Databases
StackOverflow2013
Postsdbo
PODrawing lines with Bresenham's Line Algorithm
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PODrawing lines with Bresenham's Line Algorithm
    primarykey
    data
    text
    <p>My computer graphics homework is to implement OpenGL algorithms using only the ability to draw points.</p> <p>So obviously I need to get <code>drawLine()</code> to work before I can draw anything else. <code>drawLine()</code> has to be done using integers only. No floating point.</p> <p>This is what I was taught. Basically, lines can be broken up into 4 different categories, positive steep, positive shallow, negative steep and negative shallow. This is the picture I am supposed to draw:</p> <p><img src="https://i.imgur.com/xePCI.png" alt="expected result"></p> <p>and this is the picture my program is drawing:</p> <p><img src="https://i.imgur.com/2bcLp.png" alt="actual result"></p> <p>The colors are done for us. We are given vertices and we need to use Bresenham's Line algorithm to draw the lines based on the start and end points.</p> <p>This is what I have so far:</p> <pre><code>int dx = end.x - start.x; int dy = end.y - start.y; //initialize varibales int d; int dL; int dU; if (dy &gt; 0){ if (dy &gt; dx){ //+steep d = dy - 2*dx; dL = -2*dx; dU = 2*dy - 2*dx; for (int x = start.x, y = start.y; y &lt;= end.y; y++){ Vertex v(x,y); drawPoint(v); if (d &gt;= 1){ d += dL; }else{ x++; d += dU; } } } else { //+shallow d = 2*dy - dx; dL = 2*dy; dU = 2*dy - 2*dx; for (int x = start.x, y = start.y; x &lt;= end.x; x++) { Vertex v(x,y); drawPoint(v); // if choosing L, next y will stay the same, we only need // to update d by dL if (d &lt;= 0) { d += dL; // otherwise choose U, y moves up 1 } else { y++; d += dU; } } } } else { if (-dy &gt; dx){ cout &lt;&lt; "-steep\n"; //-steep d = dy - 2*dx; //south dL = 2*dx; //southeast dU = 2*dy - 2*dx; for (int x = start.x, y = start.y; y &gt;= end.y; --y){ Vertex v(x,y); drawPoint(v); //if choosing L, next x will stay the same, we only need //to update d if (d &gt;= 1){ d -= dL; } else { x++; d -= dU; } } } else { cout &lt;&lt; "-shallow\n"; //-shallow d = 2*dy - dx; dL = 2*dy; dU = 2*dy - 2*dx; for (int x = start.x, y = start.y; x &lt;= end.x; x++){ Vertex v(x,y); drawPoint(v); if (d &gt;= 0){ d += dL; } else { --y; d -= dU; } } } } </code></pre> <p>I know my error is going to be something silly, but I honestly cannot figure out what I am doing wrong. Why are some of the lines drawn incorrectly as shown above?</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USIn silico
    1. USToastyMallows
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. PODrawing lines with Bresenham's Line Algorithm
      1. This table or related slice is empty.
      1. VTUpMod
    1. COwhat is not working?
      singulars
      1. PODrawing lines with Bresenham's Line Algorithm
      1. USMitch Wheat
    2. COIf you look at the two pictures in the paragraph above my code, the "2nd quadrant" and the "4th quadrant" are not drawing right. I put those in quotes because point (0,0) is in the bottom left hand corner of the picture. Basically, the first else statement contains the code that isn't working correctly, -shallow and -steep.
      singulars
      1. PODrawing lines with Bresenham's Line Algorithm
      1. USToastyMallows
    3. COrun your code on 2 simple examples, one that works, and the one that doesn't. Say (0,0), (5, 5) and (0,5), (5,0), trace through both and see were they diverge. This will be a much better exercise than having someone on SO do it for you.
      singulars
      1. PODrawing lines with Bresenham's Line Algorithm
      1. USMK.
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload