Transparency is lost after resizing a PNG with tRNS chunk

[Socolin]

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of ImageSharp
• I have verified if the problem exist in both DEBUG and RELEASE mode
• I have searched open and closed issues to ensure it has not already been reported

ImageSharp version

3.1.11

Other ImageSharp packages and versions

none

Environment (Operating system, version and so on)

Linux Ubuntu 24.04

.NET Framework version

net9.0

Description

Hello,

We found an image that is losing transparency after resizing it. After digging more into it the only "special" things seems to be there is a png:tRNS chunk on it.

Steps to Reproduce

I cannot share this image, so I tried to recreate the image with the same properties, the problem seems to be the there with the test image, but I'm not an expert about this. (to be transparent, I used Claude to generate this image: https://claude.ai/public/artifacts/8c3ecd97-14dc-4f37-852e-3cdaea076671)

Source image: (2x2 pixels, displayed at 64x64)

Result after resize to 4x4

ImageSharp

When using ImageSharp with the following piece of code, the result does not have any transparency

using var a = Image.Load("test_trns.png");
using var b = a.Clone(x => x.Resize(4, 4));
b.Save("test.imagesharp.png");

ImageMagick

When using image magick convert test_trns_fixed.png -resize 4x4 test.imagemagick.png it's works as expected

Images

No response

[JimBobSquarePants]

I'm not sure I would classify this as a bug.

Image.Load will load the image and populate its metadata with the following characteristics:

• Pixel format: Rgba32 — due to the presence of a tRNS chunk
• Metadata PngColorType: Rgb
• Metadata TransparentColor: white

This is a relatively specialist form of encoding that relies on a single transparent color rather than an alpha channel. ImageSharp, by design, will attempt to preserve the original metadata when saving, which means it will re-encode using the same color type and transparency settings.

In contrast, ImageMagick appears to be saving the image with a 16-entry color palette. This is quite unusual behavior, especially given that it results in a visible loss of color fidelity. (You can easily verify this by comparing the images side by side.) It also adds additional chunks (cHRM, bKGD, tIME) that were not in the original file.

Since you’ve resized the image and therefore altered pixel values, the original tRNS chunk is no longer valid. The resized image may contain multiple partially transparent colors, not just a single transparent color. You’ll need to update the PNG metadata accordingly before saving.

Here's how to do that:

string input = @"C:\Users\james\Downloads\test_trns_fixed.png";
string output = @"C:\Users\james\Downloads\test.imagesharp.png";

using var a = Image.Load(input);
using var b = a.Clone(x => x.Resize(4, 4));

PngMetadata m = b.Metadata.GetPngMetadata();
m.ColorType = PngColorType.RgbWithAlpha;
m.TransparentColor = null;

b.Save(output);

ImageSharp v3 does not support automatic metadata updates during transforms, but this will be addressed in the upcoming v4 release.

[Socolin]

Thanks for the quick response and fix :) I can confirm this resolves the issue on the original image