Resizing PNG removes the opacity/transparency

[fmw42]

This works just fine for me on IM 6.4.2-2


convert logo: -transparent white logo_trans.png
convert logo_trans.png -resize 50% logo_trans_50.png

[anthony]

What format did you write it in? As you mentioned GIF you are obviously writing in other formats as well as PNG.

JPEG does not support transparency, and is lossy (not exact colors)
GIF only supports boolean transparency, and a small number of colors.

See IM examples, Common Image Formats for more details of all three image file formats.

[roach]

I'm having the same problem described in the Title. It looks as though this thread has rotted away (some posts are missing?)

Anyways, I need to take an existing "watermark" image (PNG file, with Transparency), resize it to an appropriate dimension for the "target" image (the image I'm trying to watermark), then composite it over the target image, and save to an output file.

Step 1, Resize the watermark (.PNG overlay) file:

Code: Select all

convert watermark.png -resize 640x480^ -gravity center -extent 640x480 watermark_temp.png
[code]

Step 2, apply the watermark to the target image:
[code]
composite -gravity center watermark_temp.png inputfile.jpg outputfile.jpg
[code]

Seems to work, but watermark_temp.png has all of it's transparency converted to plain white, so the outputfile.jpg image basically looks like a watermark, on a white background.

I'm using ImageMagick 6.5.6-Q16 on Windows 7, via the OLE object.

Anyone have any ideas?

[Bonzo]

Try:

Code: Select all

convert inputfile.jpg ( watermark.png -resize 640x480^ ) -gravity center -composite over -matte outputfile.jpg

[fmw42]

try enabling the alpha channel as follows on your watermark image

convert inputfile.jpg \( watermark.png -channel rgba -alpha on -resize '640x480^' \) -gravity center -composite over outputfile.jpg

you can use -matte rather than -alpha on if your system is too old.

if on windows see http://www.imagemagick.org/Usage/windows/

[roach]

Thanks Bonzo! Turns out removing the "-extent 640x480" was the key. Something about the "extent" was removing the alpha channel. Since I didn't need it, I took it out, and everything works fine now.

thanks!