[SOLVED] How to draw n vertices of a VertexArray in SFML?

How to draw n vertices of a VertexArray in SFML?

I have a line-strip of say N vertices sf::VertexArray vArr(sf::PrimitiveType::Lines, N);. Now when i draw it, it draws the entire line-strip of N vertices... How do I draw n (such that n < N) vertices similar to void glDrawArrays(GLenum mode, GLint first, GLsizei count); in OpenGL.

For example if N = 20, and n = 4, the RenderTarget should draw 4 vertices of the given vertex array. In raw OpenGL, I would have just called glDrawArrays(GL_LINE_STRIP, 0, 4);... how to do it in SFML?

I tried to use raw OpenGL calls after reading this, but could not figure out how to draw the VertexArray from the SFML graphics module.

Solution

target.draw(array) calls the draw (const Drawable &drawable, const RenderStates &states=RenderStates::Default) overload of RenderTarget. This in turn calls array.draw(target, states), which has the following implementation:

void VertexArray::draw(RenderTarget& target, const RenderStates& states) const
{
    if (!m_vertices.empty())
        target.draw(m_vertices.data(), m_vertices.size(), m_primitiveType, states);
}

You do not have access to m_vertices directly, but you can calculate the correct addresses by grabbing the address of the desired start. So, you can write a selective drawing function like so:

void drawPartialVertexArray(sf::RenderTarget& target, const sf::VertexArray& va, size_t start, size_t count, const RenderStates& states = RenderStates::Default) const {
  target.draw(&va[start], count, va.getPrimitiveType(), states);
}

It is of course up to you to make sure that start and start+count are within the boundaries of va, and that count is a multiple of the vertex count as required by the primitive type.