OpenGL—ES 1.0 2d rounded rectangle

Question

How to make rounded rectangle in OpenGL, or any polygon with rounded corners?

Answer 1

#define PI_2   1.57079632679490f

#define SIN(x) SDL_sinf (x)
#define COS(x) SDL_cosf (x)

typedef struct _g2d_vertex_t g2d_vertex_t;

struct _g2d_vertex_t {

    float x, y;
};

// pVertices - destination buffer
// nVertices - buffer size
// dx - width
// dy - height
// r - radius
// returnes the number of used vertices
int
__cdecl buildRoundedRect (g2d_vertex_t * pVertices, int nVertices, float dx, float dy, float r) {

    float a, da;
    int i1, i2, i3, i4, n;

    if (nVertices < 4) { return 0; }

    if (nVertices == 4) {

        pVertices [0].x = 0.f; pVertices [0].y = 0.f;
        pVertices [1].x = dx;  pVertices [1].y = 0.f;
        pVertices [2].x = dx;  pVertices [2].y = dy;
        pVertices [3].x = 0.f; pVertices [3].y = dy;

        return nVertices;
    }

    n = nVertices >> 2;

    if (r > dx / 2.f) { r = dx / 2.f; }
    if (r > dy / 2.f) { r = dy / 2.f; }

    a = 0.f;
    da = PI_2 / (float) (n - 1);

    for (i1 = 0, i2 = (n << 1) - 1, i3 = n << 1, i4 = (n << 2) - 1; i1 < n; i1++, i2--, i3++, i4--, a += da) {

        float cosA = COS (a), sinA = SIN (a);

        pVertices [i1].x = (dx - r) + r * cosA; pVertices [i1].y = (dy - r) + r * sinA;
        pVertices [i2].x =     r    - r * cosA; pVertices [i2].y = (dy - r) + r * sinA;
        pVertices [i3].x =     r    - r * cosA; pVertices [i3].y =     r    - r * sinA;
        pVertices [i4].x = (dx - r) + r * cosA; pVertices [i4].y =     r    - r * sinA;
    }

    return n << 2;
}

void drawRoundedRect () {

    g2d_vertex_t vertices [50];

    glColor3f (0.3f, 0.5f, 0.2f);

    glVertexPointer (2, GL_FLOAT, 0, vertices);
    glEnableClientState (GL_VERTEX_ARRAY);

    glDrawArrays (GL_LINE_LOOP, 0, buildRoundedRect (vertices, 50 /* max count of vertices to use: 4 - 50 */, 150.f, 80.f, 20.f));
}