# Update and Render logic with interpolation

I am working with C and SDL2 and just wanted some insight as to how to properly apply interpolation to the rendering.

EDIT - some clarification, update() input() and render() are just hypothetical stubs, the real code is messy and long as I learn SDL2.

while (!quit) {
current = SDL_GetTicks();
elapsed = current - previous;
previous = current;
lag += elapsed;

input();

while (lag >= ms_per_update)
{
update();
lag -= ms_per_update;
}

interpolation = (float)lag / (float)ms_per_update;

printf("Interploation: %f\r\n", interpolation);

render(interpolation);
}


This is something that has always kind of baffled me -

lets say that in update() if SPACE pressed we start a jump animation and movement. So sprite.y += 2 to move it.

Where does that movement of one pixel happen? Does it happen in update()?

If the movement happens in update() then how do i properly apply the interpolation in render() since my item has already been moved (during the update)?

• Lets say i have a bullet at (x=10, y=10)
• It's velocity is 10 pixels per update.
• use clicks button to shoot
• first update bullet is moved to (x=20, y=10)
• render(interpolation) // lets say it is 0.5) // lots of on screen lag from awesome effects
• second update bullet is moved to (x=30, y=10)
• third update bullet is moved to (x=40, y=10)
• render(interpolation) // lets say it is .09 from lots of lag

How is the interpolation value applied to the bullet so that it appears to pass smoothly across the screen. What information is known during render to apply the interpolation? Would a struct containing the origin point, current velocity (or amount moved since last render), and current location suffice?