READ_PNG

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 as well in three IDL variables. READ_PNG supports 1, 2, 3 and 4 channel images with channel depths of 8 or 16 bits.

Syntax

Result = READ_PNG ( Filename [, R, G, B] [,/ORDER] [, /VERBOSE] [, TRANSPARENT=variable] )

or

READ_PNG, Filename, Image [, R, G, B] [,/ORDER] [, /VERBOSE] [, TRANSPARENT=variable]]

Return Value

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).

Arguments

Filename

A scalar string containing the full pathname of the PNG file to read.

R, G, B

Named variables that will contain the Red, Green, and Blue color vectors if a color table exists.

Keywords

ORDER

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.

VERBOSE

Set this keyword to produce additional diagnostic output during the read operation.

TRANSPARENT

Set this keyword equal to a named variable that will contain an array of pixel index values that are to be treated as "transparent" for the purposes of image display. If there are no transparent values then TRANSPARENT will be set to a long-integer scalar with the value 0.

Examples

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) 
WRITE_PNG,'rgb_image.png',rgbdata 
WRITE_PNG,'ci_image.png',cidata,red,green,blue 
;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 
      HELP,s,/STRUCTURE 
      IF (s.HAS_PALETTE) THEN BEGIN 
         img = READ_PNG(names[i],rpal,gpal,bpal) 
         HELP,img,rpal,gpal,bpal 
      ENDIF ELSE BEGIN 
         img = READ_PNG(names[i]) 
         HELP,img 
      ENDELSE 
   ENDIF ELSE BEGIN 
      PRINT,names[i],' is not a PNG file' 
   ENDELSE 
ENDFOR 
END 

Version History

Introduced: 5.2

See Also

WRITE_PNG, QUERY_PNG