implement spatial (vs pixel) dimensions through tags for PNG files

Imager/Changes

@@ -461,6 +461,9 @@ Revision history for Perl extension Imager.
         - freetype 2 support
         - exposed the matrix_transform() function to the OO interface
         - added Imager::Matrix2d convenience class
+        - support for setting the resolution when writing to PNG
+        - retrieve physical resolution to tags when reading from PNG
+        - found an XS bug in the interface to i_tags_add()
 
 =================================================================
 

Imager/Imager.pm

@@ -3376,6 +3376,26 @@ the first block of the first gif comment before each image.
 Where applicable, the ("name") is the name of that field from the GIF89 
 standard.
 
+Some standard tags will be implemented as time goes by:
+
+=over
+
+=item i_xres
+
+=item i_yres
+
+The spatial resolution of the image in pixels per inch.  If the image
+format uses a different scale, eg. pixels per meter, then this value
+is converted.  A floating point number stored as a string.
+
+=item i_aspect_only
+
+If this is non-zero then the values in i_xres and i_yres are treated
+as a ratio only.  If the image format does not support aspect rations
+then this is scaled so the smaller value is 72dpi.
+
+=back
+
 =head1 BUGS
 
 box, arc, circle do not support antialiasing yet.  arc, is only filled

Imager/Imager.xs

@@ -2613,7 +2613,7 @@ i_tags_add(im, name, code, data, idata)
         else
           name = NULL;
         if (SvOK(ST(3)))
-          name = SvPV(ST(3), len);
+          data = SvPV(ST(3), len);
         else {
           data = NULL;
           len = 0;

Imager/TODO

@@ -25,6 +25,8 @@ MultiImage & metadata support:
   local errors?
 - SEE design/represent.txt for proposed new structure and
   interface design that takes these factors into account.
+- define common i_* tags for specifying attribute common among images
+  like spatial resolution
 
 New Features:
 - Add mng support.

Imager/png.c

@@ -110,6 +110,8 @@ check_if_png(char *file_name, FILE **fp) {
    return(!png_sig_cmp((png_bytep)buf, (png_size_t)0, PNG_BYTES_TO_CHECK));
 }
 
+static void get_png_tags(i_img *im, png_structp png_ptr, png_infop info_ptr);
+
 /* Read a PNG file.  You may want to return an error code if the read
  * fails (depending upon the failure).  There are two "prototypes" given
  * here - one where we are given the filename, and we need to open the
@@ -246,6 +248,9 @@ i_readpng(int fd) {
   /* read rest of file, and get additional chunks in info_ptr - REQUIRED */
 
   png_read_end(png_ptr, info_ptr); 
+
+  get_png_tags(im, png_ptr, info_ptr);
+
   /* clean up after the read, and free any memory allocated - REQUIRED */
   png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp)NULL);
 
@@ -263,6 +268,10 @@ i_writepng(i_img *im,int fd) {
   png_infop info_ptr;
   int width,height,y;
   volatile int cspace,channels;
+  double xres, yres;
+  int aspect_only, have_res;
+  double offx, offy;
+  char offunit[20] = "pixel";
 
   mm_log((1,"i_writepng(0x%x,fd %d)\n",im,fd));
 
@@ -331,6 +340,28 @@ i_writepng(i_img *im,int fd) {
   png_set_IHDR(png_ptr, info_ptr, width, height, 8, cspace,
 	       PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_BASE);
 
+  have_res = 1;
+  if (i_tags_get_float(&im->tags, "i_xres", 0, &xres)) {
+    if (i_tags_get_float(&im->tags, "i_yres", 0, &yres))
+      ; /* nothing to do */
+    else
+      yres = xres;
+  }
+  else {
+    if (i_tags_get_float(&im->tags, "i_yres", 0, &yres))
+      xres = yres;
+    else
+      have_res = 0;
+  }
+  if (have_res) {
+    aspect_only = 0;
+    i_tags_get_int(&im->tags, "i_aspect_only", 0, &aspect_only);
+    xres /= 0.0254;
+    yres /= 0.0254;
+    png_set_pHYs(png_ptr, info_ptr, xres + 0.5, yres + 0.5, 
+                 aspect_only ? PNG_RESOLUTION_UNKNOWN : PNG_RESOLUTION_METER);
+  }
+
   png_write_info(png_ptr, info_ptr);
   if (!im->virtual && im->type == i_direct_type && im->bits == i_8_bits) {
     for (y = 0; y < height; y++) 
@@ -432,6 +463,8 @@ i_readpng_scalar(char *data, int length) {
     for (y = 0; y < height; y++) { png_read_row(png_ptr,(png_bytep) &(im->idata[channels*width*y]), NULL); }
   mm_log((1,"made it to here 2\n"));
   png_read_end(png_ptr, info_ptr); 
+  get_png_tags(im, png_ptr, info_ptr);
+
   mm_log((1,"made it to here 3\n"));
   png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp)NULL);
   mm_log((1,"made it to here 4\n"));
@@ -516,3 +549,21 @@ i_readpng_scalar(char *data, int length) {
 
 /*   return im; */
 /* } */
+
+static void get_png_tags(i_img *im, png_structp png_ptr, png_infop info_ptr) {
+  png_uint_32 xres, yres;
+  int unit_type;
+  if (png_get_pHYs(png_ptr, info_ptr, &xres, &yres, &unit_type)) {
+    mm_log((1,"pHYs (%d, %d) %d\n", xres, yres, unit_type));
+    if (unit_type == PNG_RESOLUTION_METER) {
+      i_tags_set_float(&im->tags, "i_xres", 0, xres * 0.0254);
+      i_tags_set_float(&im->tags, "i_yres", 0, xres * 0.0254);
+    }
+    else {
+      i_tags_addn(&im->tags, "i_xres", 0, xres);
+      i_tags_addn(&im->tags, "i_yres", 0, yres);
+      i_tags_addn(&im->tags, "i_aspect_only", 0, 1);
+    }
+  }
+}
+

Imager/t/t102png.t

@@ -7,7 +7,7 @@
 # (It may become useful if the test is moved to ./t subdirectory.)
 use lib qw(blib/lib blib/arch);
 
-BEGIN { $| = 1; print "1..10\n"; }
+BEGIN { $| = 1; print "1..12\n"; }
 END {print "not ok 1\n" unless $loaded;}
 use Imager qw(:all);
 
@@ -36,10 +36,14 @@ i_box_filled($timg, 0, 0, 20, 20, $green);
 i_box_filled($timg, 2, 2, 18, 18, $trans);
 
 if (!i_has_format("png")) {
-  for (2..10) {
+  for (2..12) {
     print "ok $_ # skip no png support\n";
   }
 } else {
+  Imager::i_tags_add($img, "i_xres", 0, "300", 0);
+  Imager::i_tags_add($img, "i_yres", 0, undef, 200);
+  # the following confuses the GIMP
+  #Imager::i_tags_add($img, "i_aspect_only", 0, undef, 1);
   open(FH,">testout/t102.png") || die "cannot open testout/t102.png for writing\n";
   binmode(FH);
   i_writepng($img,fileno(FH)) || print "not ";
@@ -57,14 +61,21 @@ if (!i_has_format("png")) {
   print i_img_diff($img, $cmpimg)
     ? "not ok 4 # saved image different\n" : "ok 4\n";
 
+  my %tags = map { Imager::i_tags_get($img, $_) }
+    0..Imager::i_tags_count($img) - 1;
+  abs($tags{i_xres} - 300) < 1 or print "not ";
+  print "ok 5 # i_xres: $tags{i_xres}\n";
+  abs($tags{i_yres} - 200) < 1 or print "not ";
+  print "ok 6 # i_yres: $tags{i_yres}\n";
+
   open FH, "> testout/t102_trans.png"
     or die "Cannot open testout/t102_trans.png: $!";
   binmode FH;
   if (i_writepng($timg, fileno(FH))) {
-    print "ok 5\n";
+    print "ok 7\n";
   }
   else {
-    print "ok 5 # skip - png transparency not yet implemented\n";
+    print "ok 7 # skip - png transparency not yet implemented\n";
   }
   close FH;
 
@@ -74,10 +85,10 @@ if (!i_has_format("png")) {
   $cmpimg=i_readpng(fileno(FH)) || print "not ";
   close(FH);
 
-  print "ok 6\n";
+  print "ok 8\n";
   print "# png average mean square pixel difference: ",sqrt(i_img_diff($timg,$cmpimg))/150*150,"\n";
   print i_img_diff($timg, $cmpimg)
-	? "not ok 7 # saved image different\n" : "ok 7\n";
+	? "not ok 9 # saved image different\n" : "ok 9\n";
 
   # REGRESSION TEST
   # png.c 1.1 would produce an incorrect image when loading images with
@@ -88,24 +99,24 @@ if (!i_has_format("png")) {
   # 1.1 may segfault here (it does with libefence)
   my $pimg = i_readpng(fileno(FH))
     or print "not ";
-  print "ok 8\n";
+  print "ok 10\n";
   close FH;
   open FH, "< testimg/palette_out.png"
     or die "cannot open testimg/palette_out.png: $!\n";
   binmode FH;
   my $poimg = i_readpng(fileno(FH))
     or print "not ";
-  print "ok 9\n";
+  print "ok 11\n";
   close FH;
   if (i_img_diff($pimg, $poimg)) {
     print <<EOS;
-not ok 10 # regression or you may need a more recent libpng
+not ok 12 # regression or you may need a more recent libpng
 # this tests a bug in Imager's png.c v1.1
 # if also tickles a bug in libpng before 1.0.5, so you may need to
 # upgrade libpng
 EOS
   }
   else {
-    print "ok 10\n";
+    print "ok 12\n";
   }
 }

Imager/tags.c

@@ -216,3 +216,85 @@ int i_tags_delbycode(i_img_tags *tags, int code) {
   return count;
 }
 
+int i_tags_get_float(i_img_tags *tags, char *name, int code, double *value) {
+  int index;
+  i_img_tag *entry;
+
+  if (name) {
+    if (!i_tags_find(tags, name, 0, &index))
+      return 0;
+  }
+  else {
+    if (!i_tags_findn(tags, code, 0, &index))
+      return 0;
+  }
+  entry = tags->tags+index;
+  if (entry->data)
+    *value = atof(entry->data);
+  else
+    *value = entry->idata;
+
+  return 1;
+}
+
+int i_tags_set_float(i_img_tags *tags, char *name, int code, double value) {
+  char temp[40];
+
+  sprintf(temp, "%.30g", value);
+  if (name)
+    i_tags_delbyname(tags, name);
+  else
+    i_tags_delbycode(tags, code);
+
+  return i_tags_add(tags, name, code, temp, strlen(temp), 0);
+}
+
+int i_tags_get_int(i_img_tags *tags, char *name, int code, int *value) {
+  int index;
+  i_img_tag *entry;
+
+  if (name) {
+    if (!i_tags_find(tags, name, 0, &index))
+      return 0;
+  }
+  else {
+    if (!i_tags_findn(tags, code, 0, &index))
+      return 0;
+  }
+  entry = tags->tags+index;
+  if (entry->data)
+    *value = atoi(entry->data);
+  else
+    *value = entry->idata;
+
+  return 1;
+}
+
+int i_tags_get_string(i_img_tags *tags, char *name, int code, 
+                      char *value, size_t value_size) {
+  int index;
+  i_img_tag *entry;
+
+  if (name) {
+    if (!i_tags_find(tags, name, 0, &index))
+      return 0;
+  }
+  else {
+    if (!i_tags_findn(tags, code, 0, &index))
+      return 0;
+  }
+  entry = tags->tags+index;
+  if (entry->data) {
+    size_t cpsize = value_size < entry->size ? value_size : entry->size;
+    memcpy(value, entry->data, cpsize);
+    if (cpsize == value_size)
+      --cpsize;
+    value[cpsize] = '\0';
+  }
+  else {
+    sprintf(value, "%d", entry->data);
+  }
+
+  return 1;
+}
+