#+TITLE: SDL Tutorial Part 0x00 - Boilerplate, Windowing and Rendering
#+DATE: <2016-09-14 Sun 21:02>
#+TAGS: tutorial programming game-development c


#+BEGIN_EXPORT html
<aside class="legacy-warning">
  <p>This is one of my older tutorials and follows a style unlike
    my current one. I also no longer hold the same claims I made about the SDL
    documentation that I originally made in this article. I think it's perfectly
    fine. You just need to spend some time looking around because it's not
    organized like other documentation is. For that reason, I have no plans to
    continue this tutorial series unless someone specifically asks for me to
    continue it.</p>
</aside>
#+END_EXPORT


SDL2 is my favorite graphics library right now. It might not be as powerful as
something like raw OpenGL, but it's simple. Simple enough that you can just
pick it up and start using it. There's a glaring issue with it, though. The
documentation is horrible. Absolutely horrible. A lot of it is unfinished, and
it doesn't look like it's getting attention any time soon. The SDL1.2
documentation wasn't as bad, but that version of the library is vastly outdated
by today's standards. So here's my take on a tutorial for SDL - part 0x00 of a
I-don't-know-how-long-I'm-going-to-drag-this-on series. My examples are going
to be written in C, because the constructs I show here can still be used
verbatim in C++ (and probably SDL's other language bindings as well). This
tutorial will be covering the little boilerplate that SDL requires, as well as
the basics of windowing and rendering. Let's get into it.

The first thing you have to worry about is installing and setting up SDL2.0. I
won't cover it in detail because it's something you should be able to figure out
yourself. If your operating system doesn't provide a means of package
management, you should be able to find a download on the [[https://libsdl.org/][official website]].

You should also figure out how to link SDL2 when you're compiling, nothing I
teach here will work if it isn't properly linked. If you're having trouble with
anything, fire up your favorite search engine or feel free to [[http://tsar-fox.com/][contact me]].

Alright, assuming you've successfully installed SDL, let's get to actually
programming. As with any C library, the first thing you should worry about is
including the header files. While SDL provides header files for specific
subsystems, we're not going to worry about that right now. There's a header
file that contains everything and we're going to use that for right now.

#+BEGIN_SRC c :hl_lines 0
#include <SDL2/SDL.h>
#+END_SRC

Depending on how header files are organized on your system, you may have to use
this instead:

#+BEGIN_SRC c :hl_lines 0
#include <SDL.h>
#+END_SRC

That will give us function prototypes and type definitions for everything in
the SDL library, but we have to initialize SDL before we can really do anything
with it. This is actually really simple, done with a single function call.

#+BEGIN_SRC c :hl_lines 0
SDL_Init(SDL_INIT_VIDEO);
#+END_SRC

=SDL_Init= takes a flag as a parameter so it knows which subsystems to
initialize. /A/ parameter. One, not several. This might be a bit confusing to
some, especially if you're not familiar with bitwise arithmetic, but
=SDL_INIT_VIDEO= is nothing more than a preprocessor macro representing some
number. =SDL_Init= interprets that number, and initializes the subsystems
associated with it. We don't write the number out in our code, though (or at
least you shouldn't). We use the macros, but there aren't macros for every
combination of subsystems you can come up with. Does this mean that SDL can only
initialize one subsystem at a time? Not at all, to combine macros and represent
multiple subsystems, you would hook them together with the [[https://en.wikipedia.org/wiki/Bitwise_operation#OR][bitwise OR operator]].
(=|=, not =||=). For example, if we wanted to initialize SDL's video AND audio
subsystems, we would do this

#+BEGIN_SRC c :hl_lines 0
SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO);
#+END_SRC

But we're not going to be working with audio just yet.

=SDL_Init= also returns an integer value, and it's pretty important. If it's
zero, SDL was initialized properly. Great! But if SDL can't be initialized for
some reason, it will return a negative number. This is where another SDL
function comes into the picture. =SDL_GetError= takes no arguments, but will
return a string literal explaining what went wrong. So if we wanted to do some
error checking (which you always should), we could do this:

#+BEGIN_SRC c :hl_lines 0
if (SDL_Init(SDL_INIT_VIDEO)) {
    fprintf(stderr, "Here's the error: %s\n",
            SDL_GetError());
    return 1;
}
#+END_SRC

You can pretty much do anything here to handle the error. I'm using =fprintf(3)=
for simplicity, but SDL provides more advanced logging features which I'll cover
in a later tutorial.

In SDL, you need to be responsible and clean up after yourself. For every
function that initializes or creates something, there is a complementary
function that deinitializes or destroys it. The function to counter =SDL_Init=
is =SDL_Quit=. It takes no parameters and returns nothing, you can just call it
and be done with it. With that covered, we've learned SDL's few lines of
boilerplate code. Much more appealing than something like Direct3D, eh? If you
compile and run the program right now, nothing interesting will happen. It
initialized and deinitialized SDL (unless something went wrong), but didn't
bother creating windows or doing anything because we didn't tell it to. Let's
change that. We first have to know about two important typedefs in SDL.
*SDL_Window, and SDL_Renderer.* =SDL_Window= is self-explanatory, it's a struct
representing a window, and =SDL_Renderer= is how you would put something into a
window. Renderers in SDL are capable of hardware acceleration and vertical-sync,
which is why SDL2 is awesome and SDL1.2 is left in the dust. These are just
structs, though, they don't do anything by themselves. Let's create a window,
and capture it in a =SDL_Window= struct:

#+BEGIN_SRC c :hl_lines 0
SDL_Window *my_cool_window = SDL_CreateWindow("A Cool Window",
                                              SDL_WINDOWPOS_UNDEFINED,
                                              SDL_WINDOWPOS_UNDEFINED,
                                              400, 400, SDL_WINDOW_SHOWN);
#+END_SRC

Whoa, that's a mouthful, but it isn't as complicated as it looks. The first
argument is just a title for the window; you can name it whatever you want. The
next two arguments are X and Y values for where the window should be placed on
the screen. People usually don't care about this; you can use
=SDL_WINDOWPOS_UNDEFINED= if you don't. After that are the window's width and
height. I'm choosing to make my window 400 by 400 pixels, but you can choose
whatever size works best for your program. There are ways to change it later on,
too. Finally, we get to a flag. Its usage is similar to the flag we used with
=SDL_Init=, you use a bitwise OR to combine flags. We're not doing anything
fancy just yet, though, so =SDL_WINDOW_SHOWN= on its own will suffice. It
ensures that the window will be visible, rather than minimized.

As you can hopefully tell from the example code above, =SDL_CreateWindow=
returns a pointer of type =SDL_Window=. If a window cannot be created, though,
it will return =NULL=. You should always do error checking, so throw something
in to see if =my_cool_window= (or whatever you named your window variable) is
=NULL=.

#+BEGIN_SRC c :hl_lines 0
if (!my_cool_window) {
    fprintf(stderr, "Window couldn't be created. %s\n",
            SDL_GetError());
    return 1;
}
#+END_SRC

There's =SDL_GetError= again! He's our friend and you should be using him every
time you do error checking.

Remember how I said that SDL has a complementary function to destroy anything
that is created? This is no exception. =SDL_DestroyWindow= is very similar to
=SDL_Quit=, but it takes a =SDL_Window= pointer as an argument.

#+BEGIN_SRC c :hl_lines 0
SDL_DestroyWindow(my_cool_window);
#+END_SRC

It's sad to see him go, but we're done so we need to free the resources. At this
point, you can compile and run the C source file. It still kinda sucks, though.
When you run it, the window pops up and immediately goes away. One useful
function is =SDL_Delay=. It might seem mundane now, but it will become quite
important when we need to cap our program's framerate. It takes a number of
milliseconds as a parameter and temporarily stops your program so that SDL and
your computer can take a short break. Alright. So when we put that in our code
(after the window creation but before the window destruction), compile, and run
it, we get this:

#+CAPTION: A screenshot of the window on my system.
[[./sdl-tutorial-part-0x00-window.png]]

I'm running i3wm, so it will probably look slightly different for you, but
we've finally got a window! It still sucks, though. It doesn't do anything,
it doesn't even clear itself! Let's make it white and learn a little bit
about renderers in SDL.

If we want to do stuff within a window, we have two options. One is to create an
=SDL_Surface= from the window and draw to the surface, which is the sucky legacy
way of doing it so we're going to pretend that I didn't mention it, or you can
create a =SDL_Renderer=, which we're going to do because it's so much more
capable. We're going to use another SDL function call to create a renderer, and
we'll capture it in a =SDL_Renderer= pointer:

#+BEGIN_SRC c :hl_lines 0
SDL_Renderer *my_cool_renderer;
SDL_CreateRenderer(my_cool_window, -1,
                   SDL_RENDERER_ACCELERATED);
if (!my_cool_renderer) {
    fprintf(stderr, "There was an error %s\n",
            SDL_GetError());
}
#+END_SRC

Alright, so clearly the first argument is the window we want to create a
renderer for. The second is more complicated. It's the index of the driver to
initialize, which you probably don't care too much about so you can just put -1
in there to have it use the first one that's available. The last is a flag,
which you probably know so much about by now. Finally, as you can imagine,
=SDL_CreateRenderer= returns =NULL= if a renderer cannot be created. This should
all seem pretty familiar, it's the same format as creating a window - create a
struct pointer to capture it, use a function call to create it, and do some
basic error checking. Dead simple, and it just gets better from here.

Once again, don't forget to clean up after yourself. The function to remove a
renderer when you're done with it is =SDL_DestroyRenderer=. It just takes a
=SDL_Renderer= pointer as an argument.

Now we've got a renderer, but if we compile and run our code - the effect is
the same because we haven't used it for anything. So let's change that and
learn a little bit about drawing in SDL.

Renderers in SDL have a color associated with them, which they use when drawing
primitive geometry like lines and quadrilaterals. It doesn't affect textures,
but you'll probably end up using primitive geometry at some point so it's good
to know. =SDL_SetRenderDrawColor= changes the aforementioned color. We're
actually not going to be drawing any primitive geometry in this tutorial, but
I'm bringing this up because the renderer will use its associated color when
clearing the screen. So more about the function - it takes a renderer as an
argument, followed by red, green, blue, and alpha (transparency) values. I'm
going to make mine white (0xFF, 0xFF, 0xFF, 0xFF), but feel free to experiment.
After that, we'll be calling =SDL_RenderClear=, which takes a renderer as an
argument and, as I briefly mentioned a few lines ago, fills it with whatever
color the renderer is currently associated with. And finally, we'll call
=SDL_RenderPresent= to update the screen. This is where some people get a little
confused. Basically, in SDL you draw everything and /then/ update the screen,
meaning that you have as much time as you want to make the scene perfect before
you have to show it to the user. And we're pretty much done! Let's look at the
basic program:

#+BEGIN_SRC c :hl_lines 0
#include <stdio.h>
#include <SDL2/SDL.h>

int main(int argc, char *argv[]) {
    SDL_Window *my_cool_window;
    SDL_Renderer *my_cool_renderer;

    if (SDL_Init(SDL_INIT_VIDEO)) {
       fprintf(stderr, "ERROR: %s\n", SDL_GetError());
       return 1;
    }

    my_cool_window = SDL_CreateWindow("Bush Did Harambe",
                                      SDL_WINDOWPOS_UNDEFINED,
                                      SDL_WINDOWPOS_UNDEFINED,
                                      400, 400, SDL_WINDOW_SHOWN);
    if (!my_cool_window) {
        fprintf(stderr, "ERROR: %s\n", SDL_GetError());
        return 1;
    }

    my_cool_renderer = SDL_CreateRenderer(my_cool_window,
                                          -1,
                                          SDL_RENDERER_ACCELERATED);
    if (!my_cool_renderer) {
       fprintf(stderr, "ERROR: %s\n", SDL_GetError());
       return 1;
    }

    SDL_SetRenderDrawColor(my_cool_renderer, 0xFF, 0xFF, 0xFF, 0xFF);
    SDL_RenderClear(my_cool_renderer);
    SDL_RenderPresent(my_cool_renderer);

    SDL_Delay(4000);
    SDL_DestroyRenderer(my_cool_renderer);
    SDL_DestroyWindow(my_cool_window);
    SDL_Quit();
    return 0;
}
#+END_SRC

Let's run it:

#+CAPTION: A screenshot of the completed window on my system.
[[./sdl-tutorial-part-0x00-finished-window.png]]

To recap, we learned about:

* SDL Functions

- *SDL_Init*: Used to initialize SDL. Takes a flag as a parameter. Returns 0 if
  it succeeds, or a negative value if it fails.
- *SDL_Quit*: Complements =SDL_Init=. Takes no parameters and returns nothing.
- *SDL_CreateWindow*: Creates a window and returns a pointer to it, or =NULL= if
  it fails. Takes a title, X and Y positions, width, height, and a flag as
  parameters.
- *SDL_DestroyWindow*: Complements =SDL_CreateWindow=, takes a =SDL_Window=
  pointer as an argument and returns nothing.
- *SDL_CreateRenderer*: Called to create a renderer, and returns a pointer to
  it, or =NULL= if it fails. Takes the =SDL_Window= pointer for the window you
  want to create a renderer for, an index (usually -1), and a flag as
  parameters.
- *SDL_DestroyRenderer*: Complements =SDL_CreateRenderer=. Takes a
  =SDL_Renderer= pointer as an argument and returns nothing
- *SDL_Delay*: Takes a number of milliseconds as an argument, and proceeds to
  wait for that period of time.
- *SDL_SetRenderDrawColor*: Takes a renderer, red, green, blue, and alpha values
  as arguments, and changes the color associated with the given renderer.
- *SDL_RenderClear*: Takes a renderer as an argument and fills it with whatever
  color is currently associated with that renderer.
- *SDL_RenderPresent*: "Refreshes" the renderer, presenting the image to the
  user.

* Type Definitions

- *SDL_Window*: Captures the result of =SDL_CreateWindow=.
- *SDL_Renderer*: Captures the result of =SDL_CreateRenderer=.

And if you would like to read more, here are some additional resources:

- [[https://wiki.libsdl.org/SDL_Init#Remarks][SDL Documentation - SDL_Init (Remarks)]]
- [[https://wiki.libsdl.org/SDL_WindowFlags][SDL Documentation - Window Flags]]
- [[https://wiki.libsdl.org/SDL_RendererFlags][SDL Documentation - Renderer Flags]]

You're still reading? Well, this is my first tutorial ever. If you have any
feedback, be it positive or negative, I'd love to hear it! I hope this tutorial
was helpful, there are many more to come.