Generate a plane with triangle strips

Question

What would be the best algorithm to generate a list of vertices to draw a plane using triangle strips?

I'm looking for a function which receives the plane's width and height and returns a float array containing correctly indexed vertices.

width represents the number of vertices per row.

height represents the number of vertices per column.

float* getVertices( int width, int height ) {
    ...
}

void render() {
    glEnableClientState(GL_VERTEX_ARRAY);
    glVertexPointer(3, GL_FLOAT, 0, getVertices(width,heigth));
    glDrawArrays(GL_TRIANGLE_STRIP, 0, width*height);
    glDisableClientState(GL_VERTEX_ARRAY);
}

Answer 1

none of the code above gives a correct mesh generation. A very good article about how to make a strip of triangles on a simple plane: http://www.learnopengles.com/android-lesson-eight-an-introduction-to-index-buffer-objects-ibos/

Here is my test code that actually tested and fully working:

int plane_width = 4; // amount of columns
int plane_height = 2; // amount of rows

int total_vertices = (plane_width + 1) * (plane_height + 1);
planeVert = new CIwFVec2[total_vertices];
memset(planeVert, 0, sizeof(CIwFVec2) * total_vertices);

int numIndPerRow = plane_width * 2 + 2;
int numIndDegensReq = (plane_height - 1) * 2;
int total_indices = numIndPerRow * plane_height + numIndDegensReq;

planeInd = new uint16[total_indices];

make_plane(plane_width, plane_height, planeVert, planeInd);

...

void make_plane(int width, int height, CIwFVec2 *vertices, uint16 *indices)
{
width++;
height++;

int size = sizeof(CIwFVec2);
// Set up vertices
for(int y = 0; y < height; y++)
{
    int base = y * width;
    for(int x = 0; x < width; x++)
    {
        int index = base + x;
        CIwFVec2 *v = vertices + index;
        v->x = (float) x;
        v->y = (float) y;
        Debug::PrintDebug("%d: %f, %f", index, v->x, v->y);
    }
}

Debug::PrintDebug("-------------------------");

// Set up indices
int i = 0;
height--;
for(int y = 0; y < height; y++)
{
    int base = y * width;

    //indices[i++] = (uint16)base;
    for(int x = 0; x < width; x++)
    {
        indices[i++] = (uint16)(base + x);
        indices[i++] = (uint16)(base + width + x);
    }
    // add a degenerate triangle (except in a last row)
    if(y < height - 1)
    {
        indices[i++] = (uint16)((y + 1) * width + (width - 1));
        indices[i++] = (uint16)((y + 1) * width);
    }
}

for(int ind=0; ind < i; ind++)
    Debug::PrintDebug("%d ", indices[ind]);
}

Answer 2

I was doing something similar and using the first two answers I have come up with this (tested, C#, XNA)

        // center x,z on origin
        float offset = worldSize / 2.0f, scale = worldSize / (float)vSize;

        // create local vertices
        VertexPositionColor[] vertices = new VertexPositionColor[vSize * vSize];

        for (uint z = 0; z < vSize; z++) {
            for (uint x = 0; x < vSize; x++) {
                uint index = x + (z * vSize);
                vertices[index].Position = new Vector3((scale*(float)x) - offset, 
                                                       heightValue, 
                                                       (scale*(float)z) - offset);
                vertices[index].Color = Color.White;
            }
        }

        // create local indices
        var indices = new System.Collections.Generic.List<IndexType>();

        for (int z = 0; z < vSize - 1; z++) {
            // degenerate index on non-first row
            if (z != 0) indices.Add((IndexType)(z * vSize));

            // main strip
            for (int x = 0; x < vSize; x++) {
                indices.Add((IndexType)(z * vSize + x));
                indices.Add((IndexType)((z + 1) * vSize + x));
            }

            // degenerate index on non-last row
            if (z != (vSize-2)) indices.Add((IndexType)((z + 1) * vSize + (vSize - 1)));
        }

This is easily convertable to c++, just make indices an std::vector.

The notable features for my solution are that: a) It doesn't need to change the winding order per substrip - adding two points creates two degenerate triangles, so the order is correct for the next substrip. b) You should conditionally add the first and last dg triangle vertices.

Answer 3

Here is some code that does this (not tested, but you get the idea at least):

void make_plane(int rows, int columns, float *vertices, int *indices) {
    // Set up vertices
    for (int r = 0; r < rows; ++r) {
        for (int c = 0; c < columns; ++c) {
            int index = r*columns + c;
            vertices[3*index + 0] = (float) c;
            vertices[3*index + 1] = (float) r;
            vertices[3*index + 2] = 0.0f;
        }
    }

    // Set up indices
    int i = 0;
    for (int r = 0; r < rows - 1; ++r) {
        indices[i++] = r * columns;
        for (int c = 0; c < columns; ++c) {
            indices[i++] = r * columns + c;
            indices[i++] = (r + 1) * columns + c;
        }
        indices[i++] = (r + 1) * columns + (columns - 1);
    }
 }

The first loop sets up the vertex array in a standard rectangular grid. There are R*C vertices.

The second loop sets up the indices. In general, there are two vertices per square in the grid. Each vertex will cause a new triangle to be drawn (with the previous two vertices), so each square is drawn with two triangles.

The first and last vertex at the start and end of each row is duplicated. This means there are two triangles of zero area (degenerate triangles) between each row. This allows us to draw the entire grid in one big triangle strip. This technique is called stitching.

Answer 4

Thanks you all. I've coded this. Is it correct? Or is the generated strip somehow wrong?

int width;
int height;
float* vertices = 0;
int* indices = 0;

int getVerticesCount( int width, int height ) {
    return width * height * 3;
}

int getIndicesCount( int width, int height ) {
    return (width*height) + (width-1)*(height-2);
}

float* getVertices( int width, int height ) {
    if ( vertices ) return vertices;

    vertices = new float[ getVerticesCount( width, height ) ];
    int i = 0;

    for ( int row=0; row<height; row++ ) {
        for ( int col=0; col<width; col++ ) {
            vertices[i++] = (float) col;
            vertices[i++] = 0.0f;
            vertices[i++] = (float) row;
        }
    }

    return vertices;
}

int* getIndices( int width, int height ) {
    if ( indices ) return indices;

    indices = new int[ iSize ];
    int i = 0;

    for ( int row=0; row<height-1; row++ ) {
        if ( (row&1)==0 ) { // even rows
            for ( int col=0; col<width; col++ ) {
                indices[i++] = col + row * width;
                indices[i++] = col + (row+1) * width;
            }
        } else { // odd rows
            for ( int col=width-1; col>0; col-- ) {
                indices[i++] = col + (row+1) * width;
                indices[i++] = col - 1 + + row * width;
            }
        }
    }
    if ( (mHeight&1) && mHeight>2 ) {
        mpIndices[i++] = (mHeight-1) * mWidth;
    }

    return indices;
}

void render() {
    glEnableClientState( GL_VERTEX_ARRAY );
    glVertexPointer( 3, GL_FLOAT, 0, getVertices(width,height) );
    glDrawElements( GL_TRIANGLE_STRIP, getIndicesCount(width,height), GL_UNSIGNED_INT, getIndices(width,height) );
    glDisableClientState( GL_VERTEX_ARRAY );
}

With width=4 and height=4 this is what I got:

And here I'm modifying some vertex height: