新手小白求教大神，这个程序的主函数中的f是函数调用吗？

画一个[爱心]形的编程
#include
#include
static float x;
static float z;
static float y;
float f(float x, float y, float z) {
float a = x * x + 9.0f / 4.0f * y * y + z * z - 1;
return a * a * a - x * x * z * z * z - 9.0f / 80.0f * y * y * z * z * z;
}

float h(float x, float z) {
for (float y = 1.0f; y >= 0.0f; y -= 0.001f)
if (f(x, y, z) <= 0.0f)
return y;
return 0.0f;
}

int main() {
for (float z = 1.5f; z > -1.5f; z -= 0.05f) {
for (float x = -1.5f; x < 1.5f; x += 0.025f) {
float v = f(x, 0.0f, z);
if (v <= 0.0f) {
float y0 = h(x, z);
float ny = 0.01f;
float nx = h(x + ny, z) - y0;
float nz = h(x, z + ny) - y0;
float nd = 1.0f / sqrtf(nx * nx + ny * ny + nz * nz);
float d = (nx + ny - nz) * nd * 0.5f + 0.5f;
putchar(".:-=+*#%@"[(int)(d * 5.0f)]);
}
else
putchar(' ');
}
putchar('\n');
}
}这个程序的主函数中的f是函数调用吗？为什么没有括号呀？我刚学，这种表达形式没有见过，求大神指点！

只有float v =f(……)
float nx =h(……)
float nz =h(……)是函数调用

 static float x;
static float z;
static float y;
float f(float x, float y, float z) 
{
    float a = x * x + 9.0f / 4.0f * y * y + z * z - 1;
    return a * a * a - x * x * z * z * z - 9.0f / 80.0f * y * y * z * z * z;
}
float h(float x, float z) 
{
    for (float y = 1.0f; y >= 0.0f; y -= 0.001f)
        if (f(x, y, z) <= 0.0f)
            return y;
    return 0.0f;
}
int main() 
{
    for (float z = 1.5f; z > -1.5f; z -= 0.05f) 
    {
        for (float x = -1.5f; x < 1.5f; x += 0.025f) 
        {
            float v = f(x, 0.0f, z);
            if (v <= 0.0f) 
            {
                float y0 = h(x, z);
                float ny = 0.01f;
                float nx = h(x + ny, z) - y0;
                float nz = h(x, z + ny) - y0;
                float nd = 1.0f / sqrtf(nx * nx + ny * ny + nz * nz);
                float d = (nx + ny - nz) * nd * 0.5f + 0.5f;
                putchar(".:-=+*#%@"[(int)(d * 5.0f)]);
            }
            else
            {
                putchar(' ');
            }
        }
        putchar('\n');
    }
    return 0;
}