Allow input from stdin

src/main.cpp

@@ -50,8 +50,8 @@ int main(int argc, char* argv[]) {
             }
         }
         if (optind == argc) {
-            print_help();
-            return EXIT_FAILURE;
+            // no file arg => read from stdin
+            write_image(std::cout);
         }
         for (int i = optind; i < argc; ++i) {
             try {

src/pixels.cpp

@@ -8,13 +8,12 @@ static inline int t2p(int val) {
     return .5 + 5 * val / 255.0;
 }
 
-void write_image(std::ostream& os, const std::string& path) {
+static void write(std::ostream& os, FILE* in) {
     char* tc = getenv("TRUECOLOR");
     std::string truecolor {tc ? tc : ""};
     bool has_truecolor = truecolor.size() > 0;
-    std::unique_ptr<FILE, void(*)(FILE*)> infile {pm_openr(path.c_str()), pm_close};
     struct pam info;
-    pnm_readpaminit(infile.get(), &info, PAM_STRUCT_SIZE(tuple_type));
+    pnm_readpaminit(in, &info, PAM_STRUCT_SIZE(tuple_type));
     std::unique_ptr<tuple, void(*)(void*)> row1 {pnm_allocpamrow(&info), pm_freerow};
     std::unique_ptr<tuple, void(*)(void*)> row2 {pnm_allocpamrow(&info), pm_freerow};
     for (int y = 0; y < info.height; y += 2) {
@@ -40,3 +39,12 @@ void write_image(std::ostream& os, const std::string& path) {
         os << sc::io::reset << '\n';
     }
 }
+
+void write_image(std::ostream& os) {
+    write(os, stdin);
+}
+
+void write_image(std::ostream& os, const std::string& path) {
+    std::unique_ptr<FILE, void(*)(FILE*)> infile {pm_openr(path.c_str()), pm_close};
+    write(os, infile.get());
+}

src/pixels.hpp

@@ -3,6 +3,7 @@
 
 #include <iostream>
 
+void write_image(std::ostream& os);
 void write_image(std::ostream& os, const std::string& path);
 
 #endif // PIXELS_H_