# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1541096933 -3600
# Node ID b1c95dc9c07a56ff6750313a8c56d775d5c35fbd
# Parent  d3de41d851b917e4c96b12f7307bb9976f4d33ad
Introduced tracking of the vertical origin so that the background is repainted
properly, scrolling onto the screen horizontally at the appropriate level.

diff -r d3de41d851b9 -r b1c95dc9c07a examples/vga/main.c
--- a/examples/vga/main.c	Wed Oct 31 18:02:07 2018 +0100
+++ b/examples/vga/main.c	Thu Nov 01 19:28:53 2018 +0100
@@ -114,7 +114,7 @@
     uint8_t background[sprite_width * sprite_height];
     int x, y;
     int dir[] = {1, 0, -1, 0, 1}, i = 0, width, column_width;
-    int xsource, xdisplay, xorigin = 0;
+    int xsource, xdisplay, xorigin = 0, yorigin = 0;
 
     while (1)
     {
@@ -144,6 +144,18 @@
 
                 scroll_display(&display_config, dir[i], dir[i + 1]);
 
+                /* Update the vertical origin if appropriate. */
+
+                if (dir[i + 1])
+                {
+                    yorigin += dir[i + 1];
+
+                    if (yorigin < 0)
+                        yorigin += screendata_height;
+                    else if (yorigin >= screendata_height)
+                        yorigin -= screendata_height;
+                }
+
                 /* For horizontal scrolling, plot the exposed column at the left
                    (if scrolling left) or at the right (if scrolling right). */
 
@@ -164,14 +176,39 @@
                     /* Determine the location of the column to be plotted. */
 
                     xorigin += width;
+
+                    if (xorigin < 0)
+                        xorigin += screendata_width;
+                    else if (xorigin >= screendata_width)
+                        xorigin -= screendata_width;
+
                     xsource = (xdisplay + xorigin) % screendata_width;
 
+                    /* Plot a column in two pieces if the vertical origin is
+                       non-zero. The first piece is at (xdisplay, 0) and
+                       provides the lower part of the background image displaced
+                       upwards (or downwards having wrapped around) on the
+                       screen. */
+
                     copy_display_section(&display_config, screendata,
                                          screendata_width, screendata_height,
-                                         xsource, 0,
-                                         column_width, screendata_height,
+                                         xsource, yorigin,
+                                         column_width, screendata_height - yorigin,
                                          xdisplay, 0,
                                          -1, 1);
+
+                    /* The second column is at (xdisplay, h - yorigin) and
+                       provides the upper part of the background image displaced
+                       downwards (or upwards having wrapped around) on the
+                       screen. */
+
+                    if (yorigin)
+                        copy_display_section(&display_config, screendata,
+                                             screendata_width, screendata_height,
+                                             xsource, 0,
+                                             column_width, yorigin,
+                                             xdisplay, screendata_height - yorigin,
+                                             -1, 1);
                 }
             }