Allow default title format from environment variable.

Issue #67

man/timg.1.md

@@ -179,18 +179,22 @@ these file decoders (GraphicsMagick or libav respectively).
     the opposite of **-V**.
 
 **-\-title=[format-string]**
-:    Print filename as title above each image. By default, this is just the
-     filename (`%f`), but it is possible to customize the title by giving a
-     format string. In this string, the following format specifiers are expanded
+:    Print title above each image. It is possible to customize the
+     title by giving a format string. In this string, the following format
+     specifiers are expanded:
 
        * `%f` = full filename
        * `%b` = basename (filename without path)
        * `%w` = image width
        * `%h` = image height
        * `%D` = internal decoder used (image, video, ...)
 
+     If no format string is given, this is just the filename (`%f`) or, if
+     set, what is provided in the `TIMG_DEFAULT_TITLE` environment variable.
+
 **-F**
-:    Behaves like --title=\"%f\", i.e. the filename is printed as title.
+:    Behaves like --title=\"%f\", i.e. the filename is printed as title
+     (or, if set, the `TIMG_DEFAULT_TITLE` environment variable).
 
 **-f** <*filelist-file*>
 :    Read a list of image filenames to show from this file. The list needs
@@ -283,8 +287,12 @@ Exit code is
 
 # ENVIRONMENT
 
+**TIMG_DEFAULT_TITLE**
+:   The default format string used for `--title`. If not given, the default
+    title format string is \"`%f`\".
+
 **TIMG_USE_UPPER_BLOCK**
-:     If this environment variable is set to the value **1**, timg will use the
+:    If this environment variable is set to the value **1**, timg will use the
      U+2580 - 'Upper Half Block' (▀) Unicode character.
 
     To display pixels, timg uses a Unicode half block and sets the foreground

src/display-options.h

@@ -17,6 +17,8 @@
 
 #include "timg-time.h"
 
+#include <stdlib.h>
+
 #include <string>
 #include <functional>
 #include <limits>
@@ -31,6 +33,11 @@ static constexpr int kNotInitialized = std::numeric_limits<int>::min();
 // Options influencing the rendering, chosen on the command-line or
 // programmatically.
 struct DisplayOptions {
+    DisplayOptions() {
+        static constexpr const char *kFormatEnv = "TIMG_DEFAULT_TITLE";
+        if (getenv(kFormatEnv)) title_format = getenv(kFormatEnv);
+    }
+
     int width = -1;             // Output size. These need to be set to...
     int height = -1;            // ... valid values.