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SGE Fundamentals

The SGE Game Engine (“SGE”, pronounced like “Sage”) is a general-purpose 2-D game engine. It takes care of several details for you so you can focus on the game itself. This makes more rapid game development possible, and it also makes the SGE easy to learn.

The SGE is free/libre software, and the SGE documentation is free/libre as well.

Although it isn’t required, you are encouraged to release your games’ code under a free/libre software license, such as the GNU General Public License, the Expat License, or the Apache License. Doing so is easy, does not negatively affect you, and is highly appreciated as a contribution to a free society.

SGE Concepts

Events

The SGE uses an event-based system. When an event occurs, a certain event method (with a name that begins with event_) is called. To define actions triggered by events, simply override the appropriate event method.

At a lower level, it is possible to read “input events” from sge.game.input_events and handle them manually. See the documentation for sge.input for more information. This is not recommended, however, unless you are running your own loop for some reason (in which case it is necessary to do this in order to get input from the user).

Position

In all cases of positioning for the SGE, it is based on a two-dimensional graph with each unit being a pixel. This graph is not quite like regular graphs. The horizontal direction, normally called x, is the same as the x-axis on a regular graph; 0 is the origin, positive numbers are to the right of the origin, and negative numbers are to the left of the origin. However, in the vertical direction, normally called y, 0 is the origin, positive numbers are below the origin, and negative numbers are above the origin. While slightly jarring if you are used to normal graphs, this is in fact common in 2-D game development and is also how pixels in most image formats are indexed.

Except where otherwise specified, the origin is always located at the top-leftmost position of an object.

In addition to integers, position variables are allowed by the SGE to be floating-point numbers.

Z-Axis

The SGE uses a Z-axis to determine where objects are placed in the third dimension. Objects with a higher Z value are considered to be closer to the viewer and thus will be placed over objects which have a lower Z value. Note that the Z-axis does not allow 3-D gameplay or effects; it is only used to tell the SGE what to do with objects that overlap. For example, if an object called spam has a Z value of 5 while an object called eggs has a Z value of 2, spam will obscure part or all of eggs when the two objects overlap.

If two objects with the same Z-axis value overlap, the object which was most recently added to the room is placed in front.

The Game Loop

There can occasionally be times where you want to run your own loop, independent of the SGE’s main loop. This is not recommended in general, but if you must (to freeze the game, for example), you should know the general game loop structure:

while True:
    # Input events
    sge.game.pump_input()
    while sge.game.input_events:
        event = sge.game.input_events.pop(0)

        # Handle event

    # Regulate speed
    time_passed = sge.game.regulate_speed()

    # Logic (e.g. collision detection and step events)

    # Refresh
    sge.game.refresh()

sge.Game.pump_input() should be called every frame regardless of whether or not user input is needed. Failing to call it will cause the queue to build up, but more importantly, the OS may decide that the program has locked up if it doesn’t get a response for a long time.

sge.Game.regulate_speed() limits the frame rate of the game and tells you how much time has passed since the last frame. It is not technically necessary, but using it is highly recommended; otherwise, the CPU will be working harder than it needs to and if things are moving, their speed will be irregular.

sge.Game.refresh() is necessary for any changes to the screen to be seen by the user. This includes new objects, removed objects, new projections, discontinued projections, etc.

Global Variables and Constants

sge.IMPLEMENTATION

A string indicating the name of the SGE implementation.

sge.BLEND_NORMAL

Flag indicating normal blending.

sge.BLEND_RGBA_ADD

Flag indicating RGBA Addition blending: the red, green, blue, and alpha color values of the source are added to the respective color values of the destination, to a maximum of 255.

sge.BLEND_RGBA_SUBTRACT

Flag indicating RGBA Subtract blending: the red, green, blue, and alpha color values of the source are subtracted from the respective color values of the destination, to a minimum of 0.

sge.BLEND_RGBA_MULTIPLY

Flag indicating RGBA Multiply blending: the red, green, blue, and alpha color values of the source and destination are converted to values between 0 and 1 (divided by 255), the resulting destination color values are multiplied by the respective resulting source color values, and these results are converted back into values between 0 and 255 (multiplied by 255).

sge.BLEND_RGBA_SCREEN

Flag indicating RGBA Screen blending: the red, green, blue, and alpha color values of the source and destination are inverted (subtracted from 255) and converted to values between 0 and 1 (divided by 255), the resulting destination color values are multiplied by the respective resulting source color values, and these results are converted back into values between 0 and 255 (multiplied by 255) and inverted again (subtracted from 255).

sge.BLEND_RGBA_MINIMUM

Flag indicating RGBA Minimum (Darken Only) blending: the smallest respective red, green, blue, and alpha color values out of the source and destination are used.

sge.BLEND_RGBA_MAXIMUM

Flag indicating RGBA Maximum (Lighten Only) blending: the largest respective red, green, blue, and alpha color values out of the source and destination are used.

sge.BLEND_RGB_ADD

Flag indicating RGB Addition blending: the same thing as RGBA Addition blending (see sge.BLEND_RGBA_ADD) except the destination’s alpha values are not changed.

sge.BLEND_RGB_SUBTRACT

Flag indicating RGB Subtract blending: the same thing as RGBA Subtract blending (see sge.BLEND_RGBA_SUBTRACT) except the destination’s alpha values are not changed.

sge.BLEND_RGB_MULTIPLY

Flag indicating RGB Multiply blending: the same thing as RGBA Multiply blending (see sge.BLEND_RGBA_MULTIPLY) except the destination’s alpha values are not changed.

sge.BLEND_RGB_SCREEN

Flag indicating RGB Screen blending: the same thing as RGBA Screen blending (see sge.BLEND_RGBA_SCREEN) except the destination’s alpha values are not changed.

sge.BLEND_RGB_MINIMUM

Flag indicating RGB Minimum (Darken Only) blending: the same thing as RGBA Minimum blending (see sge.BLEND_RGBA_MINIMUM) except the destination’s alpha values are not changed.

sge.BLEND_RGB_MAXIMUM

Flag indicating RGB Maximum (Lighten Only) blending: the same thing as RGBA Maximum blending (see sge.BLEND_RGBA_MAXIMUM) except the destination’s alpha values are not changed.

sge.game

Stores the current sge.Game object. If there is no sge.Game object currently, this variable is set to None.

Information specific to the Pygame SGE

License

The Pygame SGE is free software: you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

The Pygame SGE is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with the Pygame SGE. If not, see <http://www.gnu.org/licenses/>.

Dependencies

Formats Support

sge.Sprite supports the following image formats:

  • PNG
  • JPEG
  • Non-animated GIF
  • BMP
  • PCX
  • Uncompressed Truevision TGA
  • TIFF
  • ILBM
  • Netpbm
  • X Pixmap

sge.Sound supports the following audio formats:

  • Uncompressed WAV
  • Ogg Vorbis

sge.Music supports the following audio formats:

  • Ogg Vorbis
  • MP3 (support limited; use not recommended)
  • MOD
  • XM
  • MIDI

For starting position in MOD files, the pattern order number is used instead of the number of milliseconds.

If Pygame is built without full image support, sge.Sprite will only be able to load uncompressed BMP images.

The pygame.mixer module, which is used for all audio playback, is optional and depends on SDL_mixer; if pygame.mixer is unavailable, sounds and music will not play.

On some systems, sge.Music attempting to load an unsupported format can crash the game. Since MP3 support is limited, it is best to avoid using it; consider using Ogg Vorbis instead.

Missing Features

sge.Sprite.draw_line(), sge.Room.project_line(), and sge.Game.project_line() support anti-aliasing for lines with a thickness of 1 only. sge.Sprite.draw_polygon(), sge.Room.project_polygon(), and sge.Game.project_polygon() support anti-aliasing for outlines of polygons with a thickness of 1 only. sge.Sprite.draw_text(), sge.Room.project_text(), and sge.Game.project_text() support anti-aliasing in all cases. No other drawing or projecting methods support anti-aliasing.

Known Problems

There is a bug in either Pygame or SDL, most likely SDL, which sometimes causes keyboard input to stop working. In Pygame programs such as this one, this occurs when pygame.display.set_mode is called multiple times, which in the Pygame SGE occurs any time either the size of the window or the video mode (windowed or fullscreen) changes. See this post from the SGE blog for more information:

https://savannah.nongnu.org/forum/forum.php?forum_id=8113

You may also be interested in this report on the Pygame issue tracker:

https://bitbucket.org/pygame/pygame/issue/212/

As mentioned in the post on the SGE blog, this is a particularly serious problem for anyone using the X Window System (e.g. pretty much any GNU/Linux user), or any other window system that gives complete control to fullscreen SDL applications. On these systems, if the game requires keyboard input to either leave fullscreen or exit, the system will become unresponsive to everything that isn’t sent directly to the kernel (such as the magic SysRq key in Linux systems).

Luckily, the bug doesn’t seem to affect mouse input, so if you allow the player to enter fullscreen mode in-game, it is highly recommended for you to provide some method of either exiting the game or exiting fullscreen with the mouse. This can be a button somewhere on the screen if the game uses the mouse cursor, or it can be a simple mouse button click otherwise.

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