GL_PRIMITIVE_RESTART Alternative

[John Bell]

Hello,

I am drawing a grid of vertices with TRIANGLE_STRIP. At the end of each row, i would call the assigned restart primitive.

WebGl does not support this. What alternatives are there to drawing a simple grid efficiently?

http://www.mbsoftworks.sk/index.php?page=tutorials&series=1&tutorial=8

Old School
glGenBuffers(1, &uiVBOIndices);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, uiVBOIndices);
int iIndices[] =
{
0, 4, 1, 5, 2, 6, 3, 7, 16, // First row, then restart
4, 8, 5, 9, 6, 10, 7, 11, 16, // Second row, then restart
8, 12, 9, 13, 10, 14, 11, 15 // Third row, no restart
};
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(iIndices), iIndices, GL_STATIC_DRAW);
glEnable(GL_PRIMITIVE_RESTART);
glPrimitiveRestartIndex(HM_SIZE_X*HM_SIZE_Y);

New School

gridIndicesBuffer = gl.createBuffer();

gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, gridIndicesBuffer);

var indices = [];

//Do stuff to indices

gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, new Uint16Array(indices), gl.STATIC_DRAW);

gridIndicesBuffer.itemSize = 1;

gridIndicesBuffer.numItems = indices.length;

gl.enable(gl.GL_PRIMITIVE_RESTART); 

gl.primitiveRestartIndex(totalIndices);

Any pointers here?

Regards,

Daniel

[Kenneth Russell]

Have you tried creating a degenerate (zero-area) triangle to move to
the appropriate vertex of the first triangle in the next row?

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[John Bell]

Yeah,

I tried going back to the origin each time.

        var indices = [];

var totalIndices = (imgWidth*imgHeight);

for(var index=0;index<(totalIndices-imgWidth);index++)

{

indices.push(index);

indices.push(index+imgWidth);

if((((index)%(imgWidth))==0) &&(index>0))

{

indices.push(index+imgWidth);

}

}

Any tips?

[Alecazam]

As Ken stated, just repeat the last index (or vertex), and repeat the first index (or vertex) of your next primitive.   That introduces a few degenerate tris, but those are quickly culled by the GPU.   I've never found primitive restart all that useful, but if you have indexed data then it's an easier way to restart the stream especially if you have numerous restarts.