ColorTable for 4bit BMP is parsed wrong

[zabulus]

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

4.0.0-alpha.0.18+1ec479e404455608db862b4d607207b4bf2063c1

Other ImageSharp packages and versions

SixLabors.ImageSharp.Drawing,3.0.0-alpha.0.1+316fb2e528e2fd131a22f4d05a9dfdc4d908dbcc

Environment (Operating system, version and so on)

Win11 24h2

.NET Framework version

.net8.0

Description

We have 4bit bmp files and during parsing, ImageSharp produces wrong palette metadata for it.

result = {ReadOnlyMemory<Color>} System.ReadOnlyMemory<Color>[21]
 [0] = {Color} 000000FF
 [1] = {Color} 111100FF
 [2] = {Color} 220011FF
 [3] = {Color} 002222FF
 [4] = {Color} 333333FF
 [5] = {Color} 444400FF
 [6] = {Color} 550044FF
 [7] = {Color} 005555FF
 [8] = {Color} 666666FF
 [9] = {Color} 777700FF
 [10] = {Color} 880077FF
 [11] = {Color} 008888FF
 [12] = {Color} 999999FF
 [13] = {Color} AAAA00FF
 [14] = {Color} BB00AAFF
 [15] = {Color} 00BBBBFF
 [16] = {Color} CCCCCCFF
 [17] = {Color} DDDD00FF
 [18] = {Color} EE00DDFF
 [19] = {Color} 00EEEEFF
 [20] = {Color} FFFFFFFF

but actual palette is just steps of gray, starting from
111111 to FFFFFF
Actual on-disk-values of bmp file

FWIW image itself is parsed correctly.

Steps to Reproduce

var image = Image.Load<Rgb24>("1.0.bmp");
var colorTable = image.Metadata.GetBmpMetadata().ColorTable.GetValueOrDefault();

Observe values of colorTable.

Images

1.0.bmp.zip

[zabulus]

Similar issue is with 8bit indexed images, also colors are shifted to one byte

[zabulus]

Looks like issue is here:

ImageSharp/src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

Line 1600 in e20e47f

Color[] colorTable = new Color[palette.Length / Unsafe.SizeOf<Bgr24>()];

Rgb24 used instead Rgb32
From documentation: https://learn.microsoft.com/en-us/windows/win32/gdi/bitmap-storage, palette is RGBQUAD array.

[JimBobSquarePants]

Thanks for the additional detective work. I'll have a look ASAP.