add PNG image encoding

include/llmr/util/image.hpp

@@ -0,0 +1,14 @@
+#ifndef LLMR_UTIL_IMAGE
+#define LLMR_UTIL_IMAGE
+
+#include <string>
+
+namespace llmr {
+namespace util {
+
+std::string compress_png(int width, int height, void *rgba);
+
+}
+}
+
+#endif

src/util/image.cpp

@@ -0,0 +1,50 @@
+#include <llmr/util/image.hpp>
+#include <png.h>
+
+
+std::string llmr::util::compress_png(int width, int height, void *rgba) {
+    png_voidp error_ptr = 0;
+    png_structp png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, error_ptr, NULL, NULL);
+    if (!png_ptr) {
+        fprintf(stderr, "Couldn't create png_ptr\n");
+        return "";
+    }
+
+    png_infop info_ptr = png_create_info_struct(png_ptr);
+    if (!png_ptr) {
+        png_destroy_write_struct(&png_ptr, (png_infopp)0);
+        fprintf(stderr, "Couldn't create info_ptr\n");
+        return "";
+    }
+
+    png_set_IHDR(png_ptr, info_ptr, width, height, 8, PNG_COLOR_TYPE_RGB_ALPHA, PNG_INTERLACE_NONE,
+                 PNG_COMPRESSION_TYPE_DEFAULT, PNG_FILTER_TYPE_DEFAULT);
+
+    jmp_buf *jmp_context = (jmp_buf *)png_get_error_ptr(png_ptr);
+    if (jmp_context) {
+        png_destroy_write_struct(&png_ptr, &info_ptr);
+        return "";
+    }
+
+    std::string result;
+    png_set_write_fn(png_ptr, &result, [](png_structp png_ptr, png_bytep data, png_size_t length) {
+        std::string *out = static_cast<std::string *>(png_get_io_ptr(png_ptr));
+        out->append(reinterpret_cast<char *>(data), length);
+    }, NULL);
+
+    struct ptrs {
+        ptrs(size_t count) : rows(new png_bytep[count]) {}
+        ~ptrs() { delete[] rows; }
+        png_bytep *rows = nullptr;
+    } pointers(height);
+
+    for (int i = 0; i < height; i++) {
+        pointers.rows[i] = (png_bytep)((png_bytep)rgba + width * 4 * i);
+    }
+
+    png_set_rows(png_ptr, info_ptr, pointers.rows);
+    png_write_png(png_ptr, info_ptr, PNG_TRANSFORM_IDENTITY, NULL);
+    png_destroy_write_struct(&png_ptr, &info_ptr);
+
+    return result;
+}