NAME
Image::Leptonica::Func::pngio
VERSION
version 0.04
pngio.c
pngio.c
Read png from file
PIX *pixReadStreamPng()
l_int32 readHeaderPng()
l_int32 freadHeaderPng()
l_int32 sreadHeaderPng()
l_int32 fgetPngResolution()
Write png to file
l_int32 pixWritePng() [ special top level ]
l_int32 pixWriteStreamPng()
l_int32 pixSetZlibCompression()
Setting flag for special read mode
void l_pngSetReadStrip16To8()
Read/write to memory
PIX *pixReadMemPng()
l_int32 pixWriteMemPng()
Documentation: libpng.txt and example.c
On input (decompression from file), palette color images
are read into an 8 bpp Pix with a colormap, and 24 bpp
3 component color images are read into a 32 bpp Pix with
rgb samples. On output (compression to file), palette color
images are written as 8 bpp with the colormap, and 32 bpp
full color images are written compressed as a 24 bpp,
3 component color image.
In the following, we use these abbreviations:
bps == bit/sample
spp == samples/pixel
bpp == bits/pixel of image in Pix (memory)
where each component is referred to as a "sample".
For reading and writing rgb and rgba images, we read and write
alpha if it exists (spp == 4) and do not read or write if
it doesn't (spp == 3). The alpha component can be 'removed'
simply by setting spp to 3. In leptonica, we make relatively
little explicit use of the alpha sample. Note that the alpha
sample in the image is also called "alpha transparency",
"alpha component" and "alpha layer."
To change the zlib compression level, use pixSetZlibCompression()
before writing the file. The default is for standard png compression.
The zlib compression value can be set [0 ... 9], with
0 no compression (huge files)
1 fastest compression
-1 default compression (equivalent to 6 in latest version)
9 best compression
Note that if you are using the defined constants in zlib instead
of the compression integers given above, you must include zlib.h.
There is global for determining the size of retained samples:
var_PNG_STRIP_16_to_8
and a function l_pngSetReadStrip16To8() for setting it.
The default is TRUE, which causes pixRead() to strip each 16 bit
sample down to 8 bps:
- For 16 bps rgb (16 bps, 3 spp) --> 32 bpp rgb Pix
- For 16 bps gray (16 bps, 1 spp) --> 8 bpp grayscale Pix
If the variable is set to FALSE, the 16 bit gray samples
are saved when read; the 16 bit rgb samples return an error.
Note: results can be non-deterministic if used with
multi-threaded applications.
On systems like windows without fmemopen() and open_memstream(),
we write data to a temp file and read it back for operations
between pix and compressed-data, such as pixReadMemPng() and
pixWriteMemPng().FUNCTIONS
fgetPngResolution
l_int32 fgetPngResolution ( FILE *fp, l_int32 *pxres, l_int32 *pyres )
fgetPngResolution()
Input: stream (opened for read)
&xres, &yres (<return> resolution in ppi)
Return: 0 if OK; 0 on error
Notes:
(1) If neither resolution field is set, this is not an error;
the returned resolution values are 0 (designating 'unknown').
(2) Side-effect: this rewinds the stream.freadHeaderPng
l_int32 freadHeaderPng ( FILE *fp, l_int32 *pw, l_int32 *ph, l_int32 *pbps, l_int32 *pspp, l_int32 *piscmap )
freadHeaderPng()
Input: stream
&w (<optional return>)
&h (<optional return>)
&bps (<optional return>, bits/sample)
&spp (<optional return>, samples/pixel)
&iscmap (<optional return>)
Return: 0 if OK, 1 on error
Notes:
(1) See readHeaderPng().l_pngSetReadStrip16To8
void l_pngSetReadStrip16To8 ( l_int32 flag )
l_pngSetReadStrip16To8()
Input: flag (1 for stripping 16 bpp to 8 bpp on reading;
0 for leaving 16 bpp)
Return: voidpixReadMemPng
PIX * pixReadMemPng ( const l_uint8 *cdata, size_t size )
pixReadMemPng()
Input: cdata (const; png-encoded)
size (of data)
Return: pix, or null on error
Notes:
(1) The @size byte of @data must be a null character.pixReadStreamPng
PIX * pixReadStreamPng ( FILE *fp )
pixReadStreamPng()
Input: stream
Return: pix, or null on error
Notes:
(1) If called from pixReadStream(), the stream is positioned
at the beginning of the file.
(2) To do sequential reads of png format images from a stream,
use pixReadStreamPng()
(3) Grayscale-with-alpha pngs (spp = 2) are converted to RGBA
on read; the returned pix has spp = 4 and equal red, green and
blue channels.
(4) spp = 1 with alpha (palette) is converted to RGBA with spp = 4.
(5) We use the high level png interface, where the transforms are set
up in advance and the header and image are read with a single
call. The more complicated interface, where the header is
read first and the buffers for the raster image are user-
allocated before reading the image, works OK for single images,
but I could not get it to work properly for the successive
png reads that are required by pixaReadStream().pixSetZlibCompression
l_int32 pixSetZlibCompression ( PIX *pix, l_int32 compval )
pixSetZlibCompression()
Input: pix
compval (zlib compression value)
Return: 0 if OK, 1 on error
Notes:
(1) Valid zlib compression values are in the interval [0 ... 9],
where, as defined in zlib.h:
0 Z_NO_COMPRESSION
1 Z_BEST_SPEED (poorest compression)
9 Z_BEST_COMPRESSION
For the default value, use either of these:
6 Z_DEFAULT_COMPRESSION
-1 (resolves to Z_DEFAULT_COMPRESSION)
(2) If you use the defined constants in zlib.h instead of the
compression integers given above, you must include zlib.h.pixWriteMemPng
l_int32 pixWriteMemPng ( l_uint8 **pdata, size_t *psize, PIX *pix, l_float32 gamma )
pixWriteMemPng()
Input: &data (<return> data of tiff compressed image)
&size (<return> size of returned data)
pix
gamma (use 0.0 if gamma is not defined)
Return: 0 if OK, 1 on error
Notes:
(1) See pixWriteStreamPng() for usage. This version writes to
memory instead of to a file stream.pixWritePng
l_int32 pixWritePng ( const char *filename, PIX *pix, l_float32 gamma )
pixWritePng()
Input: filename
pix
gamma
Return: 0 if OK; 1 on error
Notes:
(1) Special version for writing png with a specified gamma.
When using pixWrite(), no field is given for gamma.pixWriteStreamPng
l_int32 pixWriteStreamPng ( FILE *fp, PIX *pix, l_float32 gamma )
pixWriteStreamPng()
Input: stream
pix
gamma (use 0.0 if gamma is not defined)
Return: 0 if OK; 1 on error
Notes:
(1) If called from pixWriteStream(), the stream is positioned
at the beginning of the file.
(2) To do sequential writes of png format images to a stream,
use pixWriteStreamPng() directly.
(3) gamma is an optional png chunk. If no gamma value is to be
placed into the file, use gamma = 0.0. Otherwise, if
gamma > 0.0, its value is written into the header.
(4) The use of gamma in png is highly problematic. For an illuminating
discussion, see: http://hsivonen.iki.fi/png-gamma/
(5) What is the effect/meaning of gamma in the png file? This
gamma, which we can call the 'source' gamma, is the
inverse of the gamma that was used in enhance.c to brighten
or darken images. The 'source' gamma is supposed to indicate
the intensity mapping that was done at the time the
image was captured. Display programs typically apply a
'display' gamma of 2.2 to the output, which is intended
to linearize the intensity based on the response of
thermionic tubes (CRTs). Flat panel LCDs have typically
been designed to give a similar response as CRTs (call it
"backward compatibility"). The 'display' gamma is
in some sense the inverse of the 'source' gamma.
jpeg encoders attached to scanners and cameras will lighten
the pixels, applying a gamma corresponding to approximately
a square-root relation of output vs input:
output = input^(gamma)
where gamma is often set near 0.4545 (1/gamma is 2.2).
This is stored in the image file. Then if the display
program reads the gamma, it will apply a display gamma,
typically about 2.2; the product is 1.0, and the
display program produces a linear output. This works because
the dark colors were appropriately boosted by the scanner,
as described by the 'source' gamma, so they should not
be further boosted by the display program.
(6) As an example, with xv and display, if no gamma is stored,
the program acts as if gamma were 0.4545, multiplies this by 2.2,
and does a linear rendering. Taking this as a baseline
brightness, if the stored gamma is:
> 0.4545, the image is rendered lighter than baseline
< 0.4545, the image is rendered darker than baseline
In contrast, gqview seems to ignore the gamma chunk in png.
(7) The only valid pixel depths in leptonica are 1, 2, 4, 8, 16
and 32. However, it is possible, and in some cases desirable,
to write out a png file using an rgb pix that has 24 bpp.
For example, the open source xpdf SplashBitmap class generates
24 bpp rgb images. Consequently, we enable writing 24 bpp pix.
To generate such a pix, you can make a 24 bpp pix without data
and assign the data array to the pix; e.g.,
pix = pixCreateHeader(w, h, 24);
pixSetData(pix, rgbdata);
See pixConvert32To24() for an example, where we get rgbdata
from the 32 bpp pix. Caution: do not call pixSetPadBits(),
because the alignment is wrong and you may erase part of the
last pixel on each line.
(8) If the pix has a colormap, it is written to file. In most
situations, the alpha component is 255 for each colormap entry,
which is opaque and indicates that it should be ignored.
However, if any alpha component is not 255, it is assumed that
the alpha values are valid, and they are written to the png
file in a tRNS segment. On readback, the tRNS segment is
identified, and the colormapped image with alpha is converted
to a 4 spp rgba image.readHeaderPng
l_int32 readHeaderPng ( const char *filename, l_int32 *pw, l_int32 *ph, l_int32 *pbps, l_int32 *pspp, l_int32 *piscmap )
readHeaderPng()
Input: filename
&w (<optional return>)
&h (<optional return>)
&bps (<optional return>, bits/sample)
&spp (<optional return>, samples/pixel)
&iscmap (<optional return>)
Return: 0 if OK, 1 on error
Notes:
(1) If there is a colormap, iscmap is returned as 1; else 0.
(2) For gray+alpha, although the png records bps = 16, we
consider this as two 8 bpp samples (gray and alpha).
When a gray+alpha is read, it is converted to 32 bpp RGBA.sreadHeaderPng
l_int32 sreadHeaderPng ( const l_uint8 *data, l_int32 *pw, l_int32 *ph, l_int32 *pbps, l_int32 *pspp, l_int32 *piscmap )
sreadHeaderPng()
Input: data
&w (<optional return>)
&h (<optional return>)
&bps (<optional return>, bits/sample)
&spp (<optional return>, samples/pixel)
&iscmap (<optional return>; input NULL to ignore)
Return: 0 if OK, 1 on error
Notes:
(1) See readHeaderPng().
(2) png colortypes (see png.h: PNG_COLOR_TYPE_*):
0: gray; fully transparent (with tRNS) (1 spp)
2: RGB (3 spp)
3: colormap; colormap+alpha (with tRNS) (1 spp)
4: gray + alpha (2 spp)
6: RGBA (4 spp)
Note:
0 and 3 have the alpha information in a tRNS chunk
4 and 6 have separate alpha samples with each pixel.AUTHOR
Zakariyya Mughal <zmughal@cpan.org>
COPYRIGHT AND LICENSE
This software is copyright (c) 2014 by Zakariyya Mughal.
This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.