diff --git a/www/command-line-options.html b/www/command-line-options.html
index 339bd77..f26b527 100644
--- a/www/command-line-options.html
+++ b/www/command-line-options.html
@@ -1750,18 +1750,21 @@
which can be 1, 2, 4, or 8. In such files, the color samples always have
8-bit depth.</dd>
+<dt>png:compression-filter=<em class="arg">value</em></dt>
+ <dd> valid values are 0 through 7. 0-4 are the corresponding PNG filters,
+ 5 means adaptive filtering except for images with a colormap, 6 means
+ adaptive filtering for all images, 7 means MNG "loco" compression.</dd>
+
<dt>png:compression-level=<em class="arg">value</em></dt>
<dd> valid values are 0 through 9, with 0 providing the least but fastest
compression and 9 usually providing the best and always the slowest.</dd>
<dt>png:compression-strategy=<em class="arg">value</em></dt>
<dd> valid values are 0 through 4, meaning default, filtered, huffman_only,
- rle, and fixed ZLIB compression strategy.</dd>
-
-<dt>png:compression-filter=<em class="arg">value</em></dt>
- <dd> valid values are 0 through 7. 0-4 are the corresponding PNG filters,
- 5 means adaptive filtering except for images with a colormap, 6 means
- adaptive filtering for all images, 7 means MNG "loco" compression.</dd>
+ rle, and fixed ZLIB compression strategy. If you are using an old zlib
+ that does not support Z_RLE (before 1.2.0) or Z_FIXED (before 1.2.2.2),
+ values 3 and 4, respectively, will use the zlib default strategy
+ instead.</dd>
<dt>png:exclude-chunk=<em class="arg">value</em></dt>
<dt>png:include-chunk=<em class="arg">value</em></dt>
@@ -1789,8 +1792,8 @@
</pre>
<p>The critical PNG chunks <kbd>IHDR</kbd>, <kbd>PLTE</kbd>,
- <kbd>IDAT</kbd>, and <kbd>IEND</kbd> cannot be excluded. Any of
- these entries appearing in the list will be ignored.</p>
+ <kbd>IDAT</kbd>, and <kbd>IEND</kbd> cannot be excluded. Any such
+ entries appearing in the list will be ignored.</p>
<p>If the ancillary PNG <kbd>tRNS</kbd> chunk is excluded and the
image has transparency, the PNG colortype is forced to be 4 or 6
@@ -1977,7 +1980,7 @@
any other nearby pixel, producing complex 2 dimensional displacements, rather
than a simple 1 dimensional vector displacements. </p>
-<p>Alteratively rather than suppling two separate images, as of IM v6.4.4-0,
+<p>Alteratively rather than supplying two separate images, as of IM v6.4.4-0,
you can use the 'red' channel of the overlay image to specify the horizontal
or X displacement, and the 'green' channel for the vertical or Y displacement.
</p>
@@ -4908,7 +4911,8 @@
<p>For compression level 0, the Huffman-only strategy is used, which is fastest but not necessarily the worst compression.</p>
-<p>If filter-type is 4 or less, the specified filter-type is used for all scanlines:</p>
+<p>If filter-type is 4 or less, the specified PNG filter-type is used for
+all scanlines:</p>
<pre class="text">
0: none
@@ -4935,8 +4939,15 @@
<p>If the filter-type is 9 the zlib Z_RLE compression strategy is used with
adaptive PNG filtering.</p>
-<p>The quality setting has no effect on the appearance of PNG and MNG images,
-since the compression is always lossless.</p>
+<p>The quality setting has no effect on the appearance or signature of PNG
+and MNG images, since the compression is always lossless.</p>
+
+<p>Not all combinations of compression level, strategy, and PNG filter type
+can be obtained using the -quality option. For more precise control,
+you can use the PNG:compression-level=N, PNG:compression-strategy=N, and
+PNG:compression-filter=N defines, respectively, instead.
+See <a href="#define">-define</a>. Values from the defines take precedence
+over values from the -quality option.</p>
<p>For further information, see
the <a href="http://www.w3.org/pub/WWW/TR">PNG</a> specification.</p>