Emulate HAM undocumented feature

diff --git a/drawing.cpp b/drawing.cpp
index c96363c673d679ccbdc929602aabc45de88983cf..fd4ae1bae77e603de16d626c02c0fffa8d6f0aad 100644 (file)
--- a/drawing.cpp
+++ b/drawing.cpp
@@ -2621,6 +2621,7 @@ static void dummy_worker (int start, int stop, int blank)
 
 static int ham_decode_pixel;
 static unsigned int ham_lastcolor;
+static bool ham_decode_start;
 
 /* Decode HAM in the invisible portion of the display (left of VISIBLE_LEFT_BORDER),
  * but don't draw anything in.  This is done to prepare HAM_LASTCOLOR for later,
@@ -2632,7 +2633,7 @@ static void init_ham_decoding(void)
        int unpainted_amiga = unpainted + hamleftborderhidden;
 
        ham_decode_pixel = src_pixel - hamleftborderhidden;
-       ham_lastcolor = color_reg_get (&colors_for_drawing, 0);
+       ham_lastcolor = color_reg_get(&colors_for_drawing, 0);
 
        if (!bplham) {
                if (unpainted_amiga > 0) {
@@ -2703,12 +2704,21 @@ static void init_ham_decoding(void)
                        }
                }
        }
+
+       ham_decode_start = true;
 }
 
 static void decode_ham(int pix, int stoppos, int blank)
 {
        int todraw_amiga = res_shift_from_window(stoppos - pix);
 
+       if (ham_decode_start) {
+               // first HAM decoding in scanline: start from previous pixel
+               todraw_amiga++;
+               ham_decode_pixel--;
+               ham_decode_start = false;
+       }
+
        if (!bplham) {
                while (todraw_amiga-- > 0) {
                        int pv = pixdata.apixels[ham_decode_pixel];
@@ -3901,6 +3911,7 @@ static void pfield_draw_line(struct vidbuffer *vb, int lineno, int gfx_ypos, int
                        uae_u16 b4sp = dp_for_drawing->bplcon4sp;
                        uae_u16 fm = dp_for_drawing->fmode;
                        init_ham_decoding();
+                       int oham_decode_pixel = ham_decode_pixel;
                        do_color_changes(dummy_worker, decode_ham, lineno);
                        if (have_color_changes) {
                                // do_color_changes() did color changes and register changes, restore them.
@@ -3921,7 +3932,7 @@ static void pfield_draw_line(struct vidbuffer *vb, int lineno, int gfx_ypos, int
                                set_res_shift();
                        }
                        hposblank = ohposblank;
-                       ham_decode_pixel = src_pixel;
+                       ham_decode_pixel = oham_decode_pixel;
                        bplham = dp_for_drawing->ham_at_start;
                        setbplmode();
                }