GetBitmap: improve no-data exception message

#42

Author: swharden

src/Spectrogram.Tests/AddTests.cs

@@ -13,7 +13,7 @@ internal class AddTests
         public void Test_No_Data()
         {
             SpectrogramGenerator sg = new(44100, 2048, 1000);
-            var bmp = sg.GetBitmap();
+            Assert.Throws<InvalidOperationException>(() => sg.GetBitmap());
         }
     }
 }

src/Spectrogram/Image.cs

@@ -20,7 +20,7 @@ public static Bitmap GetBitmap(
             int rollOffset = 0)
         {
             if (ffts.Count == 0)
-                throw new ArgumentException("This Spectrogram contains no FFTs (likely because no signal was added)");
+                throw new ArgumentException("Not enough data in FFTs to generate an image yet.");
 
             int Width = ffts.Count;
             int Height = ffts[0].Length;

src/Spectrogram/SpectrogramGenerator.cs

@@ -269,8 +269,14 @@ public List<double[]> GetMelFFTs(int melBinCount)
         /// <param name="roll">Behavior of the spectrogram when it is full of data. 
         /// Roll (true) adds new columns on the left overwriting the oldest ones.
         /// Scroll (false) slides the whole image to the left and adds new columns to the right.</param>
-        public Bitmap GetBitmap(double intensity = 1, bool dB = false, double dBScale = 1, bool roll = false) =>
-            Image.GetBitmap(FFTs, Colormap, intensity, dB, dBScale, roll, NextColumnIndex);
+        public Bitmap GetBitmap(double intensity = 1, bool dB = false, double dBScale = 1, bool roll = false)
+        {
+            if (FFTs.Count == 0)
+                throw new InvalidOperationException("Not enough data to create an image. " +
+                    $"Ensure {nameof(Width)} is >0 before calling {nameof(GetBitmap)}().");
+
+            return Image.GetBitmap(FFTs, Colormap, intensity, dB, dBScale, roll, NextColumnIndex);
+        }
 
         /// <summary>
         /// Create a Mel-scaled spectrogram.