Hi, Gritz: Thank you for the interesting post. I happen to be working on a project in which I need to composite a text onto a background image. But both the text and background image are generated by C++ code, so I hope I can write a few lines of code to do the same job as what oiiotool does in your post. I have no idea how. Could you (or any other nice people) please share a C++ snippet with me? I don't need a shadowed text; just displaying it on the image I generated would suffice. Thanks a lot!
Best Regards Heng Zhou On Tue, Mar 21, 2017 at 2:00 AM, Larry Gritz <[email protected]> wrote: > 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] > > > > _______________________________________________ > Oiio-dev mailing list > [email protected] > https://urldefense.proofpoint.com/v2/url?u=http-3A__lists.openimageio.org_listinfo.cgi_oiio-2Ddev-2Dopenimageio.org&d=DwIFaQ&c=Ngd-ta5yRYsqeUsEDgxhcqsYYY1Xs5ogLxWPA_2Wlc4&r=yN_mtgnvh2k3WD8QIZam9tbZUPHJMoZc_18KfiSrxx0&m=euk9A4Ibxbdf6xWtliyUXHguHMD7O_gAzN3HZ6km25E&s=I64wq-2NHLavZXU1sdfiiXSdK59wX0HuUtIxnG4WhD0&e= > > >
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