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<title> 曲面及等值線繪圖 </title>
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<font COLOR="#0000FF">
<h1>3.2.2 曲面及等值線繪圖</h1>
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<p>如果要畫一個(gè)三維的曲面，MATLAB是以<font COLOR="#FF0000">meshgrid</font>配合與<font COLOR="#FF0000">mesh</font>或<font COLOR="#FF0000">surf</font>指令來繪圖。先要以<font COLOR="#FF0000">meshgrid</font>產(chǎn)生在x-y平面 的二維的網(wǎng)格數(shù)據(jù)，再以一組z軸的數(shù)據(jù)對應(yīng)到這個(gè)二維的網(wǎng)格，即可畫出三維的曲面。以下的例子可說明上
述的繪圖過程。 </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> x=-7.5:0.5:7.5; y=x; % </font><font COLOR="#FF0000">先產(chǎn)生</font><font COLOR="#FF0000" FACE="Times New Roman">x</font><font COLOR="#FF0000">及</font><font COLOR="#FF0000" FACE="Times New Roman">y</font><font COLOR="#FF0000">二個(gè)陣列</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> [X,Y]=meshgrid(x,y); % </font><font COLOR="#FF0000">再以</font><font COLOR="#FF0000" FACE="Times New Roman">meshgrid</font><font COLOR="#FF0000">形成二維的網(wǎng)格數(shù)據(jù)</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> R=sqrt(X.^2+Y.^2)+eps; % </font><font COLOR="#FF0000">加上</font><font COLOR="#FF0000" FACE="Times New Roman">eps</font><font COLOR="#FF0000">可避免當(dāng)</font><font COLOR="#FF0000" FACE="Times New Roman">R</font><font COLOR="#FF0000">在分母時(shí)趨近零時(shí)會(huì)無法定義</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> Z=sin(R)./R; % </font><font COLOR="#FF0000">產(chǎn)生</font><font COLOR="#FF0000" FACE="Times New Roman">z</font><font COLOR="#FF0000">軸的數(shù)據(jù)</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> mesh(X,Y,Z) % </font><font COLOR="#FF0000">將</font><font COLOR="#FF0000" FACE="Times New Roman">z</font><font COLOR="#FF0000">軸的變化值以網(wǎng)格方式畫出</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> surf(X,Y,Z) % </font><font COLOR="#FF0000">將</font><font COLOR="#FF0000" FACE="Times New Roman">z</font><font COLOR="#FF0000">軸的變化值以曲面方式畫出</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> mesh(peaks) % </font><font COLOR="#FF0000">直接將以定義的</font><font COLOR="#FF0000" FACE="Times New Roman">peaks</font><font COLOR="#FF0000">函數(shù)以網(wǎng)格方式畫出</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> title('Mesh plot of peaks')<br>
</font></p>
<p>與三維繪圖有關(guān)的還有等值線圖，相關(guān)指令為<font COLOR="#FF0000">contour</font>,<font COLOR="#FF0000">contour3</font>。<font COLOR="#FF0000">contour</font>是將等值線圖以二維圖表示，其語法有
幾個(gè)方式。一是<font COLOR="#FF0000">contour(Z)</font>,<font COLOR="#FF0000">contour(Z,n)</font>，其中<font COLOR="#FF0000">Z</font>是一個(gè)二維矩陣，而 n為等值線的數(shù)目（如果不給即以自動(dòng)方式
設(shè)定）。另一種語法則是將z軸的值對應(yīng)到指定的x,y軸的值，語法為<font COLOR="#FF0000">contour(X,Y,Z)</font>,<font COLOR="#FF0000">contour(X,Y,Z,n)</font>，其中<font COLOR="#FF0000">X</font>,<font COLOR="#FF0000">Y</font>, <font COLOR="#FF0000">Z</font>代表x,y,z軸的數(shù)據(jù)。<font COLOR="#FF0000">contour3</font>則是將等值線以三維圖表示，其語法與<font COLOR="#FF0000">contour</font>類似，只是將對應(yīng)的關(guān)鍵字<font COLOR="#FF0000">contour</font>
改成<font COLOR="#FF0000">contour3</font>，其余部份相同。 <br>
</p>
<p><a NAME="work">以下的例子可以比較<font COLOR="#FF0000" FACE="Times New Roman">contour</font><font FACE="Times New Roman">, </font><font COLOR="#FF0000" FACE="Times New Roman">contour3</font>圖示的不同：</a>
</p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> [X,Y,Z]=peaks; % x,y</font><font COLOR="#FF0000">及</font><font COLOR="#FF0000" FACE="Times New Roman">z</font><font COLOR="#FF0000">軸的數(shù)據(jù)由</font><font COLOR="#FF0000" FACE="Times New Roman">peaks</font><font COLOR="#FF0000">函數(shù)定義</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> subplot(2,2,1)</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> contour(Z,20) % </font><font COLOR="#FF0000">畫出</font><font COLOR="#FF0000" FACE="Times New Roman">peaks</font><font COLOR="#FF0000">的</font><font COLOR="#FF0000" FACE="Times New Roman">Z</font><font COLOR="#FF0000">軸二維等值線圖，</font><font COLOR="#FF0000" FACE="Times New Roman">20</font><font COLOR="#FF0000">為等值線的數(shù)目</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> subplot(2,2,2)</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> contour(X,Y,Z,20) % </font><font COLOR="#FF0000">畫出</font><font COLOR="#FF0000" FACE="Times New Roman">peaks</font><font COLOR="#FF0000">的二維等值線圖，注意</font><font COLOR="#FF0000" FACE="Times New Roman">x,y</font><font COLOR="#FF0000">軸與上圖不同</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> subplot(2,2,3)</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> contour3(Z,20) % </font><font COLOR="#FF0000">畫出</font><font COLOR="#FF0000" FACE="Times New Roman">peaks</font><font COLOR="#FF0000">的</font><font COLOR="#FF0000" FACE="Times New Roman">Z</font><font COLOR="#FF0000">軸二維等值線圖</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> subplot(2,2,4)</font> </p>
<p><font COLOR="#FF0000" FACE="Times New Roman">>> contour3(X,Y,Z,20) % </font><font COLOR="#FF0000">畫出</font><font COLOR="#FF0000" FACE="Times New Roman">peaks</font><font COLOR="#FF0000">的三維等值線圖，注意</font><font COLOR="#FF0000" FACE="Times New Roman">x,y</font><font COLOR="#FF0000">軸與上圖不同</font></p>
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