Hugh's examples

Here is how to read in and display a pseudocolour png image

IDL> pg=read_png('/home/hcp/wrk/idlcourse/colourpeng.png',r,g,b)
%%IDL> pg=rotate(pg,7) ; wont be needed after upgrade
IDL> tvlct,r,g,b
IDL> showimage,pg,win=2

Remember that on 24-bit displays, you will need to do:

IDL> device,true_color=24,decomposed=0,retain=2

for this to work.

The ppm file format stores 24 bit images. Here is how to read in a ppm file, we use the help command to see what the resulting array is like:

 
IDL> read_ppm,'/home/hcp/wrk/idlcourse/ferry.ppm',ferry
IDL> help,ferry
FERRY           BYTE      = Array[3, 593, 413]

This is an 593 X 413 pixel image containing three 'layers', red, green and blue.

We can do this to see the individual layers:

IDL> loadct,0
IDL> showimage,reform(ferry[2,*,*]),win=2
IDL> showimage,reform(ferry[1,*,*]),win=1
IDL> showimage,reform(ferry[0,*,*]),win=0

Note that the boat in the centre foreground looks dark in the red and green images but light in the blue image. To display this as a colour image on a 24-bit terminal, you just need to use the /true keyword with tv:

IDL> tv,ferry,/true