Scale pixel value up to full allocation range

src/ImageSharp/Formats/Pbm/PbmDecoderCore.cs

@@ -7,6 +7,7 @@ using SixLabors.ImageSharp.IO;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.Metadata;
 using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing;
 
 namespace SixLabors.ImageSharp.Formats.Pbm
 {
@@ -52,6 +53,23 @@ namespace SixLabors.ImageSharp.Formats.Pbm
         /// <inheritdoc/>
         Size IImageDecoderInternals.Dimensions => this.PixelSize;
 
+        private bool NeedsUpscaling
+        {
+            get
+            {
+                bool needsUpscaling = false;
+                if (this.ColorType != PbmColorType.BlackAndWhite)
+                {
+                    if (this.MaxPixelValue is not 255 and not 65535)
+                    {
+                        needsUpscaling = true;
+                    }
+                }
+
+                return needsUpscaling;
+            }
+        }
+
         /// <inheritdoc/>
         public Image<TPixel> Decode<TPixel>(BufferedReadStream stream, CancellationToken cancellationToken)
             where TPixel : unmanaged, IPixel<TPixel>
@@ -63,6 +81,10 @@ namespace SixLabors.ImageSharp.Formats.Pbm
             Buffer2D<TPixel> pixels = image.GetRootFramePixelBuffer();
 
             this.ProcessPixels(stream, pixels);
+            if (this.NeedsUpscaling)
+            {
+                this.ProcessUpscaling(image);
+            }
 
             return image;
         }
@@ -165,5 +187,13 @@ namespace SixLabors.ImageSharp.Formats.Pbm
                 PlainDecoder.Process(this.Configuration, pixels, stream, this.ColorType, this.MaxPixelValue);
             }
         }
+
+        private void ProcessUpscaling<TPixel>(Image<TPixel> image)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            int maxAllocationValue = (this.MaxPixelValue > 255) ? 65535 : 255;
+            float factor = maxAllocationValue / this.MaxPixelValue;
+            image.Mutate(x => x.Brightness(factor));
+        }
     }
 }

tests/ImageSharp.Tests/Formats/Pbm/PbmDecoderTests.cs

@@ -74,10 +74,13 @@ namespace SixLabors.ImageSharp.Tests.Formats.Pbm
         }
 
         [Theory]
+        [WithFile(BlackAndWhitePlain, PixelTypes.L8, true)]
         [WithFile(BlackAndWhiteBinary, PixelTypes.L8, true)]
+        [WithFile(GrayscalePlain, PixelTypes.L8, true)]
         [WithFile(GrayscalePlainNormalized, PixelTypes.L8, true)]
         [WithFile(GrayscaleBinary, PixelTypes.L8, true)]
         [WithFile(GrayscaleBinaryWide, PixelTypes.L16, true)]
+        [WithFile(RgbPlain, PixelTypes.Rgb24, false)]
         [WithFile(RgbPlainNormalized, PixelTypes.Rgb24, false)]
         [WithFile(RgbBinary, PixelTypes.Rgb24, false)]
         public void DecodeReferenceImage<TPixel>(TestImageProvider<TPixel> provider, bool isGrayscale)

tests/ImageSharp.Tests/Formats/Pbm/PbmRoundTripTests.cs

@@ -17,6 +17,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Pbm
         [InlineData(BlackAndWhitePlain)]
         [InlineData(BlackAndWhiteBinary)]
         [InlineData(GrayscalePlain)]
+        [InlineData(GrayscalePlainNormalized)]
         [InlineData(GrayscaleBinary)]
         public void PbmGrayscaleImageCanRoundTrip(string imagePath)
         {
@@ -36,6 +37,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Pbm
 
         [Theory]
         [InlineData(RgbPlain)]
+        [InlineData(RgbPlainNormalized)]
         [InlineData(RgbBinary)]
         public void PbmColorImageCanRoundTrip(string imagePath)
         {

tests/Images/External/ReferenceOutput/PbmDecoderTests/DecodeReferenceImage_L8_grayscale_plain.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:f8e8b8a1a05e76b1eeb577373c3a6f492e356f0dd58489afded248415cec4a07
+size 145

tests/Images/External/ReferenceOutput/PbmDecoderTests/DecodeReferenceImage_Rgb24_rgb_plain.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:c44322c4bf461acea27053057f5241afb029d9a1e66e94dcf1be6f86f7f97727
+size 152