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코딩나우(하늘아래)
코딩나우(하늘아래)

Posted on Originally published at coding-now.com

Converting WebP to JPG on Windows with no dependencies (and the alpha channel gotcha)

#windows #powershell #python #tutorial

Someone sends you a folder of .webp files and the upload form only takes JPG. You don't need ImageMagick for this: Windows has shipped a WebP decoder since 1809 (as the Store-delivered Microsoft.WebpImageExtension), and WIC exposes it to PowerShell. But the built-in path has two sharp edges I only found by testing them.

First: renaming is not converting 

$ head -c 16 photo.webp | xxd
00000000: 5249 4646 4e0a 0000 5745 4250 5650 3820  RIFF....WEBPVP8
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Rename it to photo.jpg and those bytes don't change. Most viewers sniff the content and open it anyway, which is exactly why people think the rename worked - until something that actually validates the format rejects it. Python agrees about what it really is:

>>> Image.open("renamed.jpg").format
'WEBP'
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The no-install route: WIC via PowerShell 

Add-Type -AssemblyName PresentationCore

$src   = "C:\Pictures\image.webp"
$frame = [System.Windows.Media.Imaging.BitmapDecoder]::Create([Uri]$src, 'None', 'OnLoad').Frames[0]
$enc   = New-Object System.Windows.Media.Imaging.JpegBitmapEncoder
$enc.QualityLevel = 90
$enc.Frames.Add($frame)
$out = [System.IO.File]::Create([IO.Path]::ChangeExtension($src, '.jpg'))
$enc.Save($out); $out.Close()
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BitmapDecoder.Create picks the codec by content, and on a machine with the WebP extension you can confirm which one it chose:

$dec = [System.Windows.Media.Imaging.BitmapDecoder]::Create([Uri]$src, 'None', 'OnLoad')
$dec.CodecInfo.FriendlyName   # -> Microsoft Webp Decoder
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A whole folder is the same thing in a loop:

Add-Type -AssemblyName PresentationCore

Get-ChildItem "C:\Pictures\*.webp" | ForEach-Object {
    $f = [System.Windows.Media.Imaging.BitmapDecoder]::Create([Uri]$_.FullName, 'None', 'OnLoad').Frames[0]
    $e = New-Object System.Windows.Media.Imaging.JpegBitmapEncoder
    $e.QualityLevel = 90
    $e.Frames.Add($f)
    $o = [System.IO.File]::Create([IO.Path]::ChangeExtension($_.FullName, '.jpg'))
    $e.Save($o); $o.Close()
}
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Gotcha 1: the alpha channel is gone

This is the part worth knowing before you run it over a folder of logos. The frame that comes back from the Microsoft WebP decoder is Bgr32 - no alpha. Transparent pixels arrive black, and because the data is already flattened, switching to PngBitmapEncoder doesn't save you:

$f.Format              # -> Bgr32
# transparent WebP -> PNG, then inspect the corner pixel:
#   (0, 0, 0, 255)     # opaque black, not transparent
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Pillow reads the same file as RGBA, so if transparency matters, do it in Python and decide what goes behind it:

from PIL import Image
from pathlib import Path

for src in Path(".").glob("*.webp"):
    im = Image.open(src)
    if im.mode in ("RGBA", "LA", "P"):       # composite onto white instead of black
        im = im.convert("RGBA")
        bg = Image.new("RGB", im.size, (255, 255, 255))
        bg.paste(im, mask=im.split()[3])
        im = bg
    im.convert("RGB").save(src.with_suffix(".jpg"), quality=90)
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Gotcha 2: animated WebP silently loses everything after frame 1

The decoder does expose the frames - $dec.Frames.Count returned 4 for my test file, and Pillow reported n_frames = 4 for the same one. But Frames[0] is what the snippet above encodes, and JPEG has nowhere to put the rest. If the motion matters, GIF is two lines:

Image.open("anim.webp").save("anim.gif", save_all=True, loop=0)
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And it gets bigger

WebP -> JPG is a re-encode of already-lossy data, so you pay twice: a little more generation loss, and usually a larger file. My 2.6 KB test WebP became 7.4 KB as a quality-90 JPEG. That tracks with Google's own numbers - lossy WebP is 25-34% smaller than JPEG at equivalent SSIM, lossless is 26% smaller than PNG - so converting "to save space" is backwards. Keep the original, and if size is the goal, resize instead of re-encoding.

Quick reference

Need Use
One or two files, no install Any browser-based converter that runs client-side
A folder, nothing installed PowerShell + WIC (above)
Transparency preserved or flattened your way Pillow
Animation kept Pillow -> GIF

Everything above was run on Windows 11 with Microsoft.WebpImageExtension 1.2.31: the codec name, the Bgr32 frame format, the black corner pixel, the frame counts and the file sizes are measured, not assumed.

Longer version with the non-developer paths (Paint, Photos, a drag-and-drop converter that doesn't upload): https://www.coding-now.com/en/guides/webp-to-jpg?utm_source=devto

Has the alpha-drop bitten anyone else, or is there a WIC flag I missed?

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