Add lvl-icon-extractor tool for extracting icon from level files

Rewritten from the original `level-screenshot-converter` Python script into C++ using Principia's own level reader and libpng directly

utils/Makefile

@@ -1,4 +1,4 @@
-SUBDIRS := lvledit progress-get
+SUBDIRS := lvl-icon-extractor lvledit progress-get
 
 .PHONY: all clean $(SUBDIRS)
 

utils/README.md

@@ -2,12 +2,13 @@
 This directory contains utility scripts and programs used for Principia development or for working with file formats.
 
 ## Scripts
-- `prepare_release_builds.sh`:
+- `prepare_release_builds.sh`: Download and prepare release builds when making a release
 - `set_version.lua`: Update the version number across the codebase when preparing a release (See [Making a Release](https://principia-web.se/wiki/Making_a_Release#incrementing-version-tagging)).
 - `update-sandbox-menu.sh`: Update the pre-rendered sandbox and item menu
 
 ## Programs
 Most of these programs rely on source files from the main Principia codebase and need to be compiled to run. Likely only works on Linux. You can use the `Makefile` in each directory or run `make` in the `utils` directory to build all of them.
 
+- `lvl-icon-extractor`: Extract the embedded level icon in a Principia level file
 - `lvledit`: Edit metadata of Principia level files
 - `progress-get`: Get leaderboard score for a given level from a data.bin file

utils/lvl-icon-extractor/Makefile

@@ -0,0 +1,20 @@
+BIN = ../bin
+PRINCIPIA = ../..
+PROGRAM = lvl-icon-extractor
+SRCS := main.cc $(PRINCIPIA)/src/pkgman.cc $(PRINCIPIA)/src/rand.c
+
+CXX ?= g++
+CXXFLAGS ?= -D_NO_TMS -O2 -ffunction-sections -fdata-sections
+INCLUDES ?= -I$(PRINCIPIA)/src -I$(PRINCIPIA)/lib
+LDFLAGS ?= -lz -lpng -Wl,--gc-sections
+
+all: $(BIN)/$(PROGRAM)
+
+$(BIN)/$(PROGRAM): $(SRCS)
+	@mkdir -p $(BIN)
+	$(CXX) $(CXXFLAGS) $(INCLUDES) -o $@ $^ $(LDFLAGS)
+
+clean:
+	rm $(BIN)/$(PROGRAM)
+
+.PHONY: all clean

utils/lvl-icon-extractor/README.md

@@ -0,0 +1,8 @@
+# `lvl-icon-extractor`
+Extract the embedded level icon (a 128x128 greyscale bitmap) from a Principia level file and save it as a PNG.
+
+## Usage
+
+```bash
+lvl-icon-extractor <level path> <png output path>
+```

utils/lvl-icon-extractor/main.cc

@@ -0,0 +1,82 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <png.h>
+#include "pkgman.hh"
+
+static lvledit lvl;
+
+static int write_png(const char *outpath, uint8_t *icon, int width, int height) {
+    FILE *fp = fopen(outpath, "wb");
+    if (!fp) {
+        fprintf(stderr, "could not open output file %s\n", outpath);
+        return 1;
+    }
+
+    png_structp png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
+    if (!png_ptr) {
+        fclose(fp);
+        fprintf(stderr, "png_create_write_struct failed\n");
+        return 1;
+    }
+
+    png_infop info_ptr = png_create_info_struct(png_ptr);
+    if (!info_ptr) {
+        png_destroy_write_struct(&png_ptr, NULL);
+        fclose(fp);
+        fprintf(stderr, "png_create_info_struct failed\n");
+        return 1;
+    }
+
+    if (setjmp(png_jmpbuf(png_ptr))) {
+        png_destroy_write_struct(&png_ptr, &info_ptr);
+        fclose(fp);
+        fprintf(stderr, "libpng fatal error\n");
+        return 1;
+    }
+
+    png_init_io(png_ptr, fp);
+
+    png_set_IHDR(png_ptr, info_ptr, width, height, 8,
+                 PNG_COLOR_TYPE_GRAY, PNG_INTERLACE_NONE,
+                 PNG_COMPRESSION_TYPE_DEFAULT, PNG_FILTER_TYPE_DEFAULT);
+
+    png_write_info(png_ptr, info_ptr);
+
+    for (int y = 0; y < height; ++y) {
+        // the level icon is stored upside down (GL coord style?), so let's just
+        // write rows in reverse order here...
+        png_bytep row = (png_bytep)(icon + (height - 1 - y) * width);
+        png_write_row(png_ptr, row);
+    }
+
+    png_write_end(png_ptr, NULL);
+    png_destroy_write_struct(&png_ptr, &info_ptr);
+    fclose(fp);
+    return 0;
+}
+
+int main(int argc, char **argv) {
+    if (argc != 3) {
+        fprintf(stderr, "usage: lvl-icon-extractor <level_path> <out.png>\n");
+        return 1;
+    }
+
+    const char *path = argv[1];
+    const char *out = argv[2];
+
+    if (!lvl.open_from_path(path)) {
+        fprintf(stderr, "could not open file %s\n", path);
+        return 1;
+    }
+
+    const int ICON_W = 128;
+    const int ICON_H = 128;
+
+    if (write_png(out, lvl.lvl.icon, ICON_W, ICON_H) != 0) {
+        fprintf(stderr, "failed to write png %s\n", out);
+        return 1;
+    }
+
+    return 0;
+}