# SGE Fundamentals

Contents

- SGE Fundamentals
  - SGE Concepts
    - Events
    - Position
    - Z-Axis
    - The Game Loop
  - Global Variables and Constants

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 libre software, and the SGE documentation (including all docstrings) is released to the public domain via CC0.

Although it isn’t required, you are encouraged to release your games’ code under a 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.dsp.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.dsp.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.dsp.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.``SCALE_METHODS`

  A list of specific scale methods supported by the SGE implementation.

  Note

  This list does not include the generic scale methods, `"noblur"` and `"smooth"`. It is also possible for this list to be empty.

`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.dsp.Game` object. If there is no `sge.dsp.Game` object currently, this variable is set to `None`.