Looking for a good first place to contribute to Gecko gfx?

I heard we don’t have enough mentored bugs filed for the gfx code. If you are interested in contributing to Gecko’s graphics code, read on.

Contributing to Gecko for the first time can be scary because Gecko is a complex beast and it is very easy to get overwhelmed by the amount of code. Trying to understand it all is impossible, and knowing what parts are important to understand and where to start is hard when you approach a code base for the first time.

This is why mentoring bugs is useful.
A mentored bug is a bug that is approachable for new contributors, and that has someone who feels familiar with the code volunteering to mentor.
The mentor a bug already has a good understanding of the code and can help new contributors get started, show them where to look and explain the non-trivial things that may get in the way. Actually, the mentor probably already knows how to fix the bug, but he chooses to let someone else do it because he thinks the task is a good way to get started contributing.

Josh Matthews made a pretty cool tool to help finding mentored bugs.

Enough with the introduction, let’s talk about a good place where newcomers can start contributing to Gecko’s graphics code.

In gecko we have two drawing APIs:

Thebes, which is for the most part a wrapper around Cairo, is what we have been using for a while.

Moz2D, a higher level API that has several backends (including Cairo) is a more recent API that we want to use instead of thebes. Migrating from Thebes to Moz2D is a long process that has been going on for a while and will keep us busy for while too. It is a very good place to start contributing, because a lot of it isn’t very hard. Both APIs fulfill the same roles. So in many cases it is mostly about finding uses of thebes class (such as gfxIntSize, gfxIntRect, gfxImageSurface, etc.) and replace them with Moz2D equivalent (gfx::IntSize, gfx::IntRect, gfx::DataSourceSurface, etc.).

Moving from Thebes to Moz2D is important for us to make Gecko’s code more awesome because it makes Gecko easier to maintain and lets us build our graphics stack on top of better and more future-proof foundations.

We already filed a few mentored bugs to port from Thebes to Moz2D, such as https://bugzilla.mozilla.org/show_bug.cgi?id=882113 and the dependent bugs, and already received contributions, which is great! More of these bugs will be filed as we still have a lot of Thebes in the code.

If you are interested in helping there, the rest of this post contains some useful information:

Moz2D code lives in the gfx/2d/ directory.
Thebes code lives in the gfx/thebes/ directory.
gfx2DGlue.h contains a lot of small helper to convert from one API to the other.

Edit: For the curious, more info about Moz2D here: https://wiki.mozilla.org/Platform/GFX/Moz2D

Converting the simple classes like size, point and rectangle classes is rather trivial because they mostly have the same interfaces.
A class that is also interesting to convert is Thebes’ gfxImageSurface.
gfxImageSurface represents an image which pixels are stored in memory and can be initialized and accessed through pointers (as opposed to, say a texture on the GPU or some vector shapes that haven’t been rasterized yed).
Typically we use gfxImageSurface when we have some producer code that creates an image in memory and we want to wrap it in an object:

RefPtr<gfxImageSurface> thebesSurface = new gfxImageSurface(dataPointer, size, stride, format);
// we can use this image as any other surface type and also access the buffer through pointers

Or when we want to access an image’s buffer to pass it to some other API that doesn’t talk in term of thebes surfaces (like OpenGL texture uploads).

So for these two use cases we need an image abstraction that ensures that the image data is constituted of pixels and accessible by pointers, and that is what gfxImageSurface provides.
The Moz2D equivalent is mozilla::gfx::DataSourceSurface.
It offers pretty much the same thing and passing from one to the other in the use cases described above is rather easy.

One difference though is that while gfxImageSurface can be instanciated directly with the new operator, DataSourceSurface is created using gfx::Factory::CreateDataSourceSurface and gfx::Factory::CreateWrappingDataSourceSurface (static methods so you don’t need to hold on to a Factory object).

Some pieces of advise:

  • It is very easy to fall in the snowball effect of converting one function parameter from Thebes to Moz2D and then fix all the code that depends on it, and then all the code that depends on that code, etc. Don’t hesitate to make small patches that convert a method or a class to Moz2D and then use the helpers in gfx2DGlue.h to do the glue on the calling code. Then on a subsequent patch the calling code can be ported to Moz2D, etc. This way we avoid hug monster-patches that are awful to review and rebase. Using mercurial queues help a lot with this workflow (having several patches in a certain order and going back and forth between them to modify them).
  • When the task is to do some mechanical conversion, you can try to understand the surrounding code out of curiosity but you really don’t need to. I have seen motivated new contributors trying to understand all of Gecko’s graphics code before doing a simple patch and ending up not doing the patch because understanding everything all at once is hard and discouraging. Try to limit yourself to what you think is needed for the task. When in doubt ask questions to the mentor of the bug, ask about what parts of the code are important to understand, and don’t hesitate voice what you intend to do before doing it so that the mentor can tell you if your are going in the wrong direction before you have spent too much time on it.
  • Again, ask questions. On the bug, email the mentor and on IRC (#gfx about graphics stuff #introduction about problems like building firefox). Sometimes on IRC someone asks a question and nobody answers it. This may feel intimidating and rude, but in fact it just means no-one that is watching IRC at this very moment knows the answer to your question. Perhaps the people who could answer are in a different time-zone or a different IRC channel, In any case you can send an email or ping your mentor on IRC and if he is not swamped with work he will try to help you asap or direct you to the right person to ask.

So if you are looking to contribute to gfx but haven’t yet found a starting point, this is a good one.

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