PngEncoder image size increases by 200% to 300% when specifying ColorType

[MeltyPlayer]

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

3.1.12

Other ImageSharp packages and versions

3.1.11

Environment (Operating system, version and so on)

Windows 11

.NET Framework version

.NET 10, C# 14

Description

I was trying to cut down on the size of the PNGs I was writing, so I tried looking through the PngEncoder properties to see if there were any hints I could give it as an optimization.

I noticed that PngEncoder.ColorType could be set to Rgb/RgbWithAlpha/Grayscale/GrayscaleWithAlpha, so it seemed plausible this was such a hint? (It's kind of hard to tell what PngEncoder.ColorType is meant to be, since its documentation isn't very helpful--it's just described as "the color type", which I had already inferred from the name.)

When I actually set this property, though, I find that the PNGs I write actually increase in size by anywhere from 200% to 300% (compared to when written without setting the property).

I wanted to verify if this was intended behavior, and also to ask if you could improve the documentation for this property so it's clearer how it's meant to be used?

Steps to Reproduce

1. Load one of the following images into C# via ImageSharp, it should be an Image<Rgb24>.
2. Save it via the following two methods:

image.Save(
  stream,
  new PngEncoder {
      SkipMetadata = true,
  });

image.Save(
  stream,
  new PngEncoder {
      SkipMetadata = true,
      ColorType = PngColorType.Rgb
  });

The image without ColorType set will be smaller than the input image, but the image with ColorType set to PngColorType.Rgb will be significantly larger.

Images

textures.zip

[JimBobSquarePants]

I’ve converted this to a discussion since it is a question not an issue.

What is the encoded format (color type) of your textures? My guess would be palette based in which case it would be expected that using a different color type would increase file size since you are encoding pixel data not simply and index to a palette entry.

[MeltyPlayer]

Yes, I hadn't even meant to be saving them as indexed images but that must have been what was happening before--the old version of the image uses 8 bits per pixel whereas when specifying the color type it's 24 bits per pixel.

Thanks for pointing that out, apologies for the confusion!

[JimBobSquarePants]

No need to apologise!

ImageSharp by default will attempt to use the existing color type when re-encoding including reusing the palette. For V3 I would recommend deleting the inbound color palette from the image metadata before encoding if you are doing any transforms due to the change in pixel values. The upcoming V4 release will do that automatically for you.

Palette based PNGs can be great quality with a good quantiser + dithering combo because PNG can support multiple alpha values so it might be best to stay in that format to keep file size down.