lines when resizing a png with alpha transparency

[Jonathan Ong]

not sure if i explained it correctly. here's the source png, the EFF logo, and a webp version that is a 50% resize. I used the command cwebp -resize 1000 697 eff.png -o eff2.webp which is just resizing to 50%.

notice the lines around the circle as well as inside the E.

i'm using 0.3.1 on mac os x 10.8.4

[oX Triangle]

reason: cwep try antialiasing a transparent pixel and use black for this.. hmmmmm
only way to fix this is to convert the 1bit-mask to an alphachannel (suggest: use photoshop)

[oX Triangle]

for cwep developer my suggest..
add an option to define the transparent colour via an extra parameter

[Pascal Massimino]

Jonathan,

On Sun, Jul 14, 2013 at 1:24 AM, Jonathan Ong <jonathanr...@gmail.com> wrote:

not sure if i explained it correctly. here's the source png, the EFF logo, and a webp version that is a 50% resize. I used the command cwebp -resize 1000 697 eff.png -o eff2.webp which is just resizing to 50%.

notice the lines around the circle as well as inside the E.

it looks like it's more a problem with the design of the original alpha mask in 'eff.png': the mask

is antialiased just at the white/black limit. You get the same lines if you just rescale to png

too, for instance:

  convert eff.png -scale 1000x697 eff2.png

(see attachement)

The original logo should really use 'white' as background / hidden color, so that the averaging

performed during downsampling is correct...

hope it helps,

skal

(side note: the logo's size in webp is even smaller if you use -lossless. But that's another topic)

i'm using 0.3.1 on mac os x 10.8.4

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[Jonathan Ong]

interesting. when i resized it locally, i didn't get any of those lines. 

thanks. i guess it isn't a problem with webp.

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[Pascal Massimino]

Jonathan,

On Mon, Jul 15, 2013 at 1:25 PM, Jonathan Ong <jonathanr...@gmail.com> wrote:

interesting. when i resized it locally, i didn't get any of those lines. 

thanks. i guess it isn't a problem with webp.

Actually, thinking of it, i think we can improve cwebp/dwebp to avoid this problem.

A tentative fix of cwebp is here: https://gerrit.chromium.org/gerrit/62090

(still WIP, need to iron out details like precision, speed, etc.).

I've entered the bug #160 for that: https://code.google.com/p/webp/issues/detail?id=160

Thanks for the report!