Clarify documentation in tiffio.c and pdfapp.c

* Also clean up pixaWriteMemWebPAnim()

src/pdfapp.c

@@ -31,6 +31,16 @@
  *    Image processing operations on multiple images followed by wrapping
  *    them into a pdf.
  *
+ *    There are two possible ways to specify the set of images:
+ *    (1) an array of pathnames
+ *    (2) a directory, typically with an additional pattern for selection.
+ *    We use (1) because it is both simpler and more general.
+ *
+ *    Corresponding to each function here is:
+ *    (1) the image processing function that is carried out on each image
+ *    (2) a program in prog that extracts images from a pdf and calls this
+ *        function with an array of their pathnames.
+ *
  *    |=============================================================|
  *    |                        Important notes                      |
  *    |=============================================================|

src/tiffio.c

@@ -104,15 +104,14 @@
  *          * The exception is the old-style jpeg tiff format (OJPEG), which
  *            is not supported.
  *          * We support two formats requiring external libraries: ZIP and JPEG
- *            Both libz and libjpeg are required.
+ *            All computers should have the zip library.
  *          * At present we do not support WEBP in tiff, which uses
  *            libwebp and was added in tifflib 4.1.0 in 2019.
- *  Note 3: On Windows with 2 bpp or 4 bpp images, the bytes in the
- *          tiff-compressed file depend on the pad bits, but not on the
- *          decoded raster image when read.  Because it is sometimes
- *          convenient to use a golden file with a byte-by-byte check
- *          to verify invariance, we set the pad bits to 0 before writing,
- *          in pixWriteToTiffStream().
+ *  Note 3: We set the pad bits to 0 before writing in pixWriteToTiffStream().
+ *          Although they don't affect the raster image after decompression,
+ *          it is sometimes convenient to use a golden file with a
+ *          byte-by-byte check to verify invariance.  The issue came up
+ *          on Windows for 2 and 4 bpp images.
  * </pre>
  */
 

src/webpanimio.c

@@ -204,8 +204,7 @@ WebPPicture             frame;
         return ERROR_INT("cannot initialize WebP config", __func__, 1);
     config.lossless = lossless;
     config.quality = quality;
-    enc = WebPAnimEncoderNew(w, h, &enc_options);
-    if (enc == NULL)
+    if ((enc = WebPAnimEncoderNew(w, h, &enc_options)) == NULL)
         return ERROR_INT("cannot create WebP encoder", __func__, 1);
 
     for (i = 0; i < n; i++) {
@@ -239,16 +238,17 @@ WebPPicture             frame;
             return ERROR_INT("cannot add frame to animation", __func__, 1);
         }
     }
-    /* add a blank frame */
+        /* Add a blank frame */
     if (!WebPAnimEncoderAdd(enc, NULL, duration * i, NULL)) {
-            WebPAnimEncoderDelete(enc);
-            return ERROR_INT("cannot add blank frame to animation", __func__, 1);
+        WebPAnimEncoderDelete(enc);
+        return ERROR_INT("blank frame not added to animation", __func__, 1);
     }
-    /* encode the data */
+
+        /* Encode the data */
     ret_webp = WebPAnimEncoderAssemble(enc, &webp_data);
     WebPAnimEncoderDelete(enc);
     if (!ret_webp)
-            return ERROR_INT("cannot assemble animation", __func__, 1);
+        return ERROR_INT("cannot assemble animation", __func__, 1);
 
         /* Set the loopcount if requested.  Note that when you make a mux,
          * it imports the webp_data that was previously made, including