OpenGL—ES 1.0 2d rounded rectangle

Question

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

Answer 1

I needed to draw similar rectangle, but transparent - and code above draws some of triangles overlap. Fixed that, also removed malloc, just to simplify solution. Here is my version:

typedef struct
{
    float x;
    float y;
} Vector2f;

//
//  Draws rounded rectangle.
//
//  Slightly tuned version of http://stackoverflow.com/questions/5369507/opengles-1-0-2d-rounded-rectangle
//
#define ROUNDING_POINT_COUNT 8      // Larger values makes circle smoother.
void DrawRoundRect( float x, float y, float width, float height, float* color = 0, float radius = 0.0 )
{
    Vector2f top_left[ROUNDING_POINT_COUNT];
    Vector2f bottom_left[ROUNDING_POINT_COUNT];
    Vector2f top_right[ROUNDING_POINT_COUNT];
    Vector2f bottom_right[ROUNDING_POINT_COUNT];

    if( radius == 0.0 )
    {
        radius = min(width, height);
        radius *= 0.10; // 10%
    }

    int i = 0;
    float x_offset, y_offset;
    float step = ( 2.0f * pi ) / (ROUNDING_POINT_COUNT * 4),
          angle = 0.0f;

    unsigned int index = 0, segment_count = ROUNDING_POINT_COUNT;
    Vector2f bottom_left_corner = { x + radius, y - height + radius }; 


    while( i != segment_count )
    {
        x_offset = cosf( angle );
        y_offset = sinf( angle );


        top_left[ index ].x = bottom_left_corner.x - 
                              ( x_offset * radius );
        top_left[ index ].y = ( height - ( radius * 2.0f ) ) + 
                                bottom_left_corner.y - 
                              ( y_offset * radius );


        top_right[ index ].x = ( width - ( radius * 2.0f ) ) + 
                                 bottom_left_corner.x + 
                               ( x_offset * radius );
        top_right[ index ].y = ( height - ( radius * 2.0f ) ) + 
                                 bottom_left_corner.y -
                               ( y_offset * radius );


        bottom_right[ index ].x = ( width - ( radius * 2.0f ) ) +
                                    bottom_left_corner.x + 
                                  ( x_offset * radius );
        bottom_right[ index ].y = bottom_left_corner.y + 
                                  ( y_offset * radius );


        bottom_left[ index ].x = bottom_left_corner.x - 
                                 ( x_offset * radius );
        bottom_left[ index ].y = bottom_left_corner.y +
                                 ( y_offset * radius );


        top_left[ index ].x = top_left[ index ].x;
        top_left[ index ].y = top_left[ index ].y;


        top_right[ index ].x = top_right[ index ].x;
        top_right[ index ].y = top_right[ index ].y;


        bottom_right[ index ].x = bottom_right[ index ].x ;
        bottom_right[ index ].y = bottom_right[ index ].y;


        bottom_left[ index ].x =  bottom_left[ index ].x ;
        bottom_left[ index ].y =  bottom_left[ index ].y ;

        angle -= step;

        ++index;

        ++i;
    }

    static GLubyte clr[] = { 156, 207, 255, 128 };   // Light blue, 50% transparent.

    if( color )
        glColor4fv(color);
    else
        glColor4ubv(clr);

    glBegin( GL_TRIANGLE_STRIP );
    {
        // Top
        for( i = segment_count - 1 ; i >= 0 ; i--)
        {
            glVertex2f( top_left[ i ].x, top_left[ i ].y );
            glVertex2f( top_right[ i ].x, top_right[ i ].y );
        }

        // In order to stop and restart the strip.
        glVertex2f( top_right[ 0 ].x, top_right[ 0 ].y );
        glVertex2f( top_right[ 0 ].x, top_right[ 0 ].y );

        // Center
        glVertex2f( top_right[ 0 ].x, top_right[ 0 ].y );
        glVertex2f( top_left[ 0 ].x, top_left[ 0 ].y );
        glVertex2f( bottom_right[ 0 ].x, bottom_right[ 0 ].y );
        glVertex2f( bottom_left[ 0 ].x, bottom_left[ 0 ].y );

        // Bottom
        for( i = 0; i != segment_count ; i++ )
        {
            glVertex2f( bottom_right[ i ].x, bottom_right[ i ].y );    
            glVertex2f( bottom_left[ i ].x, bottom_left[ i ].y );                                    
        }    
    }
    glEnd();
} //DrawRoundRect