Qt Quick hates me

August 17, 2012

I haven’t blogged since a while, because I do not do much KDE hacking lately, but today I’ve got around to playing with QML again.

The goal was to build something like a breadcrumb navigation widget, where the current directory is indicated by its name being written in a larger font. Here’s what I arrived at after a few minutes:

import QtQuick 1.1

Rectangle {
    width: 800
    height: 100
    color: "black"

    Row {
        anchors.horizontalCenter: parent.horizontalCenter
        anchors.top: parent.top
        spacing: 10

        Repeater {
            id: repeater
            property int currentIndex: 4
            model: ["/home", "/username", "/some", "/pseudo", "/path"]

            Text {
                text: modelData
                font.pixelSize: index == repeater.currentIndex ? 40 : 20
                color: "white"

Close enough. Just adjust that alignment:

Text {
    height: 60
    verticalAlignment: Text.AlignBottom

Gah! Can you see it? The baselines are misaligned. Usually, you lay out text such that all letters “sit” on an even line (the baseline).

The documentation does not mention any type baseline alignment in a Row element (or any type of vertical alignment in a Row element, for that matter).

I then tried to hack around this problem by dropping the Row element and laying out the Text elements manually, but the elements in the repeater won’t anchor to each other. That’s probably a problem with the elements not being fully initialized when the anchor property is evaluated, but at this point I give up.

This has been my second experience with Quick. When I tried it for the first time, my goal was a Quick version of the Planarity game. I got stuck when I noticed that Qt Quick 1 does not have any kind of line or ellipse primitive (only rectangles and images). Really?!?

So, while I understand very well and appreciate the idea behind Qt Quick, I have come to the conclusion that Qt Quick hates me. Or I have a talent of hitting a roadblock within ten minutes, regardless of the general direction.

P. S. Turns out the WordPress WYSIWYG editor hates me, too. (Well, either me or the combination of <pre> and images.)

For my thesis, I have implemented a series of numerical applications. The core application (written in C++) uses simple INI-like configuration files for reading system parameters. For example:


Nothing exciting there, reading this is a breeze with QSettings. Now I needed to do some symbolic calculations, for which I chose Python and the excellent SymPy module. To read the system configuration, I chose the standard ConfigParser module.

parser = ConfigParser.RawConfigParser()
sysType = parser.get("system", "name")
# read system parameters
sysParams = dict()
for key, value in parser.items("system"):
    if value != "name":
        sysParams[key] = float(value)

That reads the “system/name” key, and puts all other keys (which are assumed to be floats) into a dictionary, so the system-specific code does not need to deal with the ConfigParser object.

r1, r2 = params.get("r1", 1.0), params.get("r2", 1.0)
V1, V2 = params.get("V1", -0.1), params.get("V2", +0.1)

This code gets the parameters in the system-specific code. Nothing exciting here. Except, stuff doesn’t work. The results of the following calculations suddenly don’t match my expectations (or the predictions made by my other programs).

Since the code after the snippet above is about 30 lines of SymPy magic (accumulating expressions that print dozens of lines onto my terminal), I suspected that some error occurred there. After about two days of searching for the problem there, and calculating everything by hand, something occurred to me when I looked at the debug output:

DEBUG V1 = -0.1

Didn’t the configuration file say “V1=-0.05”? Let’s have a look at the parameter dict:

{'v1': -0.05, 'v2': 0.1, 'r1': 1.0, 'r2': 1.5}

See the problem? “v1” has a lower-case “v”, so params.get("V1", -0.1) fails and returns the default value (-0.1). One glimpse at the documentation says that

parser.optionxform = str

solves the problem. Gah!

Lost for words

November 4, 2011

commit 740673c11aee9762987e3a205443ce1dec11aee8
Author: Stefan Majewsky <majewsky@gmx.net>
Date:   Sat Nov 5 00:11:37 2011 +0100


diff --git a/tagaro/interface/board.cpp b/tagaro/interface/board.cpp
index 199c007..17dbdfc 100644
--- a/tagaro/interface/board.cpp
+++ b/tagaro/interface/board.cpp
@@ -42,7 +42,6 @@ Tagaro::Board::~Board()
-       QList<Tagaro::SpriteObjectItem*> m_items;
        for(QList<Tagaro::SpriteObjectItem*>::const_iterator a = m_items.constBegin(); a != m_items.constEnd(); ++a)

Is it just a feeling, or has the number of technical posts decreased on the Planet? I definitely need to change that! (Update: The post got quite long, so I’ve shortened the syndicated version.)

These days, I’m usually working on my Diplomarbeit at the Institute of Theoretical Physics at the Technical University here in Dresden. (A Diplomarbeit is about comparable to a master’s thesis, though preparation time is one year since there was no bachelor’s thesis before it.) Our working group has an in-house application for inspecting iterated functions, which is quite intuitively called Iterator. Like many scientific applications, it has on one hand a very sharp functional objective, but contains tons of different tools and plugins.

As a part-time project besides my usual work, I’m working on a Qt port of the Iterator, which is currently based on wxWidgets. Of course, a port offers the opportunity to refactor badly engineered parts of the application. It clearly shows that all involved developers are first and foremost physicists which have not received any formal training in software design. Many antipatterns can be observed in the code. The most interesting is the existence of a god class called IteratorApp. Read the rest of this entry »

Implicit cast quirks

April 15, 2011

I thought I let you know why I just had to throw away three days worth of computation data.

double importantPrefactor(bool freeEvolution);

void multistep(bool freeEvolution, /* ... */)
    /* ... */
    const double pre = importantPrefactor(free);
    /* ... */

Yes, I used the wrong identifier inside the multistep function. Still it compiles and runs without error (except for the garbage data that comes out), because there’s a goddamn implicit cast from void(*)(void*) to bool. If you don’t believe this, try this:

qDebug() << "Is there a goddamn cast?" << ((bool) ::free);

Of course the conversion gives true because the function pointer is not null. Argh!

[Before I get to the boring (hah!) numerics stuff, some news: In order to test KDE Games on a touchscreen, and because I’ve wanted a tablet for a long time now, and because I want a 3G device, I’ve now ordered a WeTab 2. More news on this coming up soon when I have it in my hands.]

I’ve been optimizing my favorite jigsaw puzzle game Palapeli lately. Today, I looked at the bevelmap generation algorithm which renders the nice three-dimensional shadows atop the jigsaw pieces. There are a lot of norm computations in there, i.e. sqrt(x^2+y^2). All bells are ringing: There must be a way to optimize this.

Now nearly everyone who has to do with numerics and programming probably knows about the fast inverse square root which approximates 1/sqrt(x) very efficiently and sufficiently precise (about 1% maximum error). Using the inverse of this fast inverse square root does not really give an improvement, though. After all, the formula 1/InvSqrt(x*x+y*y) contains at least four floating-point operation plus everything that happens in InvSqrt.

This approach is especially inefficient because, in my special case, x and y are integers, and the result is also needed as an integer. Looking around the interwebs, I’ve found this lovely page on square root calculation, which also has a special algorithm for norms.

It all starts with two approximations to sqrt(x^2+y^2). The first one is max(abs(x), abs(y)), the second one is 1/sqrt(2)*(abs(x) + abs(y)). We need both because they are good for different values of x and y. To understand this, consider the unit circles of these approximations, that is: the set of x/y points where the approximation is 1. (You will probably make a sketch as you follow the discussion. I would have done one if it wasn’t so late already.) The unit circle for the first approximation is an upright square, while the second approximation’s unit circle is diamond-shaped (i.e. turned around by 45° compared to the first one).

An approximation is good when the unit circle comes nearby or touches the actual unit circle. For the first approximation, this happens when either x or y vanishes. The second approximation, on the other hand, is good when x and y are approximately equal. Now one can show that, at these conditions, the better approximation is always bigger than the other one. This means that we can define a third approximation by taking the maximum of both approximations. The unit circle for this approximation is a regular octagon, whose inscribed circle is the actual unit circle.

The approximation is thus always too big, except for the eight touching points of octagon and real unit circle, where approximation and exact result are identical. However, one can find a factor z such that the octagon, scaled by 1/z, is inside the circle. Scaling the approximation by (1+1/z)/2 thus distributes the approximational error to both the negative and positive side. Taking all these considerations into account, I’ve arrived at the following implementation:

template<typename T> struct fastnorm_helper { typedef T fp; };
template<> struct fastnorm_helper<int> { typedef qreal fp; };

//Approximates sqrt(x^2 + y^2) with a maximum error of 4%.
//Speedup on my machine is 30% for unoptimized and up to 50% for optimized builds.
template<typename T> inline T fastnorm(T x, T y)
    //need absolute values for distance measurement
    x = qAbs<T>(x); y = qAbs<T>(y);
    //There are two simple approximations to sqrt(x^2 + y^2):
    //  max(x, y) -> good for x >> y or x << y          (square-shaped unit circle)
    //  1/sqrt(2) * (x + y) -> good for x = y (approx.) (diamond-shaped unit circle)
    //The worse approximation is always bigger. By taking the maximum of both,
    //we get an octagon-shaped upper approximation of the unit circle. The
    //approximation can be refined by a prefactor (called a) below.
    typedef typename fastnorm_helper<T>::fp fp;
    static const fp a = (1 + sqrt(4 - 2 * qSqrt(2))) / 2;
    static const fp b = qSqrt(0.5);
    const T metricOne = qMax<T>(x, y);
    const T metricTwo = b * (x + y);
    return a * qMax(metricOne, metricTwo);
    //idea for this function from http://www.azillionmonkeys.com/qed/sqroot.html
    //(though the equations there have some errors at the time of this writing)

[License: I grant any entity the right to use the above code snippet for any purpose, without any conditions, unless such conditions are required by law. (This is the best approximation to placing the code in the public domain, which is not possible in my jurisdiction.)]

I have tested this function successfully on random input with T = double, float, int32_t. For integer types, this function needs only two floating-point multiplications, and thus runs much much faster than the trivial implementation and the InvSqrt-based solution. So please take this as my contribution towards Green IT, and reduce the time your computer spends calculating norms.

P.S. The extension of the above function for three-dimensional coordinates remains as an exercise to the reader. The only hurdle is to calculate the new prefactor z.

Update: As correctly pointed out by the commenters, if all you want is to compare the norm to some radius, e.g. sqrt(x*x + y*y) > r*r, the easiest solution is to avoid the square root and test for x*x + y*y > r*r instead.

I just spent over an hour shouting at my editor, and I want to save you from that, so I’m telling ya: If you call QDeclarativeComponent::create and get the error message above, this is (unlike stated elsewhere) not an indication for a specific type of error.

In fact, the words “Component is not ready” are wrong in this case and should be replaced by “I stopped loading your QML because some errors have occurred. If you were in qmlviewer right now, it would take care of printing these errors, but you aren’t, so you need to do qDebug() << component.errors() yourself, but I won't tell ya."