How can I batch convert multiple folders of images to PNG8?

Question

I've got 4500+ PNG 24 images in 20+ folders that I wish to reduce the size of by converting them to PNG 8. (Aside: I've tried smushing and other png optimisations but the savings are not enough, a test of 2 folders in PS showed PNG 8 should be without significant degradation of the images)

When I tried PS CS3 Batch it wouldn't save over the originals and the new files it creates have no folder structure. Is there a way to fix this or another tool for the job?

I'm running OSX but have access to Windows XP/7.

Answer 1

XnView handles batch processing/conversion. Ctrl + U : "Tools -> Batch Processing..."

• Options to overwrite, use original path (as output), and/or keep subfolder structure.
• Add the "Convert > Convert to Colours" transformation from the Transformations tab. One of the parameters is bits/pixel.

Answer 2

That's a pain isn't it? Here's the trick. After recording your actions to make it png8, click the top right corner of the actions palette and choose insert menu item. Then just click file --> save. Click OK. It should now be the last sub-item within your action.

Now when you run the batch, stuff stays in its subfolder like it's supposed to.

Answer 3

Install ImageMagick and run with Powershell

 
#--------------------------------------------------------------------

# Powershell script to recursively convert image formats
# Configuration
$srcfolder = "C:\test\Animals"
$destfolder = "C:\test\Animals"
#This ps1 file will add copy files to designated folder
#Do NOT use Mogrify or the original images will be deleted
$im_convert_exe = "convert.exe -density 300"
# with VECTOR files the density setting should come BEFORE the vector file
# or the image will be blurry.
# change src_filter to the format of the source files
$src_filter = "*.eps"
# change dest_ext to the format of the destination files
$dest_ext = "png"
$options = "-depth 8 -alpha off"
$logfile = "C:\temp\convert.log"
$fp = New-Item -ItemType file $logfile -force
$count=0
foreach ($srcitem in $(Get-ChildItem $srcfolder -include $src_filter -recurse))
{
    $srcname = $srcitem.fullname

    # Construct the filename and filepath for the output
    $partial = $srcitem.FullName.Substring( $srcfolder.Length )
    $destname = $destfolder + $partial
    $destname= [System.IO.Path]::ChangeExtension( $destname , $dest_ext )
    $destpath = [System.IO.Path]::GetDirectoryName( $destname )

    # Create the destination path if it does not exist
    if (-not (test-path $destpath))
    {
        New-Item $destpath -type directory | Out-Null
    }

    # Perform the conversion by calling an external tool
    $cmdline =  $im_convert_exe + " `"" + $srcname  + "`"" + $options + " `"" + $destname + "`" " 
    #echo $cmdline
    invoke-expression -command $cmdline

    # Get information about the output file    
    $destitem = Get-item $destname

    # Show and record information comparing the input and output files
    $info = [string]::Format( "{0} `t {1} `t {2} `t {3} `t {4} `t {5}", $count, 
    $partial, $srcname, $destname, $srcitem.Length ,  $destitem.Length)
    echo $info
    Add-Content $fp $info

    $count=$count+1
} 

#--------------------------------------------------------------