diff options
Diffstat (limited to 'fitstopnm.html')
| -rw-r--r-- | fitstopnm.html | 21 |
1 files changed, 16 insertions, 5 deletions
diff --git a/fitstopnm.html b/fitstopnm.html index 7ce160f6..7d870bae 100644 --- a/fitstopnm.html +++ b/fitstopnm.html @@ -2,7 +2,7 @@ <HTML><HEAD><TITLE>Fitstopnm User Manual</TITLE></HEAD> <BODY> <H1>fitstopnm</H1> -Updated: 22 May 2007 +Updated: 2 August 2015 <BR> <A HREF="#index">Table Of Contents</A> @@ -34,6 +34,17 @@ input and produces a PPM image if the FITS file consists of 3 image planes (NAXIS = 3 and NAXIS3 = 3), or a PGM image if the FITS file consists of 2 image planes (NAXIS = 2), or if you specify the <B>-image</B> option. +<p>Note that the PPM image is highly unlikely to be a true PPM image, as it is +not normal for a FITS image to use the third axis as R, G, and B components of +the pixels. The most common interpretation when there are 3 axes is that the +third one is time. So the image is instead a pseudo-PPM in which the three +sample values of a pixel represent something other than color components, for +example gray levels at three instants (this variation on PPM is common in +programs such as <b>fitstopnm</b> that predate the PAM format). + +<p>If you work with FITS images with 3 axes, you should probably always use +the <b>-image</b> option to avoid getting an unwanted pseudo-PPM image. + <P>The program tells you what kind of PNM image it is writing. @@ -43,11 +54,11 @@ image planes (NAXIS = 2), or if you specify the <B>-image</B> option. <dt><b>-image=</b><i>N</i> -<dd>This is for FITS files with three axes. The assumption is that -the third axis is for multiple images, and this option lets you select -which one you want. +<dd>This is for FITS files with three axes. This option says that the third +axis is for multiple images, and the option value <i>N</i> tells which one you +want. -<dd><b>-omaxval=</b><i>N</i> +<dt><b>-omaxval=</b><i>N</i> <P>This is the maxval that the output PNM image is to have. |