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tIME and text position handling

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The handling of tIME and text chunks on read now records the location of the
chunks relative to PLTE and IDAT.  Behavior on write is unchanged except that if
the position was recorded on read it will be re-used.

This involves an ABI change to the png_text_struct; a one byte location field is
added (with the same meaning as the one used to record unknown chunk location.)
Because this field is only used on read there is no API change unless a png_info
from a libpng read is passed to a subsequent libpng write (this did not work
very well before 1.7; the tIME chunk could get duplicated.)

png_set_text ignores the new field, resetting it to the current position in the
read or write stream.  On write the position is set to the next location to be
written unless the write has not started (the position is before the signature)
in which case the location is set to PNG_HAVE_PLTE|PNG_AFTER_IDAT.  When the
chunk is written the position is set to the actual write location (effectively
the position is frozen.)

Signed-off-by: John Bowler <jbowler@acm.org>
This commit is contained in:
John Bowler
2015-12-29 15:30:31 -08:00
parent c90572ee77
commit 4253c4d759
7 changed files with 192 additions and 84 deletions
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19
png.h
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@@ -692,6 +692,16 @@ typedef png_sPLT_t * * png_sPLT_tpp;
* PNG_TEXT_COMPRESSION_zTXt. Note that the "compression value" is not the
* same as what appears in the PNG tEXt/zTXt/iTXt chunk's "compression flag"
* which is always 0 or 1, or its "compression method" which is always 0.
*
* The location field (added in libpng 1.7.0) records where the text chunk was
* found when png_get_text is used. When png_set_text is used the field in the
* structure passed in is ignored and, instead, the field is set to the current
* write position.
*
* Prior to 1.7.0 the write behavior was the same; the text fields were written
* (once) at the next write_info call, however the read mechanism did not record
* the chunk location so if an info_struct from read was passed to the write
* APIs the text chunks would all be written at the start (before PLTE).
*/
typedef struct png_text_struct
{
@@ -700,6 +710,7 @@ typedef struct png_text_struct
0: zTXt, deflate
1: iTXt, none
2: iTXt, deflate */
png_byte location; /* before/after PLTE or after IDAT */
png_charp key; /* keyword, 1-79 character description of "text" */
png_charp text; /* comment, may be an empty string (ie "")
or a NULL pointer */
@@ -717,8 +728,14 @@ typedef png_text * * png_textpp;
/* Supported compression types for text in PNG files (tEXt, and zTXt).
* The values of the PNG_TEXT_COMPRESSION_ defines should NOT be changed. */
#ifdef PNG_OLD_COMPRESSION_CODES_SUPPORTED
/* These values were used to prevent double write of text chunks in versions
* prior to 1.7.0. They are never set now; if you need them #define the
* _SUPPORTED macro.
*/
#define PNG_TEXT_COMPRESSION_NONE_WR -3
#define PNG_TEXT_COMPRESSION_zTXt_WR -2
#endif /* OLD_COMPRESSION_CODES */
#define PNG_TEXT_COMPRESSION_NONE -1
#define PNG_TEXT_COMPRESSION_zTXt 0
#define PNG_ITXT_COMPRESSION_NONE 1
@@ -776,7 +793,7 @@ typedef const png_unknown_chunk * png_const_unknown_chunkp;
typedef png_unknown_chunk * * png_unknown_chunkpp;
#endif
/* Flag values for the unknown chunk location byte. */
/* Flag values for the chunk location byte. */
#define PNG_HAVE_IHDR 0x01U
#define PNG_HAVE_PLTE 0x02U
#define PNG_AFTER_IDAT 0x08U