How to save an SDL surface to a PNG and get the colors right?

Question

I wrote up a simple patch to add PNG-saving capabilities to the SDL_Image library. It almost works, too. Problem is, the colors come out all scrambled, and I don't know enough C to figure out what's wrong. Can anyone take a look at this and help me fix it up?

Use case:

1. Load a 256-color PNG image with IMG_LoadPNG_RW.
2. Save it with IMG_SavePNG_RW.
3. See if they're identical or not.

Patch:

Index: IMG_png.c
===================================================================
--- IMG_png.c   (revision 4475)
+++ IMG_png.c   (working copy)
@@ -76,17 +76,25 @@
    png_infop (*png_create_info_struct) (png_structp png_ptr);
    png_structp (*png_create_read_struct) (png_const_charp user_png_ver, png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn);
    void (*png_destroy_read_struct) (png_structpp png_ptr_ptr, png_infopp info_ptr_ptr, png_infopp end_info_ptr_ptr);
+   png_structp (*png_create_write_struct) (png_const_charp user_png_ver, png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn);
+   void (*png_destroy_write_struct) (png_structpp png_ptr_ptr, png_infopp info_ptr_ptr);
    png_uint_32 (*png_get_IHDR) (png_structp png_ptr, png_infop info_ptr, png_uint_32 *width, png_uint_32 *height, int *bit_depth, int *color_type, int *interlace_method, int *compression_method, int *filter_method);
+   void (*png_set_IHDR) (png_structp png_ptr, png_infop info_ptr, png_uint_32 width, png_uint_32 height, int bit_depth, int color_type, int interlace_method, int compression_method, int filter_method);
    png_voidp (*png_get_io_ptr) (png_structp png_ptr);
    png_uint_32 (*png_get_tRNS) (png_structp png_ptr, png_infop info_ptr, png_bytep *trans, int *num_trans, png_color_16p *trans_values);
    png_uint_32 (*png_get_valid) (png_structp png_ptr, png_infop info_ptr, png_uint_32 flag);
    void (*png_read_image) (png_structp png_ptr, png_bytepp image);
+   void (*png_write_image) (png_structp png_ptr, png_bytepp image);
    void (*png_read_info) (png_structp png_ptr, png_infop info_ptr);
+   void (*png_write_info) (png_structp png_ptr, png_infop info_ptr);
+   void (*png_set_PLTE) (png_structp png_ptr, png_infop info_ptr, png_colorp palette, int num_palette);
+   void (*png_write_end) (png_structp png_ptr, png_infop info_ptr);
    void (*png_read_update_info) (png_structp png_ptr, png_infop info_ptr);
    void (*png_set_expand) (png_structp png_ptr);
    void (*png_set_gray_to_rgb) (png_structp png_ptr);
    void (*png_set_packing) (png_structp png_ptr);
    void (*png_set_read_fn) (png_structp png_ptr, png_voidp io_ptr, png_rw_ptr read_data_fn);
+   void (*png_set_write_fn) (png_structp png_ptr, png_voidp io_ptr, png_rw_ptr read_data_fn);
    void (*png_set_strip_16) (png_structp png_ptr);
    int (*png_sig_cmp) (png_bytep sig, png_size_t start, png_size_t num_to_check);
 } lib;
@@ -120,6 +128,20 @@
      SDL_UnloadObject(lib.handle);
      return -1;
     }
+    lib.png_create_write_struct =
+     (png_structp (*) (png_const_charp, png_voidp, png_error_ptr, png_error_ptr))
+     SDL_LoadFunction(lib.handle, "png_create_write_struct");
+    if ( lib.png_create_write_struct == NULL ) {
+     SDL_UnloadObject(lib.handle);
+     return -1;
+    }
+    lib.png_destroy_write_struct =
+     (void (*) (png_structpp, png_infopp))
+     SDL_LoadFunction(lib.handle, "png_destroy_write_struct");
+    if ( lib.png_destroy_write_struct == NULL ) {
+     SDL_UnloadObject(lib.handle);
+     return -1;
+    }
     lib.png_get_IHDR =
      (png_uint_32 (*) (png_structp, png_infop, png_uint_32 *, png_uint_32 *, int *, int *, int *, int *, int *))
      SDL_LoadFunction(lib.handle, "png_get_IHDR");
@@ -127,6 +149,13 @@
      SDL_UnloadObject(lib.handle);
      return -1;
     }
+    lib.png_set_IHDR =
+     (void (*) (png_structp, png_infop, png_uint_32, png_uint_32, int, int, int, int, int))
+     SDL_LoadFunction(lib.handle, "png_set_IHDR");
+    if ( lib.png_set_IHDR == NULL ) {
+     SDL_UnloadObject(lib.handle);
+     return -1;
+    }
     lib.png_get_io_ptr =
      (png_voidp (*) (png_structp))
      SDL_LoadFunction(lib.handle, "png_get_io_ptr");
@@ -155,6 +184,13 @@
      SDL_UnloadObject(lib.handle);
      return -1;
     }
+    lib.png_write_image =
+     (void (*) (png_structp, png_bytepp))
+     SDL_LoadFunction(lib.handle, "png_write_image");
+    if ( lib.png_write_image == NULL ) {
+     SDL_UnloadObject(lib.handle);
+     return -1;
+    }
     lib.png_read_info =
      (void (*) (png_structp, png_infop))
      SDL_LoadFunction(lib.handle, "png_read_info");
@@ -162,6 +198,27 @@
      SDL_UnloadObject(lib.handle);
      return -1;
     }
+    lib.png_write_info =
+     (void (*) (png_structp, png_infop))
+     SDL_LoadFunction(lib.handle, "png_write_info");
+    if ( lib.png_write_info == NULL ) {
+     SDL_UnloadObject(lib.handle);
+     return -1;
+    } 
+    lib.png_set_PLTE =
+     (void (*) (png_structp, png_infop, png_colorp, int))
+     SDL_LoadFunction(lib.handle, "png_set_PLTE");
+    if ( lib.png_set_PLTE == NULL ) {
+     SDL_UnloadObject(lib.handle);
+     return -1;
+    }
+    lib.png_write_end =
+     (void (*) (png_structp, png_infop))
+     SDL_LoadFunction(lib.handle, "png_write_end");
+    if ( lib.png_write_end == NULL ) {
+     SDL_UnloadObject(lib.handle);
+     return -1;
+    }
     lib.png_read_update_info =
      (void (*) (png_structp, png_infop))
      SDL_LoadFunction(lib.handle, "png_read_update_info");
@@ -197,6 +254,13 @@
      SDL_UnloadObject(lib.handle);
      return -1;
     }
+    lib.png_set_write_fn =
+     (void (*) (png_structp, png_voidp, png_rw_ptr))
+     SDL_LoadFunction(lib.handle, "png_set_write_fn");
+    if ( lib.png_set_write_fn == NULL ) {
+     SDL_UnloadObject(lib.handle);
+     return -1;
+    }
     lib.png_set_strip_16 =
      (void (*) (png_structp))
      SDL_LoadFunction(lib.handle, "png_set_strip_16");
@@ -472,7 +536,7 @@
         palette->colors[i].g = i;
         palette->colors[i].b = i;
     }
-       } else if (info_ptr->num_palette > 0 ) {
+    } else if (info_ptr->num_palette > 0 ) {
     palette->ncolors = info_ptr->num_palette; 
     for( i=0; i<info_ptr->num_palette; ++i ) {
         palette->colors[i].b = info_ptr->palette[i].blue;
@@ -505,18 +569,167 @@
    return(surface); 
 }

-#else
+static void png_write_data(png_structp ctx, png_bytep area, png_size_t size)
+{
+   SDL_RWops *src;

-/* See if an image is contained in a data source */
-int IMG_isPNG(SDL_RWops *src)
+   src = (SDL_RWops *)lib.png_get_io_ptr(ctx);
+   SDL_RWwrite(src, area, size, 1);
+}
+/* write a png file */
+int IMG_SavePNG_RW(SDL_Surface *surface, SDL_RWops *src)
 {
-   return(0);
-}
+    png_structp png_ptr;
+    png_infop info_ptr;
+   png_colorp palette = NULL;
+    int start;
+   int colorType;
+   int i;
+    const char *error;
+   SDL_Palette *sdlPalette;
+   png_uint_32 height = surface->h;
+   png_uint_32 width = surface->w;
+   png_bytep *volatile row_pointers;
+   int row;

-/* Load a PNG type image from an SDL datasource */
-SDL_Surface *IMG_LoadPNG_RW(SDL_RWops *src)
-{
-   return(NULL);
+   if ( !src ) {
+    /* The error message has been set in SDL_RWFromFile */
+    return -1;
+   }
+   start = SDL_RWtell(src);
+
+   if ( IMG_InitPNG() < 0 ) {
+    return -1;
+   }
+
+   /* Create and initialize the png_struct with the desired error handler
+    * functions.  If you want to use the default stderr and longjump method,
+    * you can supply NULL for the last three parameters.  We also check that
+    * the library version is compatible with the one used at compile time,
+    * in case we are using dynamically linked libraries.  REQUIRED.
+    */
+    png_ptr = NULL; info_ptr = NULL;
+
+   /* Create the PNG loading context structure */
+   png_ptr = lib.png_create_read_struct(PNG_LIBPNG_VER_STRING,
+         NULL,NULL,NULL);
+   if (png_ptr == NULL){
+    error = "Couldn't allocate memory for PNG file or incompatible PNG dll";
+    goto done;
+   }
+
+    /* Allocate/initialize the memory for image information.  REQUIRED. */
+   info_ptr = lib.png_create_info_struct(png_ptr);
+   if (info_ptr == NULL) {
+    error = "Couldn't create image information for PNG file";
+    goto done;
+   }
+
+   /* Set error handling if you are using setjmp/longjmp method (this is
+    * the normal method of doing things with libpng).  REQUIRED unless you
+    * set up your own error handlers in png_create_read_struct() earlier.
+    */
+   if ( setjmp(png_ptr->jmpbuf) ) {
+    error = "Error reading the PNG file.";
+    goto done;
+   }
+
+   /* Set up the output control */
+    lib.png_set_write_fn(png_ptr, src, png_write_data);
+
+    /* Set the image information here.  Width and height are up to 2^31,
+     * bit_depth is one of 1, 2, 4, 8, or 16, but valid values also depend on
+     * the color_type selected. color_type is one of PNG_COLOR_TYPE_GRAY,
+     * PNG_COLOR_TYPE_GRAY_ALPHA, PNG_COLOR_TYPE_PALETTE, PNG_COLOR_TYPE_RGB,
+     * or PNG_COLOR_TYPE_RGB_ALPHA.  interlace is either PNG_INTERLACE_NONE or
+     * PNG_INTERLACE_ADAM7, and the compression_type and filter_type MUST
+     * currently be PNG_COMPRESSION_TYPE_BASE and PNG_FILTER_TYPE_BASE. REQUIRED
+     */
+   sdlPalette = surface->format->palette;
+   if (sdlPalette)
+   {  
+    colorType = PNG_COLOR_TYPE_PALETTE;
+   } else if (surface->format->Amask)
+   {
+    colorType = PNG_COLOR_TYPE_RGB_ALPHA;
+   } else
+   {
+    colorType = PNG_COLOR_TYPE_RGB;
+   }
+   lib.png_set_IHDR(png_ptr, info_ptr, surface->w, surface->h, surface->format->BitsPerPixel, colorType,
+    PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_BASE);
+
+   /* set the palette if there is one.  REQUIRED for indexed-color images */
+   if (colorType == PNG_COLOR_TYPE_PALETTE)
+   {
+        palette = (png_colorp) malloc(sdlPalette->ncolors * sizeof(png_color));
+
+    for( i=0; i < sdlPalette->ncolors; ++i ) {
+        palette[i].blue = sdlPalette->colors[i].b;
+        palette[i].green = sdlPalette->colors[i].g;
+        palette[i].red = sdlPalette->colors[i].r;
+    }
+    lib.png_set_PLTE(png_ptr, info_ptr, palette, sdlPalette->ncolors);
+   }
+   else
+   { //not sure how to handle this
+//     sig_bit.red = true_red_bit_depth;
+//     sig_bit.green = true_green_bit_depth;
+//  sig_bit.blue = true_blue_bit_depth;
+       /* if the image has an alpha channel then */
+//  sig_bit.alpha = true_alpha_bit_depth;
+//  png_set_sBIT(png_ptr, info_ptr, sig_bit);
+   }
+
+    /* Write the file header information.  REQUIRED */
+    lib.png_write_info(png_ptr, info_ptr);
+
+    /* The easiest way to write the image (you may have a different memory
+     * layout, however, so choose what fits your needs best).  You need to
+     * use the first method if you aren't handling interlacing yourself.
+     */
+
+   row_pointers = (png_bytep*) malloc(sizeof(png_bytep)*height);
+   if ( (row_pointers == NULL) ) {
+    error = "Out of memory";
+    goto done;
+   }
+   for (row = 0; row < (int)height; row++) {
+    row_pointers[row] = (png_bytep)
+      (Uint8 *)surface->pixels + row*surface->pitch;
+   }
+
+    if (height > PNG_UINT_32_MAX/sizeof(png_bytep))
+   {
+    error = "Image is too tall to process in memory";
+    goto done;
+   }
+
+   /* Read the entire image in one go */
+   lib.png_write_image(png_ptr, row_pointers);
+
+   /* It is REQUIRED to call this to finish writing the rest of the file */
+   lib.png_write_end(png_ptr, info_ptr);
+
+done:  /* Clean up and return */
+   if ( png_ptr ) {
+    lib.png_destroy_write_struct(&png_ptr, &info_ptr);
+   }
+   if ( row_pointers ) {
+    free(row_pointers);
+   }
+   if (palette)
+   {
+    free(palette);
+   }
+   if ( error ) {
+    IMG_QuitPNG();
+    IMG_SetError(error);
+    return(-1);
+   } else {
+    IMG_QuitPNG();
+   }
+   return(0); 
 }

 #endif /* LOAD_PNG */
Index: SDL_image.h
===================================================================
--- SDL_image.h (revision 4475)
+++ SDL_image.h (working copy)
@@ -107,6 +107,8 @@

 extern DECLSPEC SDL_Surface * SDLCALL IMG_ReadXPMFromArray(char **xpm);

+extern DECLSPEC int SDLCALL IMG_SavePNG_RW(SDL_Surface *surface, SDL_RWops *src);
+
 /* We'll use SDL for reporting errors */
 #define IMG_SetError   SDL_SetError
 #define IMG_GetError   SDL_GetError

Answer 1

Saving SDL surfaces as PNG images is a problem that's been solved on the web repeatedly:
http://www.bishoujo.us/svn/renpy/trunk/module/
http://lists.libsdl.org/pipermail/sdl-libsdl.org/2006-May/055936.html
http://www.os4depot.net/index.php?function=showfile&file=development/example/sdlpngsavesurf.lha
http://encelo.netsons.org/programming/sdl

Rather than asking for help debugging a significant amount of code, comparing your code to those other implementations and seeing where they differ, or asking which of those other implementations is the best to use, would probably be more productive.

As an aside, patching the SDL_image library might be a less robust solution than writing separate dependent libraries to implement PNG saving. That way the next SDL upgrade won't break your code. If you were thinking of contributing the patch to SDL, all the other implementations mentioned go back a few years. I don't think that SDL_image doesn't include PNG saving for lack of anyone wanting or being able to code it.

Answer 2

I managed to get it working. I needed to call png_set_bgr() to fix the palette.