Fixed the bug where the IDHD header would return mystic data and added full decoding of it. Also added the checks to prevent unrecognized code

TEST_DecodePng/PngDecoder.cs

@@ -74,7 +74,7 @@ namespace TEST_DecodePng
 
             //As it is the first chunck, his structure is a little bit different
 
-            int imageWidth, imageHeight, bitDepth, colorType, compressionMethod, filterMethod, interfaceMethod;
+            int imageWidth, imageHeight, bitDepth, colorType, compressionMethod, filterMethod, interlaceMethod;
 
             byte[] btaImageWidth = new byte[IMAGE_WIDTH_FIELD_SIZE];
             byte[] btaImageHeight = new byte[IMAGE_HEIGHT_FIELD_SIZE];
@@ -92,10 +92,13 @@ namespace TEST_DecodePng
             //Those are in little endien notation so we need to reverse it 
             Array.Reverse(btaImageWidth);
             Array.Reverse(btaImageHeight);
-            imageWidth = BitConverter.ToInt32(btaImageWidth,0);
+            imageWidth = BitConverter.ToInt32(btaImageWidth, 0);
-            imageHeight = BitConverter.ToInt32(btaImageHeight,0);
+            imageHeight = BitConverter.ToInt32(btaImageHeight, 0);
+
+            //As specified in the documentation, images with 0 widht or height are invalid 
+            if (imageWidth == 0 || imageHeight == 0)
+                return null;
 
-            cursor++;
             bitDepth = Convert.ToInt32(content[cursor]);
             cursor++;
             colorType = Convert.ToInt32(content[cursor]);
@@ -104,7 +107,46 @@ namespace TEST_DecodePng
             cursor++;
             filterMethod = Convert.ToInt32(content[cursor]);
             cursor++;
-            interfaceMethod = Convert.ToInt32(content[cursor]);
+            interlaceMethod = Convert.ToInt32(content[cursor]);
+
+            //Those are all the rules seen on the documentation at : https://www.w3.org/TR/PNG-Chunks.html
+
+            if (compressionMethod != 0 || filterMethod != 0 || interlaceMethod != 0 && interlaceMethod != 1)
+                return null;
+            switch (colorType)
+            {
+                case 0:
+                    //Each pixel is a grayscale sample.
+                    if (bitDepth != 1 && bitDepth != 2 && bitDepth != 4 && bitDepth != 8 && bitDepth != 16)
+                        return null;
+                    break;
+                case 2:
+                    //Each pixel is an R,G,B triple.
+                    if (bitDepth != 8 && bitDepth != 16)
+                        return null;
+                    break;
+                case 3:
+                    //Each pixel is a palette index, a PLTE chunk must appear.
+                    if (bitDepth != 1 && bitDepth != 2 && bitDepth != 4 && bitDepth != 8)
+                        return null;
+                    break;
+                case 4:
+                    //Each pixel is a grayscale sample, followed by an alpha sample.
+                    if (bitDepth != 8 && bitDepth != 16)
+                        return null;
+                    break;
+                case 6:
+                    //Each pixel is an R,G,B triple, followed by an alpha sample.
+                    if (bitDepth != 8 && bitDepth != 16)
+                        return null;
+                    break;
+                default:
+                    //Only 0,2,3,4 and 6 are valid color type values
+                    return null;
+                    break;
+            }
+
+
 
             MessageBox.Show("Image width: " + imageWidth +
                 Environment.NewLine + "Image height: " + imageHeight +
@@ -112,11 +154,10 @@ namespace TEST_DecodePng
                 Environment.NewLine + "color type : " + colorType +
                 Environment.NewLine + "compression method : " + compressionMethod +
                 Environment.NewLine + "filter method : " + filterMethod +
-                Environment.NewLine + "interface method : " + interfaceMethod
+                Environment.NewLine + "interface method : " + interlaceMethod
                 );
 
 
-
             //Timing Stuff
             chrono.Stop();
             //Timing Stuff