GSoC Report 1

The first two weeks of GSoC coding period have ended. I started to work early in the bonding period and got many things done.

May 4 - May 30

I was a bit scared after Adrien ported the gamepad system to C. I am a huge fan of Vala and had never done Gtk with C before. I spent a lot of time understanding how C with GObject works. GObject has to support many language bindings and hence the C code is highly verbose. One thing for sure is that C makes us more aware of many language features that we often take for granted.

After I found and fixed a few bugs that popped up during the porting, I started working on the gamepad mapping configuration.

What is it?

The goal is to allow the user to configure what each gamepad input does. These are the main components of the gamepad mapper system:

  • Gamepad View
    Show the standard gamepad and highlight its individual inputs.

  • Gamepad Mapping Builder
    Build the SDL string from the user provided inputs.

  • Gamepad Mapper
    Coordinate the work between UI and backend, and display relevant messages.

  • Gamepad Preferences Window
    Show the gamepads1.

What is interesting?

  • How to highlight gamepad inputs?

    I could have used Adrien’s old demo code to manually draw the gamepad:

    But SVGs provide a more accurate representation of the image, and also allow us to query and highlight their elements, so I started looking for relevant libraries.

    “Do you want to render non-animated SVGs to a Cairo surface with a minimal, no-nonsense API?” - Librsvg’s README

    All Librsvg can do for us is to render SVGs! How do I get it to highlight stuff?

    I still favoured Librsvg because it is a part of GNOME. I found two functions of use in Librsvg’s Vala API, one to render an SVG and one to render an SVG sub. I knew it could be exploited somehow.

    • Try 1: Render the gamepad SVG and render the button’s sub again

      What? How did I even expect it to work?

    • Try 2: Render the gamepad SVG and render the button’s sub with Cairo.Operator.CLEAR

    • Try 3: Paint the surface with blue, render the gamepad SVG and render the button’s sub with Cairo.Operator.CLEAR

      "Hello darkness, my old friend"
      </figure>

    I tried a lot of things, but before I and darkness became best friends, I understood that with my negligible Cairo skills, I wouldn’t be able to do it. So I went to Cairo’s IRC and @psychon immediately solved my problem.

    • Try 4: Draw button’s sub to another surface and use it as a mask to paint the gamepad SVG’s surface

  • SDL string for gamepad mapping

    Here’s an example of an SDL string. My task was to build them for user’s input mappings.

    03000000790000000600000010010000,DragonRise Inc. Generic USB Joystick,platform:Linux, x:b3,a:b2,b:b1,y:b0,back:b8,start:b9, dpleft:h0.8,dpdown:h0.4,dpright:h0.2,dpup:h0.1, leftshoulder:b4,lefttrigger:b6,rightshoulder:b5,righttrigger:b7, leftstick:b10,rightstick:b11, leftx:a0,lefty:a1,rightx:a3,righty:a4,

    Games was already using the SDL strings provided in SDL GameControllerDB as defaults. I read how Games parsed those strings, and reversed engineered2 how to build them.

    Let’s take one part of it as an example: start:b9, if button 9 is pressed on this gamepad with this mapping, then it will map to the start button of the standard gamepad. So while building the mapping, if I highlight the start button on the gamepad, and if button 2 is pressed then I’ll build a string with start:b2.

    This button to button mapping example was simple but the dpad mappings was a lot more complicated, it took me a while to do it completely.

June 1 - June 15

I was fairly busy for the first two weeks of June. I made some major enhancements to the previous work but didn’t go full steam. My aim is to finish the gamepad mapping configuration for the first GSoC evalution.

What is remaining?

  • Save the mappings to user directory
  • Allow user to reset mappings to default

I also end up doing many things at once. Whenever I find bugs/enhancements in the nearby code, I start working on it. While working on gamepad mapper, I filed new bugs and dived into code that I don’t even need for this project.

I will add the working videos/gifs in the next post. Stay tuned :D


  1. We’ll get back to this in July 

  2. I had to use it somewhere, sounds so cool! 

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