chapters: take smallest timestamp in first file into account

When generating chapters mkvmerge has to take into account things such
as splitting and file linking. This requires shifting chapter
timestamps to match file timestamps. However, for files which don't
start at 0 generated chapters would be wrongfully shifted down to
below 0 causing invalid timestamps.

Fixes #2432.

NEWS.md

@@ -15,6 +15,9 @@
   so that Windows actually recognizes it. See #2415.
 * mkvmerge: MP4 reader: fixed handling of atoms whose size exceeds the parent
   atom's size. Fixes #2431.
+* mkvmerge: chapter generation: the start timestamps of chapters generated in
+  intervals was wrong for files whose smallest video timestamp was bigger than
+  0. Fixes #2432.
 
 ## Build system changes
 

src/merge/output_control.cpp

@@ -1375,7 +1375,12 @@ prepare_additional_chapter_atoms_for_rendering() {
   if (!s_chapters_in_this_file)
     s_chapters_in_this_file = std::make_shared<KaxChapters>();
 
-  auto offset   = timestamp_c::ns(g_no_linking ? g_cluster_helper->get_first_timestamp_in_file() + g_cluster_helper->get_discarded_duration() : 0);
+  static timestamp_c s_first_file_minimum_timestamp;
+
+  if (!s_first_file_minimum_timestamp.valid())
+    s_first_file_minimum_timestamp = timestamp_c::ns(g_no_linking ? g_cluster_helper->get_first_timestamp_in_file() + g_cluster_helper->get_discarded_duration() : 0);
+
+  auto offset   = timestamp_c::ns(g_no_linking ? g_cluster_helper->get_first_timestamp_in_file() + g_cluster_helper->get_discarded_duration() : 0) - s_first_file_minimum_timestamp;
   auto &edition = GetChild<KaxEditionEntry>(*s_chapters_in_this_file);
 
   if (!FindChild<KaxEditionUID>(edition))

tests/results.txt

@@ -509,3 +509,4 @@ T_660propedit_replace_one_byte_with_ebmlvoid_all_surrounding_elements_coded_size
 T_661av1_in_matroska_by_mkvmerge_28_0_0:66667a5dd1a2cafb3aa37ce41aedfbf7:passed:20181021-225056:0.008359871
 T_662cisco_talos_2018_0694:2bacc50e6e28fb2a92d2abf2583405f5:passed:20181025-215511:0.024040919
 T_663mp4_wrong_atom_size_in_track_headers:10655836da56b23c7ba3605ae8de239f:passed:20181115-195646:0.046712794
+T_664generate_chapters_interval_first_video_timestamp_bigger_than_0:0d0a9cb9f4217a364d401fb4000fdba4-true:passed:20181116-094425:0.14444877

tests/run.rb

@@ -4,6 +4,7 @@ require "digest/md5"
 require "json"
 require "json_schema"
 require "pp"
+require "rexml/document"
 require "tempfile"
 
 require_relative "test.d/controller.rb"

tests/test-664generate_chapters_interval_first_video_timestamp_bigger_than_0.rb

@@ -0,0 +1,15 @@
+#!/usr/bin/ruby -w
+
+# T_664generate_chapters_interval_first_video_timestamp_bigger_than_0
+describe "mkvmerge / generating chapters in intervals with video tracks which start at timestamp > 0"
+test_merge "data/mkv/minimum_video_timestamp_bigger_than_0.mkv", :args => "--generate-chapters interval:60s", :keep_tmp => true
+
+test "start timestamps" do
+  extract tmp, :mode => :chapters, :args => "#{tmp}.chapters"
+
+  expected = ["00:00:00.000000000", "00:01:00.000000000", "00:02:00.000000000", "00:03:00.000000000"]
+  document = REXML::Document.new(File.new("#{tmp}.chapters"))
+  actual   = REXML::XPath.match(document, "/Chapters/EditionEntry/ChapterAtom/ChapterTimeStart").map(&:text)
+
+  expected == actual
+end