matrix.html 21 KB

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  255. <h1 class="page">Matrix</h1>
  256. <div class="sect1">
  257. <h2 id="matrix"><a class="anchor" href="#matrix"></a>Matrix</h2>
  258. <div class="sectionbody">
  259. <div class="paragraph">
  260. <p>See <a href="https://javadoc.jmonkeyengine.org/v3.3.2-stable/com/jme3/math/Matrix3f.html">Javadoc of Matrix3f</a> and <a href="https://javadoc.jmonkeyengine.org/v3.3.2-stable/com/jme3/math/Matrix4f.html">Javadoc of Matrix4f</a></p>
  261. </div>
  262. <div class="sect2">
  263. <h3 id="definition"><a class="anchor" href="#definition"></a>Definition</h3>
  264. <div class="paragraph">
  265. <p>A Matrix is typically used as a <em>linear transformation</em> to map vectors to vectors. That is: Y = MX where X is a Vector and M is a Matrix applying any or all transformations (scale, rotate, translate).</p>
  266. </div>
  267. <div class="paragraph">
  268. <p>There are a few special matrices:</p>
  269. </div>
  270. <div class="paragraph">
  271. <p><em>zero matrix</em> is the Matrix with all zero entries.</p>
  272. </div>
  273. <table class="tableblock frame-all grid-all stretch">
  274. <colgroup>
  275. <col style="width: 33.3333%;">
  276. <col style="width: 33.3333%;">
  277. <col style="width: 33.3334%;">
  278. </colgroup>
  279. <thead>
  280. <tr>
  281. <th class="tableblock halign-left valign-top">0</th>
  282. <th class="tableblock halign-left valign-top">0</th>
  283. <th class="tableblock halign-left valign-top">0</th>
  284. </tr>
  285. </thead>
  286. <tbody>
  287. <tr>
  288. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  289. <p>0</p>
  290. </div></div></td>
  291. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  292. <p>0</p>
  293. </div></div></td>
  294. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  295. <p>0</p>
  296. </div></div></td>
  297. </tr>
  298. <tr>
  299. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  300. <p>0</p>
  301. </div></div></td>
  302. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  303. <p>0</p>
  304. </div></div></td>
  305. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  306. <p>0</p>
  307. </div></div></td>
  308. </tr>
  309. </tbody>
  310. </table>
  311. <div class="paragraph">
  312. <p>The <em>Identity Matrix</em> is the matrix with 1 on the diagonal entries and 0 for all other entries.</p>
  313. </div>
  314. <table class="tableblock frame-all grid-all stretch">
  315. <colgroup>
  316. <col style="width: 33.3333%;">
  317. <col style="width: 33.3333%;">
  318. <col style="width: 33.3334%;">
  319. </colgroup>
  320. <thead>
  321. <tr>
  322. <th class="tableblock halign-left valign-top">1</th>
  323. <th class="tableblock halign-left valign-top">0</th>
  324. <th class="tableblock halign-left valign-top">0</th>
  325. </tr>
  326. </thead>
  327. <tbody>
  328. <tr>
  329. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  330. <p>0</p>
  331. </div></div></td>
  332. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  333. <p>1</p>
  334. </div></div></td>
  335. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  336. <p>0</p>
  337. </div></div></td>
  338. </tr>
  339. <tr>
  340. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  341. <p>0</p>
  342. </div></div></td>
  343. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  344. <p>0</p>
  345. </div></div></td>
  346. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  347. <p>1</p>
  348. </div></div></td>
  349. </tr>
  350. </tbody>
  351. </table>
  352. <div class="paragraph">
  353. <p>A Matrix is <em>invertible</em> if there is a matrix <em>M<sup>-1</sup></em> where <em>MM<sup>-1</sup> = M<sup>-1</sup> = I</em>.</p>
  354. </div>
  355. <div class="paragraph">
  356. <p>The <em>transpose</em> of a matrix <em>M = [m<sub>ij</sub>]</em> is <em>M<sup>T</sup> = [m<sub>ji</sub>]</em>. This causes the rows of <em>M</em> to become the columns of <em>M<sup>T</sup></em>.</p>
  357. </div>
  358. <table class="tableblock frame-all grid-all stretch">
  359. <colgroup>
  360. <col style="width: 14.2857%;">
  361. <col style="width: 14.2857%;">
  362. <col style="width: 14.2857%;">
  363. <col style="width: 14.2857%;">
  364. <col style="width: 14.2857%;">
  365. <col style="width: 14.2857%;">
  366. <col style="width: 14.2858%;">
  367. </colgroup>
  368. <thead>
  369. <tr>
  370. <th class="tableblock halign-left valign-top">1</th>
  371. <th class="tableblock halign-left valign-top">1</th>
  372. <th class="tableblock halign-left valign-top">1</th>
  373. <th class="tableblock halign-left valign-top"></th>
  374. <th class="tableblock halign-left valign-top">1</th>
  375. <th class="tableblock halign-left valign-top">2</th>
  376. <th class="tableblock halign-left valign-top">3</th>
  377. </tr>
  378. </thead>
  379. <tbody>
  380. <tr>
  381. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  382. <p>2</p>
  383. </div></div></td>
  384. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  385. <p>2</p>
  386. </div></div></td>
  387. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  388. <p>2</p>
  389. </div></div></td>
  390. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  391. <p>⇒</p>
  392. </div></div></td>
  393. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  394. <p>1</p>
  395. </div></div></td>
  396. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  397. <p>2</p>
  398. </div></div></td>
  399. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  400. <p>3</p>
  401. </div></div></td>
  402. </tr>
  403. <tr>
  404. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  405. <p>3</p>
  406. </div></div></td>
  407. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  408. <p>3</p>
  409. </div></div></td>
  410. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  411. <p>3</p>
  412. </div></div></td>
  413. <td class="tableblock halign-left valign-top"><div class="content"></div></td>
  414. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  415. <p>1</p>
  416. </div></div></td>
  417. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  418. <p>2</p>
  419. </div></div></td>
  420. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  421. <p>3</p>
  422. </div></div></td>
  423. </tr>
  424. </tbody>
  425. </table>
  426. <div class="paragraph">
  427. <p>A Matrix is symmetric if <em>M</em> = <em>M<sup>T</sup></em>.</p>
  428. </div>
  429. <table class="tableblock frame-all grid-all stretch">
  430. <colgroup>
  431. <col style="width: 33.3333%;">
  432. <col style="width: 33.3333%;">
  433. <col style="width: 33.3334%;">
  434. </colgroup>
  435. <thead>
  436. <tr>
  437. <th class="tableblock halign-left valign-top">X</th>
  438. <th class="tableblock halign-left valign-top">A</th>
  439. <th class="tableblock halign-left valign-top">B</th>
  440. </tr>
  441. </thead>
  442. <tbody>
  443. <tr>
  444. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  445. <p>A</p>
  446. </div></div></td>
  447. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  448. <p>X</p>
  449. </div></div></td>
  450. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  451. <p>C</p>
  452. </div></div></td>
  453. </tr>
  454. <tr>
  455. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  456. <p>B</p>
  457. </div></div></td>
  458. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  459. <p>C</p>
  460. </div></div></td>
  461. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  462. <p>X</p>
  463. </div></div></td>
  464. </tr>
  465. </tbody>
  466. </table>
  467. <div class="paragraph">
  468. <p>Where X, A, B, and C equal numbers</p>
  469. </div>
  470. <div class="paragraph">
  471. <p>jME includes two types of Matrix classes: Matrix3f and Matrix4f. Matrix3f is a 3x3 matrix and is the most commonly used (able to handle scaling and rotating), while Matrix4f is a 4x4 matrix that can also handle translation.</p>
  472. </div>
  473. </div>
  474. <div class="sect2">
  475. <h3 id="transformations"><a class="anchor" href="#transformations"></a>Transformations</h3>
  476. <div class="paragraph">
  477. <p>Multiplying a <a href="terminology.html#vectors" class="page">Vector</a> with a Matrix allows the Vector to be transformed. Either rotating, scaling or translating that Vector.</p>
  478. </div>
  479. <div class="sect3">
  480. <h4 id="scaling"><a class="anchor" href="#scaling"></a>Scaling</h4>
  481. <div class="paragraph">
  482. <p>If a <em>diagonal Matrix</em>, defined by D = [d<sub>ij</sub>] and d<sub>ij</sub> = 0 for i != j, has all positive entries it is a <em>scaling matrix</em>. If d<sub>i</sub> is greater than 1 then the resulting Vector will grow, while if d<sub>i</sub> is less than 1 it will shrink.</p>
  483. </div>
  484. </div>
  485. <div class="sect3">
  486. <h4 id="rotation"><a class="anchor" href="#rotation"></a>Rotation</h4>
  487. <div class="paragraph">
  488. <p>A <em>rotation matrix</em> requires that the transpose and inverse are the same matrix (R<sup>-1</sup> = R<sup>T</sup>). The <em>rotation matrix</em> R can then be calculated as: R = I + (sin(angle)) S + (1 - cos(angle)S<sup>2</sup> where S is:</p>
  489. </div>
  490. <table class="tableblock frame-all grid-all stretch">
  491. <colgroup>
  492. <col style="width: 33.3333%;">
  493. <col style="width: 33.3333%;">
  494. <col style="width: 33.3334%;">
  495. </colgroup>
  496. <thead>
  497. <tr>
  498. <th class="tableblock halign-left valign-top">0</th>
  499. <th class="tableblock halign-left valign-top">u<sub>2</sub></th>
  500. <th class="tableblock halign-left valign-top">-u<sub>1</sub></th>
  501. </tr>
  502. </thead>
  503. <tbody>
  504. <tr>
  505. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  506. <p>-u<sub>2</sub></p>
  507. </div></div></td>
  508. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  509. <p>0</p>
  510. </div></div></td>
  511. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  512. <p>u<sub>0</sub></p>
  513. </div></div></td>
  514. </tr>
  515. <tr>
  516. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  517. <p>u<sub>1</sub></p>
  518. </div></div></td>
  519. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  520. <p>-u<sub>0</sub></p>
  521. </div></div></td>
  522. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  523. <p>0</p>
  524. </div></div></td>
  525. </tr>
  526. </tbody>
  527. </table>
  528. </div>
  529. <div class="sect3">
  530. <h4 id="translation"><a class="anchor" href="#translation"></a>Translation</h4>
  531. <div class="paragraph">
  532. <p>Translation requires a 4x4 matrix, where the Vector (x,y,z) is mapped to (x,y,z,1) for multiplication. The <em>Translation Matrix</em> is then defined as:</p>
  533. </div>
  534. <table class="tableblock frame-all grid-all stretch">
  535. <colgroup>
  536. <col style="width: 50%;">
  537. <col style="width: 50%;">
  538. </colgroup>
  539. <thead>
  540. <tr>
  541. <th class="tableblock halign-left valign-top">M</th>
  542. <th class="tableblock halign-left valign-top">T</th>
  543. </tr>
  544. </thead>
  545. <tbody>
  546. <tr>
  547. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  548. <p>S<sup>T</sup></p>
  549. </div></div></td>
  550. <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
  551. <p>1</p>
  552. </div></div></td>
  553. </tr>
  554. </tbody>
  555. </table>
  556. <div class="paragraph">
  557. <p>where M is the 3x3 matrix (containing any rotation/scale information), T is the translation Vector and S<sup>T</sup> is the transpose Vector of T. 1 is just a constant.</p>
  558. </div>
  559. </div>
  560. </div>
  561. <div class="sect2">
  562. <h3 id="jme-class"><a class="anchor" href="#jme-class"></a>jME Class</h3>
  563. <div class="paragraph">
  564. <p>Both Matrix3f and Matrix4f store their values as floats and are publicly available as (m00, m01, m02, …, mNN) where N is either 2 or 3.</p>
  565. </div>
  566. <div class="paragraph">
  567. <p>Most methods are straight forward, and I will leave documentation to the Javadoc.</p>
  568. </div>
  569. </div>
  570. </div>
  571. </div>
  572. </article>
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