feat: write generation parameter exif data into output png (#57)

* Write generation parameter exif data into output pngs.

This adds prompt, negative prompt (if nonempty) and other generation
parameters to the output file as a tEXt PNG block, in the same format as
AUTOMATIC1111 webui does.

In order to keep everything free of external library dependencies, I
have somewhat dirtily hacked this into the stb_image_write
implementation.

* Mention png text data in README.md, include "karras" in sampler text

* add Steps/Model/RNG to parameter string

---------

Co-authored-by: leejet <leejet714@gmail.com>

README.md

@@ -27,6 +27,7 @@ Inference of [Stable Diffusion](https://github.com/CompVis/stable-diffusion) in
     - [`DPM++ 2M v2`](https://github.com/AUTOMATIC1111/stable-diffusion-webui/discussions/8457)
     - `DPM++ 2S a`
 - Cross-platform reproducibility (`--rng cuda`, consistent with the `stable-diffusion-webui GPU RNG`)
+- Embedds generation parameters into png output as webui-compatible text string
 - Supported platforms
     - Linux
     - Mac OS

examples/main.cpp

@@ -365,6 +365,18 @@ void parse_args(int argc, const char* argv[], Option* opt) {
     }
 }
 
+std::string basename(const std::string& path) {
+    size_t pos = path.find_last_of('/');
+    if (pos != std::string::npos) {
+        return path.substr(pos + 1);
+    }
+    pos = path.find_last_of('\\');
+    if (pos != std::string::npos) {
+        return path.substr(pos + 1);
+    }
+    return path;
+}
+
 int main(int argc, const char* argv[]) {
     Option opt;
     parse_args(argc, argv, &opt);
@@ -437,7 +449,24 @@ int main(int argc, const char* argv[]) {
         return 1;
     }
 
-    stbi_write_png(opt.output_path.c_str(), opt.w, opt.h, 3, img.data(), 0);
+    std::string parameter_string = opt.prompt + "\n";
+    if (opt.negative_prompt.size() != 0) {
+        parameter_string += "Negative prompt: " + opt.negative_prompt + "\n";
+    }
+    parameter_string += "Steps: " + std::to_string(opt.sample_steps) + ", ";
+    parameter_string += "CFG scale: " + std::to_string(opt.cfg_scale) + ", ";
+    parameter_string += "Seed: " + std::to_string(opt.seed) + ", ";
+    parameter_string += "Size: " + std::to_string(opt.w) + "x" + std::to_string(opt.h) + ", ";
+    parameter_string += "Model: " + basename(opt.model_path) + ", ";
+    parameter_string += "RNG: " + std::string(rng_type_to_str[opt.rng_type]) + ", ";
+    parameter_string += "Sampler: " + std::string(sample_method_str[opt.sample_method]);
+    if (opt.schedule == KARRAS) {
+        parameter_string += " karras";
+    }
+    parameter_string += ", ";
+    parameter_string += "Version: stable-diffusion.cpp";
+
+    stbi_write_png(opt.output_path.c_str(), opt.w, opt.h, 3, img.data(), 0, parameter_string.c_str());
     printf("save result image to '%s'\n", opt.output_path.c_str());
 
     return 0;

examples/stb_image_write.h

@@ -173,7 +173,7 @@ STBIWDEF int stbi_write_force_png_filter;
 #endif
 
 #ifndef STBI_WRITE_NO_STDIO
-STBIWDEF int stbi_write_png(char const *filename, int w, int h, int comp, const void  *data, int stride_in_bytes);
+STBIWDEF int stbi_write_png(char const *filename, int w, int h, int comp, const void  *data, int stride_in_bytes, const char* parameters = NULL);
 STBIWDEF int stbi_write_bmp(char const *filename, int w, int h, int comp, const void  *data);
 STBIWDEF int stbi_write_tga(char const *filename, int w, int h, int comp, const void  *data);
 STBIWDEF int stbi_write_hdr(char const *filename, int w, int h, int comp, const float *data);
@@ -1125,9 +1125,10 @@ static void stbiw__encode_png_line(unsigned char *pixels, int stride_bytes, int
    }
 }
 
-STBIWDEF unsigned char *stbi_write_png_to_mem(const unsigned char *pixels, int stride_bytes, int x, int y, int n, int *out_len)
+STBIWDEF unsigned char *stbi_write_png_to_mem(const unsigned char *pixels, int stride_bytes, int x, int y, int n, int *out_len, const char* parameters)
 {
    int force_filter = stbi_write_force_png_filter;
+   int param_length = 0;
    int ctype[5] = { -1, 0, 4, 2, 6 };
    unsigned char sig[8] = { 137,80,78,71,13,10,26,10 };
    unsigned char *out,*o, *filt, *zlib;
@@ -1177,10 +1178,15 @@ STBIWDEF unsigned char *stbi_write_png_to_mem(const unsigned char *pixels, int s
    STBIW_FREE(filt);
    if (!zlib) return 0;
 
+   if(parameters != NULL) {
+      param_length = strlen(parameters);
+      param_length += strlen("parameters") + 1; // For the name and the null-byte
+   }
+
    // each tag requires 12 bytes of overhead
-   out = (unsigned char *) STBIW_MALLOC(8 + 12+13 + 12+zlen + 12);
+   out = (unsigned char *) STBIW_MALLOC(8 + 12+13 + 12+zlen + 12 + ((parameters)?(param_length+12):0));
    if (!out) return 0;
-   *out_len = 8 + 12+13 + 12+zlen + 12;
+   *out_len = 8 + 12+13 + 12+zlen + 12 + ((parameters)?(param_length+12):0);
 
    o=out;
    STBIW_MEMMOVE(o,sig,8); o+= 8;
@@ -1195,6 +1201,17 @@ STBIWDEF unsigned char *stbi_write_png_to_mem(const unsigned char *pixels, int s
    *o++ = 0;
    stbiw__wpcrc(&o,13);
 
+   if(parameters != NULL) {
+      stbiw__wp32(o, param_length);
+      stbiw__wptag(o, "tEXt");
+      STBIW_MEMMOVE(o, "parameters", strlen("parameters"));
+      o+=strlen("parameters");
+      *o++ = 0; // Null pyte separator
+      STBIW_MEMMOVE(o, parameters, strlen(parameters));
+      o+=strlen(parameters);
+      stbiw__wpcrc(&o, param_length);
+   }
+
    stbiw__wp32(o, zlen);
    stbiw__wptag(o, "IDAT");
    STBIW_MEMMOVE(o, zlib, zlen);
@@ -1212,11 +1229,11 @@ STBIWDEF unsigned char *stbi_write_png_to_mem(const unsigned char *pixels, int s
 }
 
 #ifndef STBI_WRITE_NO_STDIO
-STBIWDEF int stbi_write_png(char const *filename, int x, int y, int comp, const void *data, int stride_bytes)
+STBIWDEF int stbi_write_png(char const *filename, int x, int y, int comp, const void *data, int stride_bytes, const char* parameters)
 {
    FILE *f;
    int len;
-   unsigned char *png = stbi_write_png_to_mem((const unsigned char *) data, stride_bytes, x, y, comp, &len);
+   unsigned char *png = stbi_write_png_to_mem((const unsigned char *) data, stride_bytes, x, y, comp, &len, parameters);
    if (png == NULL) return 0;
 
    f = stbiw__fopen(filename, "wb");
@@ -1231,7 +1248,7 @@ STBIWDEF int stbi_write_png(char const *filename, int x, int y, int comp, const
 STBIWDEF int stbi_write_png_to_func(stbi_write_func *func, void *context, int x, int y, int comp, const void *data, int stride_bytes)
 {
    int len;
-   unsigned char *png = stbi_write_png_to_mem((const unsigned char *) data, stride_bytes, x, y, comp, &len);
+   unsigned char *png = stbi_write_png_to_mem((const unsigned char *) data, stride_bytes, x, y, comp, &len, NULL);
    if (png == NULL) return 0;
    func(context, png, len);
    STBIW_FREE(png);