The READ_PNG routine reads the image contents of a Portable Network Graphics (PNG) file and returns the image in an IDL variable. If the image contains a palette (see QUERY_PNG), it can be returned in three IDL variables. READ_PNG supports 1, 2, 3 and 4 channel images with channel depths of 8 or 16 bits.
When an image is read, it is scaled to the sBit value, if it is provided in the file. The sBit provides the actual bit depth, which can be different from the "bit depth" byte. See www.w3.org/TR/PNG-Chunks.html for more information.
Note: IDL supports version 1.2.7 of the PNG Library.
Note: Only single channel 8-bit images may have palettes. If an 8-bit, single-channel image has index values marked as “transparent,” these can be retrieved as well.
Note: To find information about a potential PNG file before trying to read its data, use the QUERY_PNG function.
Create an RGBA (16-bits/channel) and a Color Indexed (8-bit/channel) image with a palette:
rgbdata = UINDGEN(4,320,240)
cidata = BYTSCL(DIST(256))
red = indgen(256)
green = indgen(256)
blue = indgen(256)
;Query and read the data
names = ['rgb_image.png','ci_image.png','unknown.png']
FOR i=0,N_ELEMENTS(names)-1 DO BEGIN
ok = QUERY_PNG(names[i],s)
IF (ok) THEN BEGIN
IF (s.HAS_PALETTE) THEN BEGIN
img = READ_PNG(names[i],rpal,gpal,bpal)
ENDIF ELSE BEGIN
img = READ_PNG(names[i])
ENDIF ELSE BEGIN
PRINT,names[i],' is not a PNG file'
For 8-bit images, Result will be a two- or three-dimensional array of type byte. For 16-bit images, Result will be of type unsigned integer (UINT).
A scalar string containing the full pathname of the PNG file to read.
A named variable in which the image array will be stored, if READ_PNG is called as a procedure. For 8-bit images, Image will be a two- or three-dimensional array of type byte. For 16-bit images, Image will be of type unsigned integer (UINT).
R, G, B
Named variables that will contain the Red, Green, and Blue color vectors if a color table exists.
Set this keyword to indicate that the rows of the image should be read from bottom to top. The rows are read from top to bottom by default. ORDER provides compatibility with PNG files written using versions of IDL prior to IDL 5.4, which wrote PNG files from bottom to top.
Set this keyword to produce additional diagnostic output during the read operation.
Set this keyword to a named variable in which to return an array of opacity values (an opacity table) in the range of 0 to 255. A value of 0 (zero) equals full transparency (no opacity). If you specify fewer opacity values than exist in the color table, the missing values are replaced by 255. If the file does not contain an opacity table, then a scalar 0 is returned.
This keyword is valid only if Image is a single-channel (color indexed) image and the R, G, B palette is provided.