Tag: sdl2

| Justin

Using SDL2-C# to Capture Text Input

A common feature of applications and video games is to allow the player to input text for various reasons. Maybe we want to allow the player to input their character’s name in an RPG, name a city in SimCity, or type a chat message to a friend in online. Using SDL2, we can take advantage of its built-in text processing system which abstracts much of the operating system event handles and character encoding mechanisms.

On consoles such as Xbox and Playstation, text input is rather simplistic and limited to visual keypads that you select via the controller. On a PC, we have the full range of widely varying keyboards from English and Spanish to Russian and Japanese. If we want our game or application to attract users on an international scale, it’s probably in your best interest to learn here and now how to use SDL2 to accomplish this goal.

At first glance, it probably seems simple to process text input. If the user presses the ‘A’ key on the keyboard, the OS will send an event that the keyboard was just pressed, the key was ‘A’, and no modifier keys were pressed (CAPS, SHIFT, CTRL, ALT, etc…). That’s it, right? Unfortunately, there are a ton of languages on this planet, and some of them have thousands of characters in them. People who type in those languages most certainly do not have thousand-letter keyboards or entire walls of their houses dedicated as a giant keyboard. This basically means that some characters will require multiple key presses just to process. Fortunately, SDL2 handles all of this for us and simply sends us a byte array with the results.

Among SDL2’s event processing, the structure we are interested in is SDL_TextInputEvent. This event is sent through the SDL2 event processing chain whenever text is input. I have personally seen this trigger from both physical keyboards and the Windows virtual on-screen keyboard. I am sure that there are other ways to trigger this event as well. Using this event, we can get the character information that was input by the user. Here are the fields of the C-style structure that we can use:

UInt32     type          The type of the event
UInt32     timestamp     The time that the event occurred
UInt32     windowID      The ID of the window that has focus, if any
char       text          The null-terminated, UTF-8 encoded input text

After determining that this is an SDL_TextInputEvent by checking the type field, we are most interested in the text field. That field is a pointer to a character array which is encoded using the UTF-8 scheme. In my most recent cases, I was using a C# wrapper around the SDL2 library named SDL2-CS. Because C# runs in a managed, garbage collected runtime, it’s a bit tricky to get the text input from C into C# through the .NET marshaller, but here’s how to do it.

// the character array from the C-struct is of length 32
// char types are 8-bit in C, but 16-bit in C#, so we use a byte (8-bit) here
byte[] rawBytes = new byte[SDL2.SDL.SDL_TEXTINPUTEVENT_TEXT_SIZE];
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| Justin

How to load a map from Tiled Map Editor and render it with SDL2

I recently created a library named SharpTiles that is heavily based on Nick Gravelyn’s TiledLib. Both libraries will allow developers to load maps from Tiled Map Editor. However, TiledLib is based around XNA while SharpTiles is based around SharpDL (an XNA-like game framework for SDL2). I will discuss more about SharpDL in another post, but you can refer to the simple code in this post to learn how to render a map with a small subset of the library.

Load a Map

  • Window is a SharpDL object that represents a SDL_Window
  • Renderer is a SharpDL object that represents a SDL_Renderer
  • TiledMap is the main object that you will use to load a map file
// create an SDL window at position 100,100 with size 640x480
Window window = new Window("Example 1", 100, 100, 640, 480, Window.WindowFlags.Shown)

// create an SDL rendering context based on the window above
Renderer renderer = new Renderer(Renderer.RendererFlags.RendererAccelerated);

// load and create map to be displayed in the passed Renderer
TiledMap map = new TiledMap("Maps/Map1.tmx", Renderer);

Draw a Map

You can draw a map by looping through all TileLayers (layers which containd textured tiles) and all Tiles within the layers. Make sure to check if the tile is empty before drawing it. Tiled Map Editor will create empty tiles in spots that contain no textured tiles within the layer.

  • Texture is the tileset that contains the tile’s texture
  • Position.X is the X position of the tile to draw
  • Position.Y is the Y position of the tile to draw
  • SourceTextureBounds is a rectangle that indicates where in the tileset the tile’s actual texture is contained
foreach (TileLayer tileLayer in map.TileLayers)
    foreach (Tile tile in tileLayer.Tiles)
        if (!tile.IsEmpty)
            Renderer.RenderTexture(tile.Texture, (int)tile.Position.X, (int)tile.Position.Y, tile.SourceTextureBounds);
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