Odd aliasing behavior - doesn't happen with separate files

[mkeveney]

All,

I'm trying to automatically generate a page header image that simulates a top-of-page tab. Eventually I want to be able to conveniently alter colors and regenerate it.

I first used convert.exe in discreet steps to create the pieces and then combine them. This works great. (see goodheader.pngbelow).

But I really would prefer to combine it all into a single invocation of convert without using temporary files. If I do so, I get a slightly different result -- look at the right edge of the tab, and you can see ugly aliasing apparently leftover from the mask.

I think I probably need to adjust the -alpha on|off|... setting somewhere in the process, but I've tried everything I can think of.

My batch file is listed below. My results may temporarily be viewed at:

http://keveney.com/test/test.html

I'm using version 6.4.2 08/14/08.

Any advice?

Should I post this as a bug?

-Matt

---- My Windows batch file -----

rem make header image
rem
rem faint gradient background
rem 'convex' gradient nav bar
rem

rem Mask for the tab
rem
rem (I think the fill color should not be relevant here, since were' only going to use the
rem alpha channel in subsequent operations. I make it blue so I can easily see it in testing.)
rem
convert^
-size 900x100^
xc:none^
-draw "fill blue stroke-width 0 path 'M 0,100 0,75 600,75 C 620,75 630,100 650,100 Z'"^
mask.png

rem Gradient for background
rem
convert^
-size 900x100^
gradient:#FFF-#FDD^
backgradient.png

rem Gradient for tab itself
rem
convert^
-size 700x25^
gradient:#F88-#F00^
tabgradient.png

rem Create cutout in background
rem
convert^
backgradient.png^
mask.png^
-alpha on^
-compose dst_out^
-composite^
cutout.png

rem Create header from files (this works great!)
rem
convert^
-gravity southwest^
cutout.png^
tabgradient.png^
-compose dst_over^
-composite^
goodheader.png

rem Do it all in one step. (This gives weird aliasing on right edge of tab!)
rem
convert^
-size 900x100^
gradient:#FFF-#FDD^
^
xc:none^
-draw "fill blue stroke-width 0 path 'M 0,100 0,75 600,75 C 620,75 630,100 650,100 Z'"^
-alpha on^
-compose dst_out^
-composite^
^
-size 700x25^
gradient:#F88-#F00^
-gravity southwest^
-compose dst_over^
-composite^
^
badheader.png

[el_supremo]

You need to parenthesize the operations so that they don't interfere with one another.
The way I approach it is to start with the last convert command and work backwards:

Code: Select all

convert -gravity southwest cutout.png tabgradient.png -compose dst_over -composite goodheader.png

Now parenthesize each of the input filenames taking care to put spaces around the parentheses:

Code: Select all

convert -gravity southwest ( cutout.png ) ( tabgradient.png ) ^
 -compose dst_over -composite goodheader.png

Now replace tabgradient.png with the convert operations which made it:

Code: Select all

convert -gravity southwest ( cutout.png ) ( -size 700x25 gradient:#F88-#F00 ) ^
 -compose dst_over -composite goodheader.png

Do the same for cutout.png:

Code: Select all

convert -gravity southwest ( backgradient.png mask.png -alpha on -compose dst_out -composite ) ^
 ( -size 700x25 gradient:#F88-#F00 ) -compose dst_over -composite goodheader.png

This has introduced two new input filenames so parenthesize them:

Code: Select all

convert -gravity southwest ( ( backgradient.png ) ( mask.png ) -alpha on -compose dst_out -composite ) ^
 ( -size 700x25 gradient:#F88-#F00 ) -compose dst_over -composite goodheader.png

and now replace those filenames as well:

Code: Select all

convert -gravity southwest ( ( -size 900x100 gradient:#FFF-#FDD ) ^
 ( -size 900x100 xc:none^
 -draw "fill blue stroke-width 0 path 'M 0,100 0,75 600,75 C 620,75 630,100 650,100 Z'" ) ^
 -alpha on -compose dst_out -composite ) ^
 ( -size 700x25 gradient:#F88-#F00 ) ^
 -compose dst_over -composite goodheader.png

and let it rip. It produces the "good" version of the final output file, or it did for me anyway .

Pete

[mkeveney]

Excellent! Thank you!

In the mean time, I figured out that I can also work around the problem by negating my mask and adjusting the compose parameter appropriately. The following batch file gives good results with or without the temporary files.

I haven't examined the logic in depth, but I'll bet that the inverted mask technique somehow works even with the operations in the un-grouped sequence.

-Matt

---- My batch file that works around the problem -----

rem make header image
rem
rem faint gradient background
rem 'convex' gradient nav bar
rem

rem First create mask for the tab
rem
rem (I think the fill color should not be relevant here, since were' only going to use the
rem alpha channel in subsequent operations. I make it blue so I can easily see it in testing.)

rem trying a 'negative' mask
rem
convert^
-size 900x100^
xc:none^
-draw "fill blue stroke-width 0 path 'M 0,0 0,75 600,75 C 620,75 630,100 650,100 L 900,100 900,0 Z'"^
mask.png

rem gradient for background
rem
convert^
-size 900x100^
gradient:#FFF-#FDD^
backgradient.png

rem gradient for tab itself
rem
convert^
-size 700x25^
gradient:#F88-#F00^
tabgradient.png

rem create cutout in background
rem
convert^
mask.png^
backgradient.png^
-alpha on^
-compose atop^
-composite^
cutout.png

rem create header from files (this works great!)
rem
convert^
-gravity southwest^
cutout.png^
tabgradient.png^
-compose dst_over^
-composite^
goodheader.png

rem Do it all in one step.
rem
rem (This gives weird aliasing on right edge of tab)
rem
convert^
-gravity southwest^
-size 900x100^
xc:none^
-draw "fill blue stroke-width 0 path 'M 0,0 0,75 600,75 C 620,75 630,100 650,100 L 900,100 900,0 Z'"^
-size 900x100^
gradient:#FFF-#FDD^
-alpha on^
-compose atop^
-composite^
-size 700x25^
gradient:#F88-#F00^
-compose dst_over^
-composite^
badheader.png