Changeset 1899Tweet

Timestamp:

11/06/07 09:33:55 (6 years ago)

Author:

sam

Message:

• New gradient pattern to make gamma issues more obvious.

Location:

www/study

Files:

• index.html (modified) (3 diffs)
• pat005.png (added)
• study.py (modified) (1 diff)

www/study/index.html

@@ -43 +43 @@
 
 <p> This document makes a lot of assumptions, such as the fact that input
-images are made of pixels that have either one (luminance) or three (red,
+images are made of pixels that have either one (gray level) or three (red,
 green and blue) values uniformly spread between 0 and 1 (with regards to
 human contrast perception). Real life is more complicated than that, but
@@ -142 +142 @@
 </p>
 
-<h3> 2.2. Gamma considerations </h3>
-
-<p> If your display is not very good, you might see slightly different
-shades of gray for the following patterns, despite being made of 50%
+<p> Not bad for a start. But there is a lot to improve. </p>
+
+<h3> 2.2. Screen artifacts </h3>
+
+<p> If your screen’s quality is not very good, you might experience slightly
+different shades of gray for the following patterns, despite being made of 50%
 black and 50% white pixels: </p>
 
 <p style="text-align: center;">
   <img src="pat003.png" width="240" height="80"
-       class="inline" alt="introducing gamma" />
-</p>
-
-<p> But more importantly, if you are reading this document on a computer
+       class="inline" alt="screen imperfections" />
+</p>
+
+<h3> 2.3. Gamma considerations </h3>
+
+<p> More importantly, if you are reading this document on a computer
 screen, you may have noticed that the above 50% pattern was closer to a 0.73
-grayscale (left) than to the expected 0.5 value (right). If you are reading
-a printed copy, it might be a different matter. </p>
+grayscale (left) than to the intuitively expected 0.5 value (right). If you
+are reading a printed copy, it might be a different matter. </p>
 
 <p style="text-align: center;">
@@ -167 +171 @@
 white pattern is equivalent to a gray value of 0.73 instead of 0.5. Conversely,
 it clearly means that a gray value of 0.5 should not be approached with a 50%
-black and white dither patterns. </p>
+dither pattern. </p>
 
 <p> So, instead of using 25%, 50% and 75% patterns (which give non-uniform
-gray values of 0.53, 0.73 and 0.88), let’s try with 6.25%, 25% and 50%
-patterns (which give the better spread gray values of 0.28, 0.53 and 0.73).
-As you can see, the result is visually less appealing. But the important
-part here is that color accuracy is higher:  </p>
+gray values of 0.53, 0.73 and 0.88), one should rather use 6.25%, 25% and 50%
+patterns, which give the better spread gray values of 0.28, 0.53 and 0.73
+and result far more accurate gradients. This is especially obvious when
+observing the high intensity drop between the 25% pattern and black (top row):
+</p>
+
+<p style="text-align: center;">
+  <img src="pat005.png" width="400" height="240"
+       class="inline" alt="better gradients" />
+</p>
+
+<p> Here is the result on Lenna. As you can see, the result is visually less
+appealing than with the “incorrect” colours. But when seen from a distance,
+there is no doubt this version is more accurate: </p>
 
 <p style="text-align: center;">

www/study/study.py

@@ -147 +147 @@
 dest.writePng("pat004.png")
 
+# Pattern 5: gamma-corrected 50% gray, black-white halftone, 50% gray
+dest = Image((400, 240))
+for y in range(80):
+    for x in range(400):
+        if x < 80:
+            c = 0.
+        elif x < 160:
+            c = ((x + y) & 1) and (y & 1)
+        elif x < 240:
+            c = (x + y) & 1
+        elif x < 320:
+            c = ((x + y) & 1) or (y & 1)
+        else:
+            c = 1.
+        dest.setGray(x, y, c)
+for y in range(80, 160):
+    for x in range(400):
+        dest.setGray(x, y, x / 80 / 4.)
+for y in range(160, 240):
+    for x in range(400):
+        if x < 80:
+            c = 0.
+        elif x < 160:
+            c = (((x + y) & 3) == 1) and ((y & 3) == 1)
+        elif x < 240:
+            c = ((x + y) & 1) and (y & 1)
+        elif x < 320:
+            c = (x + y) & 1
+        else:
+            c = 1.
+        dest.setGray(x, y, c)
+dest.writePng("pat005.png")
+
 # Output 6: gamma-aware 20/40/60/80% threshold:
 dest = Image((w, h))