Ignore:
Timestamp:
Nov 6, 2007, 9:33:55 AM (13 years ago)
Author:
Sam Hocevar
Message:
  • New gradient pattern to make gamma issues more obvious.
File:
1 edited

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  • www/study/index.html

    r1898 r1899  
    4343
    4444<p> This document makes a lot of assumptions, such as the fact that input
    45 images are made of pixels that have either one (luminance) or three (red,
     45images are made of pixels that have either one (gray level) or three (red,
    4646green and blue) values uniformly spread between 0 and 1 (with regards to
    4747human contrast perception). Real life is more complicated than that, but
     
    142142</p>
    143143
    144 <h3> 2.2. Gamma considerations </h3>
    145 
    146 <p> If your display is not very good, you might see slightly different
    147 shades of gray for the following patterns, despite being made of 50%
     144<p> Not bad for a start. But there is a lot to improve. </p>
     145
     146<h3> 2.2. Screen artifacts </h3>
     147
     148<p> If your screen’s quality is not very good, you might experience slightly
     149different shades of gray for the following patterns, despite being made of 50%
    148150black and 50% white pixels: </p>
    149151
    150152<p style="text-align: center;">
    151153  <img src="pat003.png" width="240" height="80"
    152        class="inline" alt="introducing gamma" />
    153 </p>
    154 
    155 <p> But more importantly, if you are reading this document on a computer
     154       class="inline" alt="screen imperfections" />
     155</p>
     156
     157<h3> 2.3. Gamma considerations </h3>
     158
     159<p> More importantly, if you are reading this document on a computer
    156160screen, you may have noticed that the above 50% pattern was closer to a 0.73
    157 grayscale (left) than to the expected 0.5 value (right). If you are reading
    158 a printed copy, it might be a different matter. </p>
     161grayscale (left) than to the intuitively expected 0.5 value (right). If you
     162are reading a printed copy, it might be a different matter. </p>
    159163
    160164<p style="text-align: center;">
     
    167171white pattern is equivalent to a gray value of 0.73 instead of 0.5. Conversely,
    168172it clearly means that a gray value of 0.5 should not be approached with a 50%
    169 black and white dither patterns. </p>
     173dither pattern. </p>
    170174
    171175<p> So, instead of using 25%, 50% and 75% patterns (which give non-uniform
    172 gray values of 0.53, 0.73 and 0.88), let’s try with 6.25%, 25% and 50%
    173 patterns (which give the better spread gray values of 0.28, 0.53 and 0.73).
    174 As you can see, the result is visually less appealing. But the important
    175 part here is that color accuracy is higher:  </p>
     176gray values of 0.53, 0.73 and 0.88), one should rather use 6.25%, 25% and 50%
     177patterns, which give the better spread gray values of 0.28, 0.53 and 0.73
     178and result far more accurate gradients. This is especially obvious when
     179observing the high intensity drop between the 25% pattern and black (top row):
     180</p>
     181
     182<p style="text-align: center;">
     183  <img src="pat005.png" width="400" height="240"
     184       class="inline" alt="better gradients" />
     185</p>
     186
     187<p> Here is the result on Lenna. As you can see, the result is visually less
     188appealing than with the “incorrect” colours. But when seen from a distance,
     189there is no doubt this version is more accurate: </p>
    176190
    177191<p style="text-align: center;">
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