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3 May 2024 : Day 235 #
As discussed at the start of the week, this is going to be my last developer diary post for a little bit. But I want to make absolutely clear that this is a temporary pause. I'm heading to the HPC Days Conference in Durham next week where I'll be giving a talk on Matching AI Research to HPC Resource. I'm expecting it to be a packed schedule and, as is often the case with this kind of event, I'm not expecting to be able to fit in my usual gecko work alongside this. So there will be a pause, but I'll be back on the 18th May to restart from where I'm leaving things today.

During the pause I'm hoping write up some of the blog posts that I've put on hold while working on the gecko engine, so things may not be completely silent around here. But the key message I want to get across is that I'm not abandoning gecko. Not at all. I'll be back to it in no time.

Nevertheless, as I write this I'm still frustrated and stumped by my lack of progress with the offscreen rendering. So I'm also hoping that a break will give me the chance to come up with some alternative approaches. Over the last couple of days I attempted to capture the contents of the surface used to transfer the offscreen texture onscreen, but the results were inconclusive at best.

On the forums I received helpful advice from Tone (tortoisedoc). Tone highlighted the various areas where texture decoding can go awry:
two things can go wrong (assuming the data is correct in the texture):
  • alignment of data (start, stride)
  • format of pixel (ARGB / RGBA etc)
Which one is the texture you are look at configured with?

Is the image the same the browser would show? Which image are you displaying in the embedded view?

These are all great points and great questions. Although I know what sort of image I'm asking for (either RGB or RGBA in UNSIGNED_BYTE format) the results don't seem to be matching that. It's made more complex by the fact that the outline of the image is there (which suggests things like start and stride are correct) but still many of the pixels are just blank. It's somewhat baffling and I think the reason might be more to do with an image that doesn't exist rather than an image which is being generated in the wrong format.

But I could be wrong and I'll continue to consider these possibilities and try to figure out the underlying reason. I really appreciate the input and as always it gives me more go to on.

But today I've shifted focus just briefly by giving the code a last review before the pause: sifting through the code again to try to spot differences.

There were many places I thought there might be differences, such as the fact that the it wasn't clear to me that the GLContext->mDesc.isOffscreen flag was being set. But stepping through the application in the debugger showed it was set after all. So no fix to be had there.

The only difference I can see — and it's too small to make a difference I'm sure — is that the Surface capabilities are set at slightly different places. It's possible that in the gap between where it's set in ESR 78 and the place it's set slightly later in ESR 91, something makes use of it and a difference results.

I'm not convinced to be honest, but to avoid even the slightest doubt I've updated the code so that the values are now set explicitly:
GLContext::GLContext(const GLContextDesc& desc, GLContext* sharedContext,
                     bool useTLSIsCurrent)
    : mDesc(desc),
          StaticPrefs::gfx_work_around_driver_bugs_AtStartup()) {
  mCaps.any = true;
  mCaps.color = true;


bool GLContext::InitImpl() {
  // TODO: Remove SurfaceCaps::any.
  if (mCaps.any) {
    mCaps.any = false;
    mCaps.color = true;
    mCaps.alpha = false;
The updated code is built and right now transferring over to my development phone. As I say though, this doesn't look especially promising to me. I'm not holding my breath for a successful render.

And I was right: no improvements after this change. Argh. How frustrating.

I'll continue looking through the fine details of the code. There has to be a difference in here that I'm missing. And while it doesn't make for stimulating diary entries, just sitting and sifting through the code seems like an essential task nonetheless.

If you'd like to read any of my other gecko diary entries, they're all available on my Gecko-dev Diary page.

If you've got this far, then not only do you have my respect and thanks, but I'd also urge you to return on the 18th May when I'll be picking things up from where I've left them today.


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