CODEWITHSUNDEEP
objective-ciosipadopengl-es
Jameson
Jameson

Reputation: 968

Error with GL_TRIANGLE_STRIP and vertex array

I have this method that prepares the coordinates in the posCoords array. It works properly about 30% of the time, then the other 70% the first few triangles are messed up in the grid. The entire grid is drawn using GL_TRIANGLE_STRIP. I'm pulling my hair out trying to figure out whats wrong. Any ideas?

enter image description here

enter image description here

if(!ES2) {
    glEnableClientState(GL_VERTEX_ARRAY);
    glEnableClientState(GL_TEXTURE_COORD_ARRAY);
}

int cols = floor(SCREEN_WIDTH/blockSize);
int rows = floor(SCREEN_HEIGHT/blockSize);
int cells = cols*rows;
NSLog(@"Cells: %i", cells);
coordCount =  /*Points per coordinate*/2 * /*Coordinates per cell*/ 2 * cells + /* additional coord per row */2*2*rows;

NSLog(@"Coord count: %i", coordCount);

if(texCoords) free(texCoords);
if(posCoords) free(posCoords);
if(dposCoords) free(dposCoords);

texCoords = malloc(sizeof(GLfloat)*coordCount);
posCoords = malloc(sizeof(GLfloat)*coordCount);
dposCoords = malloc(sizeof(GLfloat)*coordCount);

int index = 0;


float lowY, hiY = 0;
int x,y = 0;
BOOL drawLeftToRight = YES;
for(y=0;y<SCREEN_HEIGHT;y+=blockSize) {

    lowY = y;
    hiY = y + blockSize;

    // Draw a single row
    for(x=0;x<=SCREEN_WIDTH;x+=blockSize) {
        CGFloat px,py,px2,py2 = 0;

        // Top point of triangle
        if(drawLeftToRight) {
            px = x;
            py = lowY;

            // Bottom point of triangle
            px2 = x;
            py2 = hiY;
        }
        else {
            px = SCREEN_WIDTH-x;
            py = lowY;

            // Bottom point of triangle
            px2 = SCREEN_WIDTH-x;
            py2 = hiY;
        }

        // Top point of triangle
        posCoords[index] = px;
        posCoords[index+1] = py;

        // Bottom point of triangle
        posCoords[index+2] = px2;
        posCoords[index+3] = py2;

        texCoords[index] = px/SCREEN_WIDTH;
        texCoords[index+1] = py/SCREEN_HEIGHT;
        texCoords[index+2] = px2/SCREEN_WIDTH;
        texCoords[index+3] = py2/SCREEN_HEIGHT;

        index+=4;
    }

    drawLeftToRight = !drawLeftToRight;



}

Upvotes: 1

Views: 1204

Answers (3)

Jameson
Jameson

Reputation: 968

Found the issue: the texture buffer was overflowing in to the vertex buffer, it was random because some background tasks where shuffling memory around on a timer (sometimes)

Upvotes: 0

ioquatix
ioquatix

Reputation: 1476

I tend to draw left to right and at the end of the row add a degenerate triangle, then left to right again.

e.g. [1, 2, 3, 4, 5, 6; 6 10; 10, 11, 8, 9, 6, 7]

The middle part is called a degenerate triangle (e.g. triangles of zero area).

Also, if I had to take a guess at why you are seeing various kinds of corruption, I'd check to make sure that your vertices and indices are exactly what you expect them to be - normally you see that kind of corruption when you don't specify indices correctly.

Upvotes: 0

Ian Bullard
Ian Bullard

Reputation: 11

With a triangle strip the last vertex you add replaces the the oldest vertex used so you're using bad vertices along the edge. It's easier to explain with your drawing.

  • Triangle 1 uses vertices 1, 2, 3 - valid triangle
  • Triangle 2 uses vertices 2, 3, 4 - valid triangle
  • Triangle 3 uses vertices 4, 5, 6 - valid triangle
  • Triangle 4 uses vertices 5, 6, 7 - straight line, nothing will be drawn
  • Triangle 5 uses vertices 6, 7, 8 - valid

etc.

If you want your strips to work, you'll need to pad your strips with degenerate triangles or break your strips up.

Upvotes: 1

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