Let's see if this comes through the mail list with images intact. If not, I'll
make it into a wiki page or something.
---
Let's say you have a text image, white on black, and you want to composite it
over an existing background image. Let's quickly generate one to have it as an
example:
$ oiiotool -create 128x96 3 -text:x=20:y=50:size=32:color=1,0,0 "Hello" -d
uint8 -o text.tif
And let's make an RGBA file for our background using something from our
testsuite:
$ cp tahoe-tiny.tif ./bg.tif
Now, we could naively composite it:
$ oiiotool text.tif bg.tif --over -o comp.tif
oiiotool ERROR: over : images must have alpha channels
Oops. Well, adding alpha to the background would be easy,
oiiotool ... bg.tif -ch R,G,B,A=1.0 ...
But what about the foreground?
Well, if we really were just using oiiotool to make the text in the first
place, we could have given it an alpha channel:
$ oiiotool -create 128x96 4 -text:x=20:y=50:size=32:color=1,0,0,1 "Hello"
-d uint8 -o textrgba.tif
But let's suppose for a minute that we didn't have that luxury, we have an RGBA
image of text and that's that. We can construct an alpha channel from the
luminance, then thresholding it, like this, step by step (first, an invalid
command line that explains, then a fully valid command line):
oiiotool text.tif # read input
--dup # duplicate it on the stack
--chsum:weight=.2126,.7152,.0722 # compute luminance as single
channel
--mulc 20 # multiply it...
--clamp:min=0:max=1 # ...and clamp to threshold
--chappend # mash the original RGB with the
luminance-computed alpha
--chnames R,G,B,A # make sure that new channel has the right
name
bg.tif # read the background
--ch R,G,B,A=1.0 # add an alpha channel to the backgound
(opaque)
--over # composite
-o comp1.tif # output
$ oiiotool text.tif --dup --chsum:weight=.2126,.7152,.0722 --mulc 20
--clamp:min=0:max=1 --chappend -chnames R,G,B,A bg.tif -ch R,G,B,A=1.0 --over
-o comp1.tif
The purpose of the --mulc and --clamp is so that a dimmer text color (less than
luminance 1.0) won't make a semi-transparent alpha and show the background
color through the text.
This is fairly nice, but a little naive about the text readability. What we
really want is a bit of a blurred drop shadow, black rim around the letters,
for better visibility against light backgrounds. So let's add a blur to the
alpha we are generating, thus making the alpha image extend past the edges of
the letters themselves to suppress part of the background:
$ oiiotool text.tif --dup --chsum:weight=.2126,.7152,.0722 --blur 5x5
--mulc 20 --clamp:min=0:max=1 --chappend -chnames R,G,B,A bg.tif -ch
R,G,B,A=1.0 --over -o comp2.tif
Voilà! Much nicer! Adjust the blur size and mulc threshold to suit your taste.
--
Larry Gritz
[email protected]
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