Convert -resize but keep channel depth for blue at 2 bits

[cpc664]

Hello,

using convert v6.8.9-9, I want to resize an image with these characteristics (source is a PNG of an Amstrad CPC screen capture) :

Colorspace: sRGB
Depth: 8-bit
Channel depth:
red: 8-bit
green: 8-bit
blue: 2-bit

The result with -resize 21% -depth 8 -type palette comes with a blue channel depth of 8bit.

How can I keep a channel depth for blue at 2 bits ? Thanks.

[fmw42]

You may have to resize, separate channels and set the depth as required, then combine the channels again.

This seems to work for me (unix syntax)

Code: Select all

convert rose: -depth 8 -resize 200% -separate \
\( -clone 0 -depth 8 \) \
\( -clone 1 -depth 8 \) \
\( -clone 2 -depth 2 \) \
-delete 0-2 -combine -set colorspace sRGB \
rose.png

Code: Select all

identify -verbose rose.png

Depth: 8-bit
Channel depth:
red: 8-bit
green: 8-bit
blue: 2-bit

[cpc664]

Thanks, it works but the resulting image is bigger.

My primary goal was to have a lesser resolution image with a smaller file size for web use.

[snibgo]

My primary goal was to have a lesser resolution image with a smaller file size for web use.

For PNG files, I suggest you use the pngcrush program.

[cpc664]

At the moment I use a small script with optipng and advpng :

#! /bin/sh
optipng -i 0 -o 7 $1
advpng -z -4 -i 50 $1