The png module includes a number of functions for dealing
with PNG images and colormaps. Use require("png") to load it.
Read an image from a PNG file
img = png_read (file [,&image_type])
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)
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
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).
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;
}
png_read_flipped, png_write, png_rgb_to_r, png_rgb_to_g, png_rgb_to_b
Read an image from a PNG file
img = png_read_flipped (file [,&image_type])
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.
png_read, png_write_flipped
Write an image to a PNG file
png_write (file, image [,has_alpha])
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
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.
png_write_flipped, png_read
Write an image to a PNG file
png_write_flipped (file, image [,has_alpha])
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.
png_read, png_write_flipped
Convert an RGB image to grayscale
gray = png_rgb_to_gray (rgb)
This function converts an RGB image represented by an array (typically 2d) to a corresponding array of 8 bit integers representing the grayscale values. The grayscale values are formed by averaging the R, G, and B channels.
png_gray_to_rgb
Convert a grayscale image to RGB
rgb = png_gray_to_rgb (gray, colormap)
This function creates an RGB image, represented by an array of 32 bit integers from a grayscale image using the specified colormap. The shape of the of the RGB array will be that of the input grayscale array. 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.
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.
png_rgb_to_gray, png_get_colormap
Retrieve a colormap
cmap = png_get_colormap (String_Type name)
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
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.
png_gray_to_rgb, png_add_colormap, png_get_colormap_names
Add a color map
png_add_colormap (name, cmap)
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.
png_get_colormap
Get the red channel of an RGB image
r = png_get_r (rgb)
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);
png_get_g, png_get_b, png_get_a
Get the green channel of an RGB image
r = png_get_g (rgb)
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);
png_get_r, png_get_b, png_get_a
Get the blue channel of an RGB image
r = png_get_b (rgb)
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);
png_get_r, png_get_g, png_get_a
Get the alpha channel of an RGB image
r = png_get_a (rgb)
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);
png_get_g, png_get_b, png_get_a