[Interest] QOpenGLWidget and text

[Matthew Woehlke]

On 2015-04-04 05:57, Till Oliver Knoll wrote:
> Am 03.04.2015 um 17:26 schrieb Matthew Woehlke:
>> On 2015-04-02 13:45, Till Oliver Knoll wrote:
>>> ..... But what's next? A "drawButton"? "drawBitmap"? In a Widget?!
>>
>> Yes. And why not?
> 
> Because the class name sais *Widget, and not *Painter or *WhateverItem.
> 
> A Widget's responsibility - much like the one of a View; we'll come to that in a second... - is to provide a /surface/ to draw onto, and to manage the underlying system resources (we'll deliberately skip over other functionality, such as layout management).

Please don't be overly pedantic. The point isn't whether the ability is
particularly a member of QOpenGLWidget, but whether it is available *at
all* (in a non-deprecated class, anyway) without the user having to
concoct a complicated system for providing it.

Hopefully Qt3D will bring back these features. It's still a shame that
5.4 gets left out in the cold.

> The important point to take away here is that a /Widget/ itself never exposes functionality to /draw/ something (exceptions might prove my thesis ;)). You may draw /onto/ a Widget - but /how/, that's the responsibility of some other class.
> 
>> QGraphicsView already does this sort of thing.
> 
> It's funny that you mention that /View/ class, since it does exactly /not/ "this sort of thing".

Actually it does. It does it via a helper class, but as above, that's
not really the point. (And anyway, what I meant was 'the QGV-based
graphics framework'.)

> That sounds to me you really want to look into Qt3D, if you want this kind of convenience.

Well, yes, but doesn't help 5.4.

>> The problem is I don't *want* to draw "over" GL¹. I *need* to draw *in*
>> GL, specifically I need to specify the text location in terms of the GL
>> modelview space.
>> ...
>> Overpainting techniques are fine... *for overpainting*. That's not what
>> I'm doing.
> 
> Clearly I cannot predict your application's requirements, but "overpainting", that's exactly what you were doing apparently, all the time. You said so yourself, you were using
> 
>  http://doc.qt.io/qt-4.8/qglwidget.html#renderText

True, and it does happen that the QGLWidget method is also technically
overpainting. Fortunately for my use case, this was not an issue (the
bit I'm drawing is itself an overlay, albeit a 3D one that needs to be
rendered with GL), but for this case it would be technically preferable
to actually rendering the text into the GL depth buffer (if unlikely to
make a practical difference).

In fact that has been the case for every time I've used renderText (in
multiple applications); I am drawing annotations *in GL world
coordinates* that are expected to be visible 'on top' anyway.

> I've never used this method myself, but from what I gather you specify the text coordinates in /window/ coordinates - /not/ world (or model) coordinates!

That is *one option*, and not the one I use. Look again; there is an
overload that takes *world* coordinates. This is the one I use, and I
can assure you it works.

> You get exactly the same result (probably even a better one due to better anti-aliasing) by using a QPainter and "overpaint" at the same window coordinates.

Actually, not better; exactly the same :-).

-- 
Matthew