Fix Applesoft shape table visualizer

An edge case wasn't being handled correctly.

SourceGen/RuntimeData/Apple/VisShapeTable.cs

@@ -25,28 +25,33 @@ namespace RuntimeData.Apple {
     /// limited to hi-res graphics.  It's really just a list of vectors.
     /// </summary>
     /// <remarks>
-    /// See the Applesoft BASIC Programming Reference Manual, pages 91-100, for a full
-    /// description of the format.
+    /// <para>See the Applesoft BASIC Programming Reference Manual, pages 91-100, for a full
+    /// description of the format.</para>
     ///
-    /// Table format:
+    /// <para>Table format:</para>
+    /// <code>
     ///  +00    number of shapes
-    ///  +01    (unused)
+    ///  +01    (undefined)
     ///  +02/03 offset from start of table to first shape
     ///  +04/05 offset from start of table to second shape
     ///   ...
     ///  +xx    shape #1 data
     ///  +yy    shape #2 data
+    /// </code>
     ///
-    /// Shape data is a series of bytes ending in $00.  Each byte holds three vectors,
+    /// <para>Shape data is a series of bytes ending in $00.  Each byte holds three vectors,
     /// CCBBBAAA.  AAA and BBB specify a direction (up/right/down/left) and whether or
     /// not to plot a point.  CC cannot specify whether to plot and cannot move up (a 00
-    /// in CC means "do nothing").
+    /// in CC means "do nothing").</para>
+    /// <para>The shape indices should start at 1, as is done in Applesoft, but an earlier
+    /// version of this visualizer started at 0.  So we're stuck with it in the name of
+    /// backward compatibility.</para>
+    /// </remarks>
     ///
     /// TODO: optionally render as it would on the hi-res screen.  Some shapes draw with
     ///   HCOLOR=white but use alternating vertical lines to render multiple colors.
     /// TODO: support ROT, using Applesoft-style ugly rotation handling.  Could also support
     ///   SCALE but that's only interesting w.r.t. hi-res color changes.
-    /// </remarks>
     public class VisShapeTable : MarshalByRefObject, IPlugin, IPlugin_Visualizer {
         // IPlugin
         public string Identifier {
@@ -69,7 +74,7 @@ namespace RuntimeData.Apple {
                     new VisParamDescr("File offset (hex)",
                         P_OFFSET, typeof(int), 0, 0x00ffffff, VisParamDescr.SpecialMode.Offset, 0),
                     new VisParamDescr("Image index",
-                        P_INDEX, typeof(int), 0, 256, 0, 0),
+                        P_INDEX, typeof(int), 0, 255, 0, 0),
                 }),
             new VisDescr(VIS_GEN_SHAPE_TABLE_SHAPE, "Apple II Shape Table Shape", VisDescr.VisType.Bitmap,
                 new VisParamDescr[] {
@@ -190,14 +195,27 @@ namespace RuntimeData.Apple {
                     break;
                 }
 
-                int bits = val & 0x07;
-                DrawVector(bits, false, ref xc, ref yc, ref xmin, ref xmax, ref ymin, ref ymax, vb);
-                bits = (val >> 3) & 0x07;
-                DrawVector(bits, false, ref xc, ref yc, ref xmin, ref xmax, ref ymin, ref ymax, vb);
-                bits = (val >> 6) & 0x03;
-                DrawVector(bits, true, ref xc, ref yc, ref xmin, ref xmax, ref ymin, ref ymax, vb);
+                // "If all the remaining sections of the byte contain only zeroes, then those
+                // sections are ignored."  We ignore C if it's zero, and if B and C are both zero
+                // then both parts are ignored.
+                int abits = val & 0x07;
+                int bbits = (val >> 3) & 0x07;
+                int cbits = val >> 6;
+
+                DrawVector(abits, ref xc, ref yc, ref xmin, ref xmax, ref ymin, ref ymax, vb);
+                if (bbits != 0 || cbits != 0) {
+                    DrawVector(bbits, ref xc, ref yc, ref xmin, ref xmax, ref ymin, ref ymax, vb);
+                }
+                if (cbits != 0) {
+                    DrawVector(cbits, ref xc, ref yc, ref xmin, ref xmax, ref ymin, ref ymax, vb);
+                }
             }
 
+            // Return true if we actually plotted something.
+            if (xmax < 0 || ymax < 0) {
+                mAppRef.ReportError("Shape definition doesn't draw anything");
+                return false;
+            }
             return true;
         }
 
@@ -206,7 +224,7 @@ namespace RuntimeData.Apple {
         /// min/max values are updated if a point is plotted -- no need to expand the bitmap
         /// outside the range of actual plotted points.
         /// </summary>
-        private void DrawVector(int bits, bool isC, ref int xc, ref int yc,
+        private void DrawVector(int bits, ref int xc, ref int yc,
                 ref int xmin, ref int xmax, ref int ymin, ref int ymax, VisBitmap8 vb) {
             if ((bits & 0x04) != 0) {
                 if (vb != null) {
@@ -228,12 +246,8 @@ namespace RuntimeData.Apple {
             }
             switch (bits & 0x03) {
                 case 0x00:
-                    if (isC) {
-                        // do nothing
-                    } else {
-                        // move up
-                        yc--;
-                    }
+                    // move up
+                    yc--;
                     break;
                 case 0x01:
                     // move right

docs/sgmanual/visualization.html

@@ -225,8 +225,15 @@ Neither has any effect on black &amp; white bitmaps.</p>
 half-pixel shifts are not represented.</p>
 
 <p>The VisShapeTable script renders Applesoft shape tables, which can
-have multiple vector shapes.  The only parameter other than the offset
-is the shape number.</p>
+have multiple vector shapes.  There are two generators:</p>
+<ul>
+  <li><b>Shape Table</b> - full table.  Set the offset to the first
+    byte of the table, and set the image index to the desired shape
+    index.  (Note: indices here start at zero, but in Applesoft programs
+    they start at 1.)</li>
+  <li><b>Shape Table Shape</b> - single shape.  Set the offset to the
+    first byte of the shape definition.</li>
+</ul>
 
 
 <h3>Atari 2600 : Atari/Vis2600</h3>