QUERY_PNG

QUERY_PNG is a method of obtaining information about a PNG image file without having to read the file. See QUERY_* Routines for more information.

When an image is read by READ_PNG, 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.

Examples

Query included in creating RGBA (16-bit/channel) and Color Indexed (8-bits/channel) image.

PRO ex_query_png

rgbdata = UINDGEN(4,320,240)

cidata = BYTSCL(DIST(256))

red = indgen(256)

green = indgen(256)

blue = indgen(256)

tempdir = GETENV('IDL_TMPDIR')

WRITE_PNG, tempdir+'rgb_image.png',rgbdata

WRITE_PNG, tempdir+'ci_image.png',cidata,red,green,blue

; Query and Read the data:

names = [tempdir+'rgb_image.png',$

         tempdir+'ci_image.png',$

         tempdir+'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

Syntax

Result = QUERY_PNG ( Filename [, Info] )

Return Value

This routine returns a long with the value of 1 (one) if the query was successful (and the file type was correct) or 0 (zero) on failure.

Arguments

Filename

A scalar string containing the pathname of the PNG file to query.

Info

Returns an anonymous structure containing information about the image in the file. The Info.TYPE field will return the value ‘PNG’.

Note: See General Query * Routine Info Structures for detailed structure information.

Keywords

None

Version History

5.2	Introduced