lots of changes after a lot of time

rewrote some functions to use an accompanying function that returns a string; this prepares for splitting us into a library
fixed weird bug caused by CImg update
some endls have been replaced with \n
slightly more consistent function naming
more comments

1.3 part 1 out of ?

.github/workflows/ci.yml

@@ -0,0 +1,22 @@
+name: TerminalImageViewer CI
+
+on:
+  push:
+    branches: [ "master" ]
+  pull_request:
+    branches: [ "*" ]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v3
+    - name: Install dependencies
+      run: sudo apt-get install -qy imagemagick
+    - name: Build
+      run: make -C src
+    - name: Test
+      run: ./src/tiv -w 80 -h 24 /usr/share/pixmaps/debian-logo.png

src/tiv.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2023 Aaron Liu
+ * Copyright (c) 2017-2023, Stefan Haustein, Aaron Liu
  *
  *     This file is free software: you may copy, redistribute and/or modify it
  *     under the terms of the GNU General Public License as published by the
@@ -14,10 +14,8 @@
  *     You should have received a copy of the GNU General Public License
  *     along with this program.  If not, see <https://www.gnu.org/licenses/>.
  *
- * This file incorporates work covered by the following copyright and
+ * Alternatively, you may copy, redistribute and/or modify this file under
- * permission notice:
+ * the terms of the Apache License, version 2.0:
- *
- *     Copyright (c) 2017—2021, Stefan Haustein
  *
  *     Licensed under the Apache License, Version 2.0 (the "License");
  *     you may not use this file except in compliance with the License.
@@ -42,17 +40,18 @@
 #include <string>
 #include <vector>
 
-// CImg, the superior grafiks library
+// This #define tells CImg that we use the library without any display options --
+// just for loading images.
 #define cimg_display 0
 #include "CImg.h"
 // CImg defines its own min and max macros to compile, so we need to undef them
 #undef min
 #undef max
 
-// First include for detecting console output size,
-// everything else for exit codes
 #ifdef _POSIX_VERSION
+// Console output size detection
 #include <sys/ioctl.h>
+// Exit codes
 #include <sysexits.h>
 #endif
 
@@ -74,9 +73,6 @@
 #define EX_CONFIG 78    /* configuration error */
 #endif
 
-// using namespace std; // haha nope, bad style
-// especially when we're also using the CImg namespace
-
 // Implementation of flag representation for flags in the main() method
 constexpr int FLAG_FG = 1;         // emit fg color
 constexpr int FLAG_BG = 2;         // emit bg color
@@ -85,15 +81,18 @@ constexpr int FLAG_24BIT = 8;      // 24-bit color mode
 constexpr int FLAG_NOOPT = 16;     // Only use the same half-block character
 constexpr int FLAG_TELETEXT = 32;  // Use teletext characters
 
-// Steps (@TODO: Figure out what exactly they represent)
+// Color saturation value steps from 0 to 255
 constexpr int COLOR_STEP_COUNT = 6;
 constexpr int COLOR_STEPS[COLOR_STEP_COUNT] = {0, 0x5f, 0x87, 0xaf, 0xd7, 0xff};
 
+// Grayscale saturation value steps from 0 to 255
 constexpr int GRAYSCALE_STEP_COUNT = 24;
 constexpr int GRAYSCALE_STEPS[GRAYSCALE_STEP_COUNT] = {
     0x08, 0x12, 0x1c, 0x26, 0x30, 0x3a, 0x44, 0x4e, 0x58, 0x62, 0x6c, 0x76,
     0x80, 0x8a, 0x94, 0x9e, 0xa8, 0xb2, 0xbc, 0xc6, 0xd0, 0xda, 0xe4, 0xee};
 
+// An interleaved map of 4x8 bit character bitmaps (each hex digit represents a row) 
+// to the corresponding unicode character code point.  
 constexpr unsigned int BITMAPS[] = {
     0x00000000, 0x00a0,