workshop02_myFirstSprite.adoc

My First Sprite

links.adoc

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

Where you should already be

• Have a working build system, that uses SDL

  • setup on your present machine using your automation script

  • from Workshop 1

  • you shouldn’t need to change the conanfile.txt or CMakeLists.txt files

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

• SDL2 can already load some images, which we can then use as Sprites

  • only very limited forms of .bmp files

  • 24bpp `.bmp files

1. Create a Window

• add code using SDL_CreateWindow to create a window

  • be careful where/when in your program you put this

  • it would be good to have it in a function

1.1 Check for errors

• you should check for errors and do something appropriate

  • bad return values

• SDL has some assert functions that you could use

  • look them up, which would make sense for your program

  • or handle errors differently

2. Create a Renderer

• To draw stuff in the window we need a renderer

• Use SDL_CreateRenderer to create a renderer for our window

• Check it for errors

• What "kind" of renderer makes most sense

2.1 Combine/Simplify

• SDL can make a connected window and a renderer in a single function call

  • use this instead

3. Clear and Present the renderer

• Our render function should do 3 things:

  1. Clear the "backbuffer"

  2. Draw some stuff

  3. Show the "backbuffer"

     • make the "backbuffer" the "frontbuffer"

3.1 Clear and Present the renderer

• Look up the appropriate functions in SDL2

• Use them to make the window various colours:

  1. Red

  2. Green

  3. Blue

  4. Yellow

  5. Grey

4. Game Loop

• Our present program doesn’t have a Game Loop

  • it just runs and then exits

• Add a Game Loop

  • keep it simple initially

  • it should call (at least) 3 functions

    1. process_input

    2. update

    3. render

• Add a log for each time this loop goes around

  • what "priority" should this log be?

4.1 Make the window colour change over time

• Store the colour in a variable

  • what "scope" should this variable be in?

  • or - how do our functions access its data

• Change the variable over time

  • where should this happen?

• Use the variable when drawing

  • wher should this happen?

4.2 WARNING: global variables

• So far we have no classes

• Quite likely you will use Global variables for your program at this point

• WARNING: Global variables are, in general, BAD practice

• We should try to move away from this in future

5. Input Handling

• Our Game loop doesn’t exit well at the present

• SDL can give us input from the user

• Look up SDL_PollEvent in the documentation

• Use it in your code (in process_input) to stop your program running when:

  • the user closes the window

  • the user presses escape down

• Log appropriately

6. Loading a Sprite

• SDL can load some bitmap files with the SDL_LoadBMP function

  • we’ll move to a better image loader later

• Look up the documentation for SDL_LoadBMP

• Make your program load an image

  • make sure you have one in the appropriate format

  • where will your program look

• What "type" does SDL_LoadBMP give you

• Log and handles errors appropriately

  • this WILL help you now and in future

6.1 Creating a texture

• "Textures" exist in GPU memory

• "Surfaces" exist in CPU memory

• SDL_LoadBMP doesn’t give you a texture

• SDL_CreateTextureFromSurface will help you create a "texture"

  • what should you do with the surface after you’ve finished with it

  • check the documentation

• Log and handles errors appropriately

  • this WILL help you now and in future

6.2 Drawing the texture

• SDL can draw a texture with the SDL_RenderCopy function

• Look up the documentation

• Use SDL_RenderCopy in your program

• Log and handles errors appropriately

  • this MAY help you now and in future

7. Change the sprite size and position

• SDL_RenderCopy can control the size and position of the sprite

• Read the documentation

• Change the size and position

• What units are these in?

8. Make the sprite position and size independent of window resolution

• Change the Window size for your program

  • try a variety of options, including fullscreen

• Make your sprite always appear relatively the same size