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17 Apr 2024 : Day 219 #
Today has been a bit of a disheartening day from a Gecko perspective. So disheartening in fact that I considered pausing this blog, taking some time off and then trying to fix it in private to avoid having to admit how badly things are going right now.

But it's important to show the downs as well as the ups. Software development can be a messy business at times, things rarely go to plan and even when they do there's still often an awful lot of angst and frustration preceding the enjoyment of getting something working.

So here it is, warts and all.

Overnight I rebuilt the packages with the new patches installed. Running the WebView showed no changes in the output: still a white screen, no page rendering or coloured backgrounds. I can live with that, it's not what I wanted but it's also no worse than before.

So I decided to head off in the direction that was set last Thursday when I laid plans to check the implementation that happens between the DeclarativeWebContainer::clearWindowSurface() method and the CompositorOGL::EndFrame() method. This direction is in response to the useful discussion in last week's Sailfish Community Meeting.

First up I wanted to establish what was happening on the window side, so starting at clearWindowSurface() I added some code first to change the colour used to clear the texture from white to green.

This didn't affect the rendering, which remains white. Okay, so far so normal. That's a little unexpected, but nevertheless adds new and useful information.

I then added some code to read off the colour at the centre of the texture. This is pretty simple code and, as before, I added some debug output lines so we can find out what's going wrong. Here's what the method looks like now:
void DeclarativeWebContainer::clearWindowSurface()
    // The GL context should always be used from the same thread in which it 
    was created.
    Q_ASSERT(m_context->thread() == QThread::currentThread());
    QOpenGLFunctions_ES2* funcs = 

    funcs->glClearColor(0.0, 1.0, 0.0, 0.0);

    QSize screenSize = QGuiApplication::primaryScreen()->size();
    size_t bufferSize = screenSize.width() * screenSize.height() * 4;
    uint8_t* buf = static_cast<uint8_t*>(calloc(sizeof(uint8_t), bufferSize));

    funcs->glReadPixels(0, 0, screenSize.width(), screenSize.height(),
                            GL_RGBA, GL_UNSIGNED_BYTE, buf);

    int xpos = screenSize.width() / 2;
    int ypos = screenSize.height() / 2;
    int pos = xpos * ypos * 4;

    volatile char red = buf[pos];
    volatile char green = buf[pos + 1];
    volatile char blue = buf[pos + 2];
    volatile char alpha = buf[pos + 3];

    printf(&quot;Colour: (%d, %d, %d, %d)\n&quot;, red, green, blue, alpha);
Everything after the call to swapBuffers() is newly added.

Executing this I was surprised to find that there was no new output from this. So I tried to set a breakpoint on it, but gdb claims it doesn't exist in the binary. A bit of reflection made me realise why: this is code in sailfish-browser, which forms part of the browser, not the WebView.

So for the WebView I'll need to find the similar equivalent call. Before trying to find out where the correct call lives, I thought I'd first see what happens when I run the browser instead of the WebView. Will gdb have more luck with that?

And this is where things start to go wrong. When running with the browser the method is called once. The screen turns green. But there's no other rendering. The fact the screen goes green is good. The fact there's no other rendering is bad. Very bad.

It means that over the last few weeks while I've been trying to fix the WebView render pipeline, I've been doing damage to the browser render pipeline in the process. I honestly thought they didn't interact at this level, but it turns out I was wrong.

So now I have both a broken WebView and a broken browser to fix. Grrrr.

While I was initially quite downhearted about this, on reflection it's not quite as bad as it sounds. The working browser version is still safely stored in the repository, so if necessary I can just revert back. But I've also got all of the changes on top of this easily visible using git locally on my system. So I can at least now work through the changes to see whether I can figure out what's responsible.

I'm not going to make more progress on this tonight, so I'll have to return to it tomorrow to try to understand what I've broken.

So very frustrating. But that's software development for you.

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


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