ACC SHELL
png_read
SYNOPSIS
Read an image from a PNG file
USAGE
img = png_read (file [,&image_type])
DESCRIPTION
The `png_read' function reads the image from the specified file
and returns it. If a second argument is present, then it must be a
reference to a variable that will be set to an integer representing
the type of image read:
PNG_COLOR_TYPE_RGB (RGB image)
PNG_COLOR_TYPE_RGBA (RGB image with an alpha channel)
PNG_COLOR_TYPE_GRAY (Grayscale image)
PNG_COLOR_TYPE_GRAY_ALPHA (Grayscale image with an alpha channel)
The image returned by this function is represented by a 2d array of
unsigned integers. If the image type is `PNG_COLOR_TYPE_RGB'
or `PNG_COLOR_TYPE_RGBA', then the integers will be 32 bits
wide with the most significant byte corresponding to the alpha
channel, followed by bytes for the red, green, and blue channels in
that order. In this way, the color channels may be extracted from
the image through simple bit operations, e.g.,
(img & 0x0000FF00) shr 8
will pick out the green channel. Alternatively, the
`png_rgb_get_?' functions may be used to extract the various
channels.
If the image type is `PNG_COLOR_TYPE_GRAY_ALPHA' then image
will be represented by 16 bit integers with the most significant
byte corresponding to the alpha channel and the least significant
byte representing the grayscale. If the image type is
`PNG_COLOR_TYPE_GRAY' then the image array will consist of
unsigned 8 bit integers (`UChar_Type').
EXAMPLE
define read_grayscale_image (file)
{
variable img_type;
img = png_read (file, &img_type);
if ((img_type != PNG_COLOR_TYPE_GRAY)
or (img_type != PNG_COLOR_TYPE_GRAY_ALPHA))
img = png_rgb_to_gray (img);
return img;
}
SEE ALSO
png_read_flipped, png_write, png_rgb_to_r, png_rgb_to_g, png_rgb_to_b
--------------------------------------------------------------
png_read_flipped
SYNOPSIS
Read an image from a PNG file
USAGE
img = png_read_flipped (file [,&image_type])
DESCRIPTION
This function is like the `png_read' function except that it
returns an image with the rows flipped. See the documentation for
`png_read' for more information.
SEE ALSO
png_read, png_write_flipped
--------------------------------------------------------------
png_write
SYNOPSIS
Write an image to a PNG file
USAGE
png_write (file, image [,has_alpha])
DESCRIPTION
The `png_write' function writes a 2d image to the specified
file as a PNG. The type of PNG created depends upon the data type
of the image array according to the following table:
data-type PNG-type
-----------------------------------
Char_Type (8 bit) grayscale
UChar_Type (8 bit) grayscale
Int16_Type (16 bit) grayscale
UInt16_Type (16 bit) grayscale
Int32_Type (32 bit) RGB
UInt32_Type (32 bit) RGB
If the optional parameter `has_alpha' is present and non-zero,
then an alpha channel will also be written to the file.
See the documentation for the `png_read' function for
information about how the various channels are encoded as integers.
SEE ALSO
png_write_flipped, png_read
--------------------------------------------------------------
png_write_flipped
SYNOPSIS
Write an image to a PNG file
USAGE
png_write_flipped (file, image [,has_alpha])
DESCRIPTION
This function is like the `png_write' function except that it
writes the image to the file with the rows reversed. See the
documentation for `png_write' for more information.
SEE ALSO
png_read, png_write_flipped
--------------------------------------------------------------
png_rgb_to_gray
SYNOPSIS
Convert an RGB image to grayscale
USAGE
gray = png_rgb_to_gray (rgb)
DESCRIPTION
This function converts an RGB image represented by a 2d array of 32 bit
integers to a 2d array of 8 bit integers representing the grayscale values.
The grayscale values are formed by averaging the R, G, and B channels.
SEE ALSO
png_gray_to_rgb
--------------------------------------------------------------
png_gray_to_rgb
SYNOPSIS
Convert a grayscale image to RGB
USAGE
rgb = png_gray_to_rgb (gray, colormap)
DESCRIPTION
This function creates an RGB image, represented by a 2d array of
32 bit integers from a grayscale image using the specified colormap.
The values in the input image will be linearly mapped onto the
elements of the colormap such that the minimum value in the
grayscale image will be assigned the first RGB value in the
colormap, and the maximum value will be assigned the last RGB value.
The colormap argument must either be an array of 32 bit integers
encoding the RGB information, or must be the name of a supported
color map. See the documentation for the `png_get_colormap'
function for more information about colormaps.
QUALIFIERS
The qualifiers `gmin' and `gmax' may be used to define the
minimum and maximum grayscale values used for the mapping onto the
colormap. Grayscale values outside the specified range will be clipped.
SEE ALSO
png_rgb_to_gray, png_get_colormap
--------------------------------------------------------------
png_get_colormap
SYNOPSIS
Retrieve a colormap
USAGE
cmap = png_get_colormap (String_Type name)
DESCRIPTION
This function returns a colormap of the specified name. Currently
recognized names include:
cool Linear change from blue to magenta
copper Dark to light copper brown
gebco Colors for GEBCO bathymetric charts
globe Colors for global bathy-topo relief
drywet Goes from dry to wet colors
gray Grayramp from black to white
haxby Bill Haxby's colortable for geoid & gravity
hot Black through red and yellow to white
jet Dark to light blue, white, yellow and red
no_green For those who hate green
ocean white-green-blue bathymetry scale
polar Blue via white to red
rainbow Rainbow colors magenta-blue-cyan-green-yellow-red
red2green Polar scale from red to green via white
relief Wessel/Martinez colortable for bathymetry/topography
topo Sandwell-Anderson colors for topography
sealand Smith bathymetry/topography scale
seis R-O-Y-G-B seismic tomography colors
split Polar scale like polar, but via black instead of white
wysiwyg 20 RGB colors for openwin -cubesize large and waxenvy printer
The above colormaps were derived from the GMT distribution, which is
an open source collection of tools for geographic data. See
`http://gmt.soest.hawaii.edu/' for more information about GMT.
In the current implementation, the colormap is a 256 element array
of unsigned 32 bit integers.
SEE ALSO
png_gray_to_rgb, png_add_colormap, png_get_colormap_names
--------------------------------------------------------------
png_add_colormap
SYNOPSIS
Add a color map
USAGE
png_add_colormap (name, cmap)
DESCRIPTION
This function adds a color map of the specified name to the table of
internal colormaps. The colormap is simply a 256 element unsigned
32 integer bit array that maps 256 grayscale values to RGB.
SEE ALSO
png_get_colormap
--------------------------------------------------------------
png_get_r
SYNOPSIS
Get the red channel of an RGB image
USAGE
r = png_get_r (rgb)
DESCRIPTION
This function returns the red component of the RGB image encoded as
an unsigned 32 bit integer using the algorithm:
r = typecast ((rgb shr 16) & 0xFF, UChar_Type);
SEE ALSO
png_get_g, png_get_b, png_get_a
--------------------------------------------------------------
png_get_g
SYNOPSIS
Get the green channel of an RGB image
USAGE
r = png_get_g (rgb)
DESCRIPTION
This function returns the green component of the RGB image encoded as
an unsigned 32 bit integer using the algorithm:
g = typecast ((rgb shr 8) & 0xFF, UChar_Type);
SEE ALSO
png_get_r, png_get_b, png_get_a
--------------------------------------------------------------
png_get_b
SYNOPSIS
Get the blue channel of an RGB image
USAGE
r = png_get_b (rgb)
DESCRIPTION
This function returns the blue component of the RGB image encoded as
an unsigned 32 bit integer using the algorithm:
b = typecast ((rgb & 0xFF), UChar_Type);
SEE ALSO
png_get_r, png_get_g, png_get_a
--------------------------------------------------------------
png_get_a
SYNOPSIS
Get the alpha channel of an RGB image
USAGE
r = png_get_a (rgb)
DESCRIPTION
This function returns the alpha channel of the RGB image encoded as
an unsigned 32 bit integer using the algorithm:
a = typecast ((rgb shr 24) & 0xFF, UChar_Type);
SEE ALSO
png_get_g, png_get_b, png_get_a
--------------------------------------------------------------
ACC SHELL 2018