PSP shit wave

void draw_wave(int start_y, unsigned int color, float amplitude, float frequency, float lambda, float time)
{
    sceGuDisable(GU_TEXTURE_2D);

    #define STEP_SIZE 2

    float _2A = 2.0f * amplitude;
    float k = _2PI/lambda;
    float omega = _2PI*frequency;

    int x;
    for (x = 0; x < 480; x += STEP_SIZE) {

        float y = _2A*cosf(k*x)*sinf(omega*time);

        struct ya2d_vertex_1ui3s *vertices = sceGuGetMemory(4 * sizeof(struct ya2d_vertex_1ui3s));
        vertices[0].color = color;
        vertices[0].x = x;
        vertices[0].y = 272;
        vertices[0].z = 0;

        vertices[1].color = color;
        vertices[1].x = x;
        vertices[1].y = start_y + y;
        vertices[1].z = 0;

        vertices[2].color = color;
        vertices[2].x = x + STEP_SIZE;
        vertices[2].y = 272;
        vertices[2].z = 0;

        vertices[3].color = color;
        vertices[3].x = x + STEP_SIZE;
        vertices[3].y = start_y + y;
        vertices[3].z = 0;

        sceGumDrawArray(GU_TRIANGLE_STRIP, GU_COLOR_8888|GU_VERTEX_16BIT|GU_TRANSFORM_2D, 4, 0, vertices);
    }
}