view release on metacpan or  search on metacpan

src/Source/LibPNG/pngwrite.c  view on Meta::CPAN

      display->first_row);
   png_uint_16p output_row = png_voidcast(png_uint_16p, display->local_row);
   png_uint_16p row_end;
   const int channels = (image->format & PNG_FORMAT_FLAG_COLOR) != 0 ? 3 : 1;
   int aindex = 0;
   png_uint_32 y = image->height;

   if ((image->format & PNG_FORMAT_FLAG_ALPHA) != 0)
   {
#     ifdef PNG_SIMPLIFIED_WRITE_AFIRST_SUPPORTED
         if ((image->format & PNG_FORMAT_FLAG_AFIRST) != 0)
         {
            aindex = -1;
            ++input_row; /* To point to the first component */
            ++output_row;
         }

         else
#     endif
         aindex = channels;
   }

   else
      png_error(png_ptr, "png_write_image: internal call error");

   /* Work out the output row end and count over this, note that the increment
    * above to 'row' means that row_end can actually be beyond the end of the
    * row; this is correct.
    */
   row_end = output_row + image->width * (channels+1);

   while (y-- > 0)
   {
      png_const_uint_16p in_ptr = input_row;
      png_uint_16p out_ptr = output_row;

      while (out_ptr < row_end)
      {
         const png_uint_16 alpha = in_ptr[aindex];
         png_uint_32 reciprocal = 0;
         int c;

         out_ptr[aindex] = alpha;

         /* Calculate a reciprocal.  The correct calculation is simply
          * component/alpha*65535 << 15. (I.e. 15 bits of precision); this
          * allows correct rounding by adding .5 before the shift.  'reciprocal'
          * is only initialized when required.
          */
         if (alpha > 0 && alpha < 65535)
            reciprocal = ((0xffff<<15)+(alpha>>1))/alpha;

         c = channels;
         do /* always at least one channel */
         {
            png_uint_16 component = *in_ptr++;

            /* The following gives 65535 for an alpha of 0, which is fine,
             * otherwise if 0/0 is represented as some other value there is more
             * likely to be a discontinuity which will probably damage
             * compression when moving from a fully transparent area to a
             * nearly transparent one.  (The assumption here is that opaque
             * areas tend not to be 0 intensity.)
             */
            if (component >= alpha)
               component = 65535;

            /* component<alpha, so component/alpha is less than one and
             * component*reciprocal is less than 2^31.
             */
            else if (component > 0 && alpha < 65535)
            {
               png_uint_32 calc = component * reciprocal;
               calc += 16384; /* round to nearest */
               component = (png_uint_16)(calc >> 15);
            }

            *out_ptr++ = component;
         }
         while (--c > 0);

         /* Skip to next component (skip the intervening alpha channel) */
         ++in_ptr;
         ++out_ptr;
      }

      png_write_row(png_ptr, png_voidcast(png_const_bytep, display->local_row));
      input_row += display->row_bytes/(sizeof (png_uint_16));
   }

   return 1;
}

/* Given 16-bit input (1 to 4 channels) write 8-bit output.  If an alpha channel
 * is present it must be removed from the components, the components are then
 * written in sRGB encoding.  No components are added or removed.
 *
 * Calculate an alpha reciprocal to reverse pre-multiplication.  As above the
 * calculation can be done to 15 bits of accuracy; however, the output needs to
 * be scaled in the range 0..255*65535, so include that scaling here.
 */
#define UNP_RECIPROCAL(alpha) ((((0xffff*0xff)<<7)+(alpha>>1))/alpha)

static png_byte
png_unpremultiply(png_uint_32 component, png_uint_32 alpha,
   png_uint_32 reciprocal/*from the above macro*/)
{
   /* The following gives 1.0 for an alpha of 0, which is fine, otherwise if 0/0
    * is represented as some other value there is more likely to be a
    * discontinuity which will probably damage compression when moving from a
    * fully transparent area to a nearly transparent one.  (The assumption here
    * is that opaque areas tend not to be 0 intensity.)
    *
    * There is a rounding problem here; if alpha is less than 128 it will end up
    * as 0 when scaled to 8 bits.  To avoid introducing spurious colors into the
    * output change for this too.
    */
   if (component >= alpha || alpha < 128)
      return 255;

   /* component<alpha, so component/alpha is less than one and
    * component*reciprocal is less than 2^31.
    */
   else if (component > 0)
   {
      /* The test is that alpha/257 (rounded) is less than 255, the first value
       * that becomes 255 is 65407.
       * NOTE: this must agree with the PNG_DIV257 macro (which must, therefore,
       * be exact!)  [Could also test reciprocal != 0]
       */
      if (alpha < 65407)
      {
         component *= reciprocal;
         component += 64; /* round to nearest */
         component >>= 7;
      }

      else
         component *= 255;

      /* Convert the component to sRGB. */
      return (png_byte)PNG_sRGB_FROM_LINEAR(component);
   }

   else
      return 0;
}

static int
png_write_image_8bit(png_voidp argument)
{
   png_image_write_control *display = png_voidcast(png_image_write_control*,
      argument);
   png_imagep image = display->image;
   png_structrp png_ptr = image->opaque->png_ptr;

   png_const_uint_16p input_row = png_voidcast(png_const_uint_16p,
      display->first_row);
   png_bytep output_row = png_voidcast(png_bytep, display->local_row);
   png_uint_32 y = image->height;
   const int channels = (image->format & PNG_FORMAT_FLAG_COLOR) != 0 ? 3 : 1;

   if ((image->format & PNG_FORMAT_FLAG_ALPHA) != 0)
   {
      png_bytep row_end;
      int aindex;

#     ifdef PNG_SIMPLIFIED_WRITE_AFIRST_SUPPORTED
         if ((image->format & PNG_FORMAT_FLAG_AFIRST) != 0)
         {
            aindex = -1;