Gamma correction
A gamma characteristic is a power law relationship that approximates the relationship between the encoded luminance in a television system and the actual desired image brightness. Computer graphics systems that require a linear relationship between these quantities use gamma correction.A cathode ray tube, for example, converts a video signal to light in a nonlinear way, because the electron gun it contains is a nonlinear device. The light intensity L is related to the source voltage VS by the following formula:
- L ∝ (VS)γ
where γ is the Greek letter gamma. For a CRT, γ is about 2.2. For simplicity, consider the example of a monochrome CRT. In this case, when a video signal of 0.5 (representing mid-grey) is fed to the display, the luminance is about 0.21 (resulting in a dark grey). Pure black (0.0) and pure white (1.0) are the only shades that are unaffected by gamma.
To compensate for this effect, the inverse transfer function (gamma correction) is sometimes applied to the video signal so that the end-to-end response is linear. In other words, the transmitted signal is deliberately distorted so that, after it has been distorted again by the display device, the viewer sees the correct brightness. The inverse of the function above is:
- VC ∝ (VS)(1/γ)
where VC is the corrected voltage and VS is the source voltage (e.g. from a camera or VCR). In our CRT example 1/γ is 1/2.2 or about 0.45.
A colour CRT receives three video signals (red, green and blue) and in general each colour has its own value of gamma, denoted γR, γG or γB. However, in simple display systems, a single value of γ is used for all three colours.
Other display devices have different values of gammas: for example, a Game Boy Advance display has a gamma between 3 and 4 depending on lighting conditions.
For a tutorial on gamma characteristics in computer graphics, see http://www.w3.org/TR/PNG-GammaAppendix.html .
love.html">love.html">love.html">love.html">love.html">love.html">love.html">Love in the tongue.html">tongue of hate exprest,
Feeling the infinite must be
Speaks, where expression bates its wings,
What of all words is in excess
With dailiest babble shows her sense
And while she feels the heavens lie bare,
BEGINNING OF END.
Of Love's incessant tumultuous wing;
'Twere well she should be strange with him somewhat:
That would not suffer her lay it down a whit!
And with her lightest comrade lightly walked,
And peep from some unnoticed corner of her heart.html">heart.
(There yonder, in the hollow, that's HIS cot),
She saw.html">saw him at his gate, yet stilled her tongue--
And she must punish him for doing him wrong:
And if she turned upon the brow o' the hill,
You saw she thought.html">thought he was not thought upon.
She that night woke and stirred, with no distress,
Lest perhaps her foolish heart suspect that it was sad.
Love, like a wind, shook wide your blosmy eyes,
For that you loved me.
You were so kind, so sweet, none could withhold
For that you loved me.
Like to a box of spikenard did you break
For that you loved me.
Life fell to dust without me; so you tried
For that you loved me.
You gave yourself as children give, that weep
For that you loved me.
I am no woman.html">woman, girl, nor ever knew
To her that loved me.
Have less.html">less of love, or less of woman in
That you so love me.
For, wild Penelope, the web you wove
Is this to love me,
Or what rights have I that scorn could deny?
If so you love me!
She did not love to love; but hated him
From passion taught misprision to begin;
Was because love to cast out had no skill
.
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