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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>2 Position Operations</title><link rel="stylesheet" type="text/css" href="../scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="../racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-style.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../doc-site.css" title="default"/><script type="text/javascript" src="../scribble-common.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../doc-site.js"></script><script type="text/javascript" src="../local-redirect/local-redirect.js"></script><script type="text/javascript" src="../local-redirect/local-user-redirect.js"></script><!--[if IE 6]><style type="text/css">.SIEHidden { overflow: hidden; }</style><![endif]--></head><body id="doc-racket-lang-org"><div class="tocset"><div class="tocview"><div class="tocviewlist tocviewlisttopspace"><div class="tocviewtitle"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_0");">▼</a></td><td></td><td><a href="index.html" class="tocviewlink" data-pltdoc="x">Graphics:<span class="mywbr"> </span> Legacy Library</a></td></tr></table></div><div class="tocviewsublistonly" style="display: block;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1 </td><td><a href="Basic_Commands.html" class="tocviewlink" data-pltdoc="x">Basic Commands</a></td></tr><tr><td align="right">2 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Position Operations</a></td></tr><tr><td align="right">3 </td><td><a href="Color_Operations.html" class="tocviewlink" data-pltdoc="x">Color Operations</a></td></tr><tr><td align="right">4 </td><td><a href="Draw__Clear__and_Flip_Operations.html" class="tocviewlink" data-pltdoc="x">Draw, Clear, and Flip Operations</a></td></tr><tr><td align="right">5 </td><td><a href="World_Operations.html" class="tocviewlink" data-pltdoc="x">World Operations</a></td></tr><tr><td align="right">6 </td><td><a href="Miscellaneous_Operations.html" class="tocviewlink" data-pltdoc="x">Miscellaneous Operations</a></td></tr><tr><td align="right">7 </td><td><a href="An_Example.html" class="tocviewlink" data-pltdoc="x">An Example</a></td></tr><tr><td align="right">8 </td><td><a href="A_More_Complicated_Example.html" class="tocviewlink" data-pltdoc="x">A More Complicated Example</a></td></tr><tr><td align="right">9 </td><td><a href="Protecting_Graphics_Operations.html" class="tocviewlink" data-pltdoc="x">Protecting Graphics Operations</a></td></tr><tr><td align="right">10 </td><td><a href="Mouse_Operations.html" class="tocviewlink" data-pltdoc="x">Mouse Operations</a></td></tr><tr><td align="right">11 </td><td><a href="Keyboard_Operations.html" class="tocviewlink" data-pltdoc="x">Keyboard Operations</a></td></tr><tr><td align="right">12 </td><td><a href="Flushing.html" class="tocviewlink" data-pltdoc="x">Flushing</a></td></tr><tr><td align="right">13 </td><td><a href="Graphics_Library_as_a_Unit.html" class="tocviewlink" data-pltdoc="x">Graphics Library as a Unit</a></td></tr></table></div></div></div><div class="tocsub"><div class="tocsubtitle">On this page:</div><table class="tocsublist" cellspacing="0"><tr><td><a href="#%28def._%28%28lib._graphics%2Fgraphics..rkt%29._posn%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">posn</span></span></a></td></tr><tr><td><a href="#%28def._%28%28lib._graphics%2Fgraphics..rkt%29._get-pixel%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">get-<wbr></wbr>pixel</span></span></a></td></tr><tr><td><a href="#%28def._%28%28lib._graphics%2Fgraphics..rkt%29._get-color-pixel%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">get-<wbr></wbr>color-<wbr></wbr>pixel</span></span></a></td></tr><tr><td><a href="#%28def._%28%28lib._graphics%2Fgraphics..rkt%29._test-pixel%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">test-<wbr></wbr>pixel</span></span></a></td></tr></table></div></div><div class="maincolumn"><div class="main"><div class="navsettop"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.7", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.7");">top</a></span><span class="navright"> <a href="Basic_Commands.html" title="backward to "1 Basic Commands"" data-pltdoc="x">← prev</a> <a href="index.html" title="up to "Graphics: Legacy Library"" data-pltdoc="x">up</a> <a href="Color_Operations.html" title="forward to "3 Color Operations"" data-pltdoc="x">next →</a></span> </div><h3 x-source-module="(lib "graphics/scribblings/graphics.scrbl")" x-source-pkg="htdp-doc" x-part-tag=""posn"">2<tt> </tt><a name="(part._posn)"></a>Position Operations</h3><p>A position is a pixel location within a viewport. The upper-left
corner is pixel <span style="font-style: italic"></span>(<span style="font-style: italic"></span>0<span style="font-style: italic">, </span>0<span style="font-style: italic"></span>)<span style="font-style: italic"></span>, and positions increase to the right and
down.</p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>struct</p></div></div><table cellspacing="0" cellpadding="0" class="RForeground"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define-struct.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._struct%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">struct</a></span></td><td><span class="hspace"> </span></td><td><a name="(def._((lib._graphics/graphics..rkt)._posn-y))"></a><a name="(def._((lib._graphics/graphics..rkt)._posn-x))"></a><a name="(def._((lib._graphics/graphics..rkt)._make-posn))"></a><a name="(def._((lib._graphics/graphics..rkt)._posn~3f))"></a><a name="(def._((lib._graphics/graphics..rkt)._struct~3aposn))"></a><a name="(def._((lib._graphics/graphics..rkt)._posn))"></a><span title="Provided from: graphics/graphics | Package: htdp-lib"><span class="RktSym"><a href="#%28def._%28%28lib._graphics%2Fgraphics..rkt%29._posn%29%29" class="RktValDef RktValLink" data-pltdoc="x">posn</a></span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">x</span><span class="stt"> </span><span class="RktSym">y</span><span class="RktPn">)</span></td></tr></table></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">#:extra-constructor-name</span><span class="hspace"> </span><span class="RktSym"><a href="#%28def._%28%28lib._graphics%2Fgraphics..rkt%29._make-posn%29%29" class="RktValLink" data-pltdoc="x">make-posn</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">x</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=number-types.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._real%7E3f%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">real?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">y</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=number-types.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._real%7E3f%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">real?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Represents a positions.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">((</span><a name="(def._((lib._graphics/graphics..rkt)._get-pixel))"></a><span title="Provided from: graphics/graphics | Package: htdp-lib"><span class="RktSym"><a href="#%28def._%28%28lib._graphics%2Fgraphics..rkt%29._get-pixel%29%29" class="RktValDef RktValLink" data-pltdoc="x">get-pixel</a></span></span><span class="hspace"> </span><span class="RktVar">viewport</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">p</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=data-structure-contracts.html%23%2528def._%2528%2528lib._racket%252Fcontract%252Fbase..rkt%2529._one-of%252Fc%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">one-of/c</a></span><span class="hspace"> </span><span class="RktVal">0</span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">)</span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">viewport</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="Basic_Commands.html#%28def._%28%28lib._graphics%2Fgraphics..rkt%29._viewport~3f%29%29" class="RktValLink" data-pltdoc="x">viewport?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">p</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="#%28def._%28%28lib._graphics%2Fgraphics..rkt%29._posn~3f%29%29" class="RktValLink" data-pltdoc="x">posn?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Returns the color of the pixel at position <span class="RktVar">p</span> in
<span class="RktVar">viewport</span>; <span class="RktVal">0</span> denotes white and <span class="RktVal">1</span> denotes not
white.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">((</span><a name="(def._((lib._graphics/graphics..rkt)._get-color-pixel))"></a><span title="Provided from: graphics/graphics | Package: htdp-lib"><span class="RktSym"><a href="#%28def._%28%28lib._graphics%2Fgraphics..rkt%29._get-color-pixel%29%29" class="RktValDef RktValLink" data-pltdoc="x">get-color-pixel</a></span></span><span class="hspace"> </span><span class="RktVar">viewport</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">p</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="Color_Operations.html#%28def._%28%28lib._graphics%2Fgraphics..rkt%29._rgb~3f%29%29" class="RktValLink" data-pltdoc="x">rgb?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">viewport</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="Basic_Commands.html#%28def._%28%28lib._graphics%2Fgraphics..rkt%29._viewport~3f%29%29" class="RktValLink" data-pltdoc="x">viewport?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">p</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="#%28def._%28%28lib._graphics%2Fgraphics..rkt%29._posn~3f%29%29" class="RktValLink" data-pltdoc="x">posn?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Returns an <span class="RktSym">rgb</span> value for color of the pixel at position
<span class="RktVar">p</span> in <span class="RktVar">viewport</span>.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">((</span><a name="(def._((lib._graphics/graphics..rkt)._test-pixel))"></a><span title="Provided from: graphics/graphics | Package: htdp-lib"><span class="RktSym"><a href="#%28def._%28%28lib._graphics%2Fgraphics..rkt%29._test-pixel%29%29" class="RktValDef RktValLink" data-pltdoc="x">test-pixel</a></span></span><span class="hspace"> </span><span class="RktVar">viewport</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">color</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="Color_Operations.html#%28def._%28%28lib._graphics%2Fgraphics..rkt%29._rgb~3f%29%29" class="RktValLink" data-pltdoc="x">rgb?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">viewport</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="Basic_Commands.html#%28def._%28%28lib._graphics%2Fgraphics..rkt%29._viewport~3f%29%29" class="RktValLink" data-pltdoc="x">viewport?</a></span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="argcontract"><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">color</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">:</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=data-structure-contracts.html%23%2528def._%2528%2528lib._racket%252Fcontract%252Fbase..rkt%2529._or%252Fc%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">or/c</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=data-structure-contracts.html%23%2528def._%2528%2528lib._racket%252Fcontract%252Fbase..rkt%2529._integer-in%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">integer-in</a></span><span class="hspace"> </span><span class="RktVal">0</span><span class="hspace"> </span><span class="RktVal">299</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=strings.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._string%7E3f%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">string?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym"><a href="Color_Operations.html#%28def._%28%28lib._graphics%2Fgraphics..rkt%29._rgb~3f%29%29" class="RktValLink" data-pltdoc="x">rgb?</a></span><span class="RktPn">)</span></td></tr></table></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">Returns the color that will actually be used if <span class="RktVar">color</span> is used
to draw.</div></p><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.7", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.7");">top</a></span><span class="navright"> <a href="Basic_Commands.html" title="backward to "1 Basic Commands"" data-pltdoc="x">← prev</a> <a href="index.html" title="up to "Graphics: Legacy Library"" data-pltdoc="x">up</a> <a href="Color_Operations.html" title="forward to "3 Color Operations"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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