In an earlier post, I’ve shown how to calculate an RGB↔XYZ conversion matrix. It’s only natural to follow up with a code for converting between sRGB and XYZ colour spaces. While the matrix is a significant portion of the algorithm, there is one more step necessary: gamma correction.

What is gamma correction?

Human perception of light’s brightness approximates a power function of its intensity. This can be expressed as \(P = S^\alpha\) where \(P\) is the perceived brightness and \(S\) is linear intensity. \(\alpha\) has been experimentally measured to be less than one which means that people are more sensitive to changes to dark colours rather than to bright ones.

Based on that observation, colour space’s encoding can be made more efficient by using higher precision when encoding dark colours and lower when encoding bright ones. This is akin to precision of floating-point numbers scaling with value’s magnitude. In RGB systems, the role of precision scaling is done by gamma correction. When colour is captured (for example from a digital camera) it goes through gamma compression which spaces dark colours apart and packs lighter colours more densely. When displaying an image, the opposite happens and encoded value goes through gamma expansion.

This may have been obvious, but I’ve just learned that browsers ignore Expires header when the user manually reloads the page (as in by pressing F5 or choosing Reload option).

I’ve run into this when testing how Firefox treats pages which ‘never’ expire. To my surprise, the browser made requests for files it had a fresh copy of in its cache. To see behaviour much more representative of the experience of a returning user, one should select the address bar (Alt+D does the trick) and then press Return to navigate to the current page again. Hitting Reload is more akin, though not exactly the same, to the first visit.

Of course, all of the above applies to the max-age directive of the Cache-Control header as well.

Moral of the story? Make sure you test the actual real-life scenarios before making any decisions.

Anyone can think of myriad reasons to run a Tor hidden service. Surely many unsavoury endeavours spring to mind but of course, there are as many noble ones. There are also various pragmatic causes like circumventing lousy NATs. Me? I just wanted to play around with my router.

Configuring a hidden service is actually straightforward so to make things more interesting, this article will cover configuring a hidden service on a Turris Omnia router with the help of Linux Containers to maximise isolation of components. While some steps will be Omnia-specific, most translate easily to other systems, so this post may be applicable regardless of the distribution used.

Do you recall when I decided to abuse Go’s runtime and play with string↔[]byte conversion? Fun times… I wonder if we could do the same to Java?

To remind ourselves of the ‘problem’, strings in Java are immutable but because Java has no concept of ownership or const keyword to make true on that promise, Java runtime has to make a defensive copy each time a new string is created or when string’s characters are returned.

Alas, do not despair! There is another way (exception handling elided for brevity):

private static Field getValueField() {
	final Field field = String.class.getDeclaredField("value");
	field.setAccessible(true);
	/* Test that it works. */
	final char[] chars = new char[]{'F', 'o', 'o'};
	final String string = new String();
	field.set(string, chars);
	if (string.equals("Foo") && field.get(string) == chars) {
		return field;
	}
	throw new UnsupportedOperationException(
		"UnsafeString not supported by the runtime");
}

private final static Field valueField = getValueField();

public static String fromChars(final char[] chars) {
	final String string = new String();
	valueField.set(string, chars);
	return string;
}

public static char[] toChars(final String string) {
	return (char[]) valueField.get(string);
}

However. There is a twist…

I’ve recently found myself in need of an RGB↔XYZ transformation matrix expressed to the maximum possible precision. Sources on the Internet typically limit the precision to a handful decimal places so I’ve performed do the calculations myself.

What we’re looking for is a 3-by-3 matrix \(M\) which, when multiplied by red, green and blue coordinates of a colour, produces its XYZ coordinates. In other words, a change of basis matrix from a space whose basis vectors are RGB’s primary colours: $$ M = \begin{bmatrix} X_r & X_g & X_b \\ Y_r & Y_g & Y_b \\ Z_r & Z_g & Z_b \end{bmatrix} $$