img2webp: add -exact/-noexact per-frame options

Bug: b:363409354
Change-Id: I4e7282ed2df091dbef6d79743be1c8c868c0d44a

doc/tools.md

@@ -321,10 +321,13 @@ Per-frame options (only used for subsequent images input):
 
 ```
 -d <int> ............. frame duration in ms (default: 100)
--lossless  ........... use lossless mode (default)
+-lossless ............ use lossless mode (default)
--lossy ... ........... use lossy mode
+-lossy ............... use lossy mode
 -q <float> ........... quality
 -m <int> ............. method to use
+-exact, -noexact ..... preserve or alter RGB values in transparent area
+                       (default: -noexact, may cause artifacts
+                                 with lossy animations)
 ```
 
 example: `img2webp -loop 2 in0.png -lossy in1.jpg -d 80 in2.tiff -o out.webp`

examples/img2webp.c

@@ -59,10 +59,14 @@ static void Help(void) {
 
   printf("Per-frame options (only used for subsequent images input):\n");
   printf(" -d <int> ............. frame duration in ms (default: 100)\n");
-  printf(" -lossless  ........... use lossless mode (default)\n");
+  printf(" -lossless ............ use lossless mode (default)\n");
-  printf(" -lossy ... ........... use lossy mode\n");
+  printf(" -lossy ............... use lossy mode\n");
   printf(" -q <float> ........... quality\n");
   printf(" -m <int> ............. method to use\n");
+  printf(" -exact, -noexact ..... preserve or alter RGB values in transparent "
+                                  "area\n"
+         "                        (default: -noexact, may cause artifacts\n"
+         "                                  with lossy animations)\n");
 
   printf("\n");
   printf("example: img2webp -loop 2 in0.png -lossy in1.jpg\n"
@@ -248,6 +252,10 @@ int main(int argc, const char* argv[]) {
           fprintf(stderr, "Invalid negative duration (%d)\n", duration);
           parse_error = 1;
         }
+      } else if (!strcmp(argv[c], "-exact")) {
+        config.exact = 1;
+      } else if (!strcmp(argv[c], "-noexact")) {
+        config.exact = 0;
       } else {
         parse_error = 1;   // shouldn't be here.
         fprintf(stderr, "Unknown option [%s]\n", argv[c]);

man/img2webp.1

@@ -1,5 +1,5 @@
 .\"                                      Hey, EMACS: -*- nroff -*-
-.TH IMG2WEBP 1 "July 18, 2024"
+.TH IMG2WEBP 1 "September 3, 2024"
 .SH NAME
 img2webp \- create animated WebP file from a sequence of input images.
 .SH SYNOPSIS
@@ -88,6 +88,11 @@ Specify the compression factor between 0 and 100. The default is 75.
 Specify the compression method to use. This parameter controls the
 trade off between encoding speed and the compressed file size and quality.
 Possible values range from 0 to 6. Default value is 4.
+.TP
+.B \-exact, \-noexact
+Preserve or alter RGB values in transparent area. The default is
+\fB-noexact\fP, to help compressibility. Note \fB\-noexact\fP may cause
+artifacts in frames compressed with \fB\-lossy\fP.
 
 .SH EXIT STATUS
 If there were no problems during execution, \fBimg2webp\fP exits with the value