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workshop03_movementAndTiming.adoc

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Movement And Timing

Where we want to get to

  • Create a window

  • Draw a Sprite in the window

  • Control where and what size in the window it appears

  • Make the sprite position and size independent of window resolution

  • Time how long your program is spending in each stage

  • Compensate for real-time in your Game Loop

Where you should already be

  • Have a working build system, that uses SDL

  • Have a program that draws a sprite on the screen

    • with a size and position that you can control/change

    • sprite position and size independent of window resolution

  • Be able to change the window size, including fullscreen

Activity summary

  • We’ll be adding stuff to main.cpp to do more interesting things

  • You could continue with your existing code (take a snapshot/commit)

    • or start a fresh project

1. Toggle fullscreen on a key-press

  • Allow switching to fullscreen mode on a key-press

  • Allow switching out of fullscreen mode on the same key

  • Allow the user to change the size of the window with the mouse

    • at the window edges

  • Make the window return to the size it was before fullscreen when toggling back

2. Move your sprite

  • Store the sprite position in a variable and use it in your render function

    • What information do you need in the variable?

    • What type/struct/class would make sense to store it in?

  • Change the variable and check that your sprite renders in a new, correct position

  • Add code to change the variable (once) on a key press

  • Make the sprite move the same amount every time a key is pressed down

3. Move your sprite over time

  • In your update function, change the variable by a fixed amount each call

    • What happens to the position over time?

  • Change your update function, so that your sprite orbits the centre of the window

    • You’ll want to use the present position of the sprite to support your calculation

4. Time how long your program is spending in each stage

  • Use SDL_GetTicks initially

    • Log the time (in milliseconds?) at each stage of the game loop

  • Where is burning most time?

  • Break down the step that is taking the longest

    • Add more logging (perhaps at a less-important priority and make the log output priority lower)

    • There is a specific point in our game that our program is waiting

    • What is it?

    • HINT: What "Frames Per Second" are your getting

    • Can you disable the cause?

    • What happens once you disable the cause?

5. Use better timing

  • SDL_GetTicks only has millisecond precision

  • Use the high-resolution clock instead

    • std::chrono::high_resolution_clock

    • Read the documentation of how to calculate "durations"

  • How long is each stage of your game loop taking now?

6. Compensate for real-time in your Game Loop

  • Use one of the methods outlined in lectures to manage real-time in your game loop

    • note: the slides link out to a number of external sources

  • Keeping the cause you disabled (in 4) off (V-sync off, in the driver), does your program now run at a consistent rate?

    • How long should it take for your sprite to complete an orbit?

    • Time that (or multiple orbits) with an external clock (your phone?)

  • Re-enable V-sync

    • Does your program still run correctly wrt time?