VirtualBox Failed to Save Settings

I recently installed VirtualBox configured internet connection to the internet for a virtual windows 7 machine I’ve installed using Vagrant, tried to setup USB, but got the following message: “Failed to save settings – Empty or null HostOnly interface name is not valid.”. Looking at the details, I saw the following “Result Code: E_FAIL (0x80004005) Component: NetworkAdapterWrapper …”.I don’t remember when I started to get that popup message. Couldn’t find any HostOnly adapter using the VirtualBox GUI. Finding the solution in the web is too hard. and I can’t find anything in the user guide either. But, one useful thing I know: in Linux and Unix-like system, I can find configuration files. They are usually found in directories whose names begin with ‘.’. followed by the program’s name or under “${HOME}/.config’ or directories created by the software. This time, the file was found in “${HOME}/VirtualBox VMs/<machine-name>”.
In this case: “\${HOME}/VirtualBox VMs/windows7_default_1563689015423_90469/”. In this directory there are files with the suffix ‘.vbox’. They are XML files. In one of them, named “windows7_default_1563689015423_90469.vbox”, I find the following element:

        <Adapter slot="1" MACAddress="08002718154D" cable="true" type="82540EM"&gt;
<DisabledModes&gt;
<InternalNetwork name="intnet"/&gt;
<NATNetwork name="NatNetwork"/&gt;
</DisabledModes&gt;
<HostOnlyInterface name="VirtualBox Host-Only Ethernet Adapter #2"/&gt;



Comment it out.
Problem solved.

Drawing Simple 3D Shapes in HTML5

HTML5 includes some features that allows developers to draw 3D shapes by drawing bi dimensional shapes and applying 3d transform, such as rotateX, rotateY and rotateZ. For convenience, you can shift the origin of axes using the style property If you don’t want the 3D image too flat (for example, all the faces of a cube having the same size) use perspective and perspective-origin style properties.

You can use the matrix3d style properties instead of the named transforms if, for example, you don’t want to compute angles.

The Style Properties

A style property can be defined by adding the attribute style to an HTML element, defining a CSS class or accessing a DOM node.

In this section I will explain the properties using a little Javascript program that draws a regular tetrahedron.

Drawing a tetrahedron is done by drawing 4 isosceles triangles and rotating each of them once or twice.

“perspective” and “perspective-origin”

The distance and angle from which the shape is viewed.

“perspective” holds the distance

“perspective-origin” – a position value.

For example:

        var main_div = d3.select('body')
.append('div')
.style('position','absolute')
.style('top','50px')
.style('left','50px')
.style('perspective','50px')
.style('perspective-origin','bottom left');


Transform Values: “rotateX”, “rotateY”, “rotate” and “transform-origin”

Rotate an axis. Keep the value of the rotated axis coordinate unchanged, and change the rest. The axis is rotated around the position defined by “transform-origin”

The following code adds the data for creating 4 triangles, and rotates 1 triangle 120 degrees to the right and 1 triangle 120 degrees to the left. Rotation is done around the bottom face’s centroid.

        main_div.selectAll('div').
data([{color: 'red', transform: null,upperVertexInd: true},
{color: 'black', transform: 'rotateX(90deg)', 'origin':'
100px 100px 0'},
{color: 'blue', transform: 'rotateY(120deg)',origin: cent
roidString,upperVertexInd: true},
{color: 'green', transform: 'rotateY(-120deg)',origin: ce
ntroidString,upperVertexInd: true}])
.enter()
.append('div')
.style('position','absolute')
.style('top',0)
.style('left',0)
.style('transform',d=>d.transform)
.style('transform-origin',d=>d.origin)
.style('transform-style','preserve-3d')


(To be more precise, it rotates the DIV elements)

Tarnsform Values: “matrix3d”

This matrix is used if you want to use a transformation out of the comfort zone. For example, a rotation transform with cosines and sines of the angle. The argument list contains 16 values, which are the cells of a square matrix of order 4 (4 rows and 4 columns).

This matrix will be applied on (x,y,z,w) vector to get the target vector. When rotating a 2d vector )point), our original z-coordinate will be 0, and w will be 1.

To specify the matrix:

$\left( \begin{matrix} a_0 \ a_4 \ a_8 \ a_{12} \\ a_1 \ a_5 \ a_9 \ a_{13} \\ a_2 \ a_6 \ a_{10} a_{14} \\ a_3 \ a_7 \ a_{11} \ a_{15}\end{matrix} \right)$

use

$matrix3d(a_0,a_1,a_2,...,a_{15})$

In my example, I will rotate 3 triangles, so their top vertex will go to a line perpendicular to the tetrahedron base, and passing through the base’s median.

The median of a triangle is the point where median cross its other, dividing each median at the ratio 1:2.

So, if each side of a triangle is of length 1. The height is $\sqrt(3)\over2$

Since, the height is the length of the median, the distance from a side to the centroid is the height divided by 3, and the requested sine is latex13latex 1 \over 3

The cosine is $\sqrt {1 - {1 \over 3}^2} = {\sqrt 8 \over 3}$

so, we will compute the matrix as follows:

        var rotateXCos = Math.sqrt(8) / 3;
var rotateXSin = 1 / 3;
var rotateXMat3d = [1,0,0,0,
0,rotateXCos,rotateXSin,0,
0,-rotateXSin,rotateXCos,0,
0,0,0,1];
var matrixTransformString = 'matrix3d(' + rotateXMat3d + ')';


Now, the code to draw the tetrahedron with *d3.js( is:


var side=100;
var len=100;
var height=side * Math.sqrt(3)/2;
var centroidZValue = -height / 3; // The point where medians meet.
var rotateXCos = Math.sqrt(8) / 3;
var rotateXSin = 1 / 3;
var rotateXMat3d = [1,0,0,0,
0,rotateXCos,rotateXSin,0,
0,-rotateXSin,rotateXCos,0,
0,0,0,1];
var matrixTransformString = 'matrix3d(' + rotateXMat3d + ')';
var centroidString = '150px 0 ' + centroidZValue + 'px';
var main_div = d3.select('body')
.append('div')
.style('position','absolute')
.style('top','50px')
.style('left','50px')
.style('perspective','50px')
.style('perspective-origin','bottom left');
main_div.selectAll('div').
data([{color: 'red', transform: null,upperVertexInd: true},
{color: 'black', transform: 'rotateX(90deg)', 'origin':'
100px 100px 0'},
{color: 'blue', transform: 'rotateY(120deg)',origin: cent
roidString,upperVertexInd: true},
{color: 'green', transform: 'rotateY(-120deg)',origin: ce
ntroidString,upperVertexInd: true}])
.enter()
.append('div')
.style('position','absolute')
.style('top',0)
.style('left',0)
.style('transform',d=>d.transform)
.style('transform-origin',d=>d.origin)
.style('transform-style','preserve-3d')
.append('div')
.style('transform-style','preserve-3d')
.style('position','absolute')
.style('top',0)
.style('left',0)
.style('transform',function(d){
return d.upperVertexInd?matrixTransformString:false;
})
.style('transform-origin',function(d){
return d.upperVertexInd?'0 100px 0':false;
})
.append('svg')
.append('polygon')
.attr('points',[100,100,150,100-height,200,100])
.style('fill','none')
.style('stroke',d=>d.color);



The Requested Gradient Cannot Be Found: Use the HTML5 Canvas

Some days ago I found that someone was looking for a D3.js expert. To prove one is an expert one has to pass a test, and one of the tasks on this test is to create a 3D color picker with RGB for axes.

The cube faces cannot be filled with a bi-dimensional linear gradients because such gradients are not supported. So, you have to explicitly write a loop to add the pixels.
Using SVG to add the pixels is a bad idea: SVG is an XML language, and uses a DOM tree. Using SVG will use a lot of memory and will slow down your computer. Use a canvas instead. Drawing on a canvas is done by simple Javascript commands, that add lines and shapes.
Following is a little code snippet that fills a cube face:

    for (i=x1; i<=x2;i++){
for (j=y1; j<=y2; j++){
rgb_arr[d.rgb_variable[0]]=i;
rgb_arr[d.rgb_variable[1]]=j;
ctx.fillStyle=d3.rgb(rgb_arr[0],rgb_arr[1],rgb_arr[2]);
ctx.fillRect(i,j,1,1);
}
}



Now, to get the color where the mouse points, first get the position using the mouse event’s offsetX and offsetY. These properties will hold the correct value even if the canvas is rotated.
Then you can get the RGBA values of the pixel using method getImageData of the canvas’ context. The method returns the data of a rectangle defined by 4 arguments: x,t,width and height.
Following is an example:

   canvas.on('click', function(evt){
var ctx=d3.event.target.getContext('2d');
var pixelData = ctx.getImageData(d3.event.offsetX,d3.event.offsetY,1,1).data;
});



Thunderbird Calendar Cleanup

The thought to create a copy of my Thunderbird profile did not cross my mind until the last power failure. After each power failure I found that I have to setup my mail account, and that all my events were “lost”.
Wellm the events were not exactly lost, but a new calendar id was created, which is to be used to find all the events and their relevant properties in the calendar extension’s database. The database itself is an SQLite file, and in my system is located under ‘~/.thunderbird/default-dir/calendar-data/local.sqlite’
(Change the default-dir name to your local name, it is by default the one that ends with ‘.default*.

Finding the Current Calendar Id

The calendar id is the value of the Thunderbird’s user preference ‘calendar.list.sortOrder’. You can view this variables by choosing from the menu:
preferences->preferences->preferences*, and then from the window opend, choosing the Advanced tab, and then clicking the “Config Editor* button.
You can connect to the database using:

sqlite3 /path/to/local.sqlite


From now on, I’ll assume you only have one calendar.

SQLite Tables

The tables, indexes, trigges and other database entities are stored in a table named ‘sqlite_master’. To find tables, run the query

select name from sqlite_master
where type='table';


For each table column, run the query:

pragma table_info(table_name);


The result will be:

cal_calendar_schema_version
cal_attendees
cal_recurrence
cal_properties
cal_events
cal_todos
cal_tz_version
cal_alarms
cal_relations
cal_attachments



Replace table_name by a table name, for example:

pragma table_info(cal_events)


The result will be:

0|cal_id|TEXT|0||0
1|id|TEXT|0||0
2|time_created|INTEGER|0||0
3|last_modified|INTEGER|0||0
4|title|TEXT|0||0
5|priority|INTEGER|0||0
6|privacy|TEXT|0||0
7|ical_status|TEXT|0||0
8|flags|INTEGER|0||0
9|event_start|INTEGER|0||0
10|event_end|INTEGER|0||0
11|event_stamp|INTEGER|0||0
12|event_start_tz|TEXT|0||0
13|event_end_tz|TEXT|0||0
14|recurrence_id|INTEGER|0||0
15|recurrence_id_tz|TEXT|0||0
16|alarm_last_ack|INTEGER|0||0
17|offline_journal|INTEGER|0||0


Finding the Last Calendar Id

Now, I guess that an event in the last calendar has the latest event start date.
A good query to find that event can be:

select cal_id, title, event_start
from cal_events
order by event_start;


Let us call the most recent value calendar id “old-cal-id”, and the new calendar id “new-cal-id*

Updating and Deleting

I suggest that you perform update and delete queries within transaction, so if something goes wrong you can rollback.
Begin a transaction by running the command:

begin transaction


Now, for each table that has the column cal_id, run the query:

update table_name
set cal_id="new-cal-id"
where cal_id="old-cal-id"


Replace table_name by a name of a table that has the column cal_id, for example:

update cal_event
set cal_id="new-cal-id"
where cal_id="old-cal-id";


Now, to delete the rest, run:

delete from table_name
where cal_id != "old-cal-id";


If everything’s fine, it’s time to commit your transaction by running:

COMMIT;


Main Window Operation In Matplotlib

Matplotlib is a MATLAB-like library that allows Python programmers to create images and animations. For example, you can easily draw a graphic representation of functions with Y (and maybe Z) values generated by numpy and scipy functions.
Matplotlib can also be interactive and handle events. The command mpl_connect is used for connecting an event with a callback function.

The Backend Layer

Someone on the IRC has challenged me with questions on how to perform some operations when the window is closed. In addition, I want the window title to be other than the default, “Figure 1”.

Well, the layer that handles the main window is the backend layer,
To find what backend Matplotlib uses, you can add the line
print type(fig.canvas)
The result may be something like:
<class 'matplotlib.backends.backend_gtkagg.FigureCanvasGTKAgg'>
This means that the backend used is ‘GtkAgg’.
With the function ‘dir’, I’ve found that the canvass has a function named get_toplevel, and the returned value of fig.canvass.get_toplevel() is an object of type gtk.Window.
This object has the methods of a GTK window. So you can change its title with the ‘set_titlemethod. For example: fig.canvas.get_toplevel().set_title(‘Rubic Cube’) You can tell your application what to do when the user closes the window, by calling its 'connect' method, with 'destroy' for first arguments. For example: fig.canvas.get_toplevel().connect(‘destroy’, destroyFunc, ‘Goodbye, cruel world!’) destroyFunc is a function that accept 2 arguments (3 if a class member): the widget where the event has occurred and additional user defined data.
More about Python FTK can be found at http://www.pygtk.org/pygtk2tutorial/index.html

Last but not least, you can specify the backend Matplotlib will use, by calling the ‘use’ method of matplotlib.
For example:
matplotlib.use('GTKAgg')

Note: This method should be called before importing ‘pyplot’.

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Overcoming PDF Problems With Firefox

The other day I downloaded “Fahrenheit 451” by Ray Bradbury as a free eBook, and tried to read it. I found that I cannot read much more than the titles of each chapter.

I tried double-clicking and dragging, and saw that something appears on the screen, organized in paragraphs.

Right Click->Inspect Element

Now, a sub-window is opened at the bottom of the screen:

You can see in that sub-window that some text in English appears in a ‘div’ element just under another ‘div’ element of class ‘textLayer’. To the right of the ‘Inspect Element’ sub-window you can see the CSS rules:

As you can see, the value of property ‘color’ is ‘transparent’, You can edit that property using the color selector or by overriding the text value.
Let’s set it to ‘black’.

Now, you can see the paragraphs.
You can change the opacity property of ‘div.textLayer’ from 0.2 to a higher value (up to 1) in order to read the text better.
Firefox is a great PDF viewer, but my browser couldn’t save the document with the changes: the result was a corrupt file that cannot be opened. I’d written about it to the newsgroup ‘mozilla.wishlist’ found on server ‘news.mozilla.org’. and they opened a ticket.

Before you upload an internet site, you better test it on your local machine. To do that, you should allocate an IP address known as a loopback address that does not require a modem for access. If you’ve installed Apache Httpd server, you’ll probably get an HTML page that reads “It works” upon connecting to “http://localhost” or http://127.0.0.1” from the web browser. But what if you want to create another site? How to make your server recognize an IP?

In this post I will describe by example the process of adding a local IP.

Step 1: Associate an IP with a Domain Name

If you want to create a domain name such as ‘example.coq‘, add a line for it in /etc/hosts in the format:

<inet-addr>   <alias>

For example:

127.0.0.2               example.coq

Step 2: Attach the IP Address To a Network Interface

To make an address available to internet servers, attach it to a network interface.

A network interface is the identifier followed by colons at the beginning of blocks returned by the command ifconfigFor example:

In this block lo0 is an interface name.

lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> metric 0 mtu 16384
options=600003<RXCSUM,TXCSUM,RXCSUM_IPV6,TXCSUM_IPV6>
inet6 ::1 prefixlen 128
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x2
inet 127.0.0.1 netmask 0xff000000 

From the prefix ‘lo’ of the interface name, you can know it is a loopback interface.

You can attach an address, login as root, and run ifconfig like in the following example:

ifconfig lo0 127.0.0.2 netmask 255.255.255.255 alias

This will attach IP address 127.0.0.2 to the loopback interface ‘lo0’

You can make the operating system add it each type you start your computer, by adding the following line to /etc/rc.conf:

ifconfig_lo0_alias0="inet 127.0.0.2 netmask 255.255.255.255"

You can learn more about virtual hosts from the section ‘11.6 Virtual Hosts” of the FreeBSD Handbook

Step 3: Start a Listening Server

Now, you can start a server that will listen on your address. You can do it by adding a virtual host in apache httpd, create a server in ‘node.js’, etc.

If you’ve installed ‘Apache24’ from the ports, you can find documentation in '/usr/local/share/doc/apache24'`. In addition, you can find documentation in the httpd site.

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