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">To very directly answer your question, "eye" is the eye position (or camera if you prefer), "center" is a point towards which the eye is looking, and therefore the look vector points from eye towards center. This look vector is the negative
z-axis of OpenGL. The "up" vector specifies the direction of the positive y-axis. The x-axis is then defined as y-cross-z, which is the default OpenGL right-handed system. HOWEVER, OSG takes it a step further and maps the y-axis to the look vector (into the
screen). With this simple rotation, x points right and z points up, which remains right-handed. I'm guessing this is done because many OSG applications represent physical (real-world) systems, in which the z-axis often does point up. (Robert or someone else
can correct me if that's not the reason)<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Ravi<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Mon, Aug 13, 2018 at 10:07 AM Rowley, Marlin R <<a href="mailto:marlin.r.rowley@lmco.com" target="_blank">marlin.r.rowley@lmco.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">All,<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I’m very confused by the parameters passed into this function. I am going over some of the legacy code and saw this line of code:<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-autospace:none">
<span style="font-size:9.5pt;font-family:Consolas;color:black">osg::</span><span style="font-size:9.5pt;font-family:Consolas;color:#2B91AF">Matrixd</span><span style="font-size:9.5pt;font-family:Consolas;color:black"> frustumView;</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:9.5pt;font-family:Consolas;color:black">frustumView.makeLookAt(VIEW_POS, mDirection, mUp);</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:9.5pt;font-family:Consolas;color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">saw the docs, it’s described like this:<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:9.5pt;font-family:Consolas;color:black">void makeLookAt(const Vec3d& eye,const Vec3d& center,const Vec3d& up);</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:9.5pt;font-family:Consolas;color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:9.5pt;font-family:Consolas;color:black">So in this context, I would think that “eye” represents the look vector. However, it seems to work like a position
vector. The “center” parameter expects a direction vector (0, -1, 0) where the look vector is along the -yaxis.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:9.5pt;font-family:Consolas;color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:9.5pt;font-family:Consolas;color:black">This matrix will not translate to OpenGL properly since it’s coordinate system is based off of a right handed coordinate
frame.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:9.5pt;font-family:Consolas;color:black"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:9.5pt;font-family:Consolas;color:black">Questions:</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:9.5pt;font-family:Consolas;color:black"> </span><o:p></o:p></p>
<ol start="1" type="1">
<li class="m4618417125473015712m4382851675941503044msolistparagraph" style="mso-list:l0 level1 lfo1">
Am I interpreting these parameters properly?<o:p></o:p></li><li class="m4618417125473015712m4382851675941503044msolistparagraph" style="mso-list:l0 level1 lfo1">
How would I convert this system to the OpenGL right-handed system?<o:p></o:p></li></ol>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:#1F497D">----------------------------------------</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:#1F497D">Marlin Rowley</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:#1F497D">Software Engineer, Staff</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><i><span style="font-size:12.0pt;font-family:"Times New Roman",serif;color:#1F497D">Missiles and Fire Control</span></i><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:#1F497D">972-603-1931 (office)</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:#1F497D">214-926-0622 (mobile)</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><a href="mailto:marlin.r.rowley@lmco.com" target="_blank"><span style="color:#0563C1">marlin.r.rowley@lmco.com</span></a></span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal">_______________________________________________<br>
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<a href="mailto:osg-users@lists.openscenegraph.org" target="_blank">osg-users@lists.openscenegraph.org</a><br>
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