From d72807dd672d0c6f5c39d1d8d6bf4729b7dad284 Mon Sep 17 00:00:00 2001
From: Lior Halphon <LIJI32@gmail.com>
Date: Sat, 3 Jun 2017 16:42:42 +0300
Subject: [PATCH] =?UTF-8?q?Implemented=20LCD=20first-frame-skip=20behavior?=
 =?UTF-8?q?,=20fixes=20a=20visual=20glitch=20in=20Pok=C3=A9mon=20Pinball?=
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---
 Core/display.c | 8 +++++++-
 Core/gb.h      | 8 ++++++++
 Core/memory.c  | 3 +++
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/Core/display.c b/Core/display.c
index 94a73b4..2ca78d8 100755
--- a/Core/display.c
+++ b/Core/display.c
@@ -318,7 +318,13 @@ static void update_display_state(GB_gameboy_t *gb, uint8_t cycles)
             if (gb->io_registers[GB_IO_STAT] & 0x20) {
                 gb->stat_interrupt_line = true;
             }
-            display_vblank(gb);
+            if (gb->frame_skip_state == GB_FRAMESKIP_LCD_TURNED_ON) {
+                gb->frame_skip_state = GB_FRAMESKIP_FIRST_FRAME_SKIPPED;
+            }
+            else {
+                gb->frame_skip_state = GB_FRAMESKIP_SECOND_FRAME_RENDERED;
+                display_vblank(gb);
+            }
         }
         
         /* Handle STAT changes for lines 0-143 */
diff --git a/Core/gb.h b/Core/gb.h
index edf5214..43a64d7 100644
--- a/Core/gb.h
+++ b/Core/gb.h
@@ -365,6 +365,14 @@ struct GB_gameboy_internal_s {
         bool stat_interrupt_line;
         uint8_t effective_scx;
         uint8_t current_window_line;
+        /* The LCDC will skip the first frame it renders after turning it on, unless the previous
+           frame was skipped as well.
+           See https://www.reddit.com/r/EmuDev/comments/6exyxu/ */
+        enum {
+            GB_FRAMESKIP_LCD_TURNED_ON,
+            GB_FRAMESKIP_FIRST_FRAME_SKIPPED,
+            GB_FRAMESKIP_SECOND_FRAME_RENDERED,
+        } frame_skip_state;
     );
 
     /* Unsaved data. This includes all pointers, as well as everything that shouldn't be on a save state */
diff --git a/Core/memory.c b/Core/memory.c
index da9f0e5..18c46eb 100644
--- a/Core/memory.c
+++ b/Core/memory.c
@@ -439,6 +439,9 @@ static void write_high_memory(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
                     /* It appears that there's a slight delay after enabling the screen? */
                     /* Todo: verify this. */
                     gb->display_cycles = gb->cgb_double_speed? -2 : -4;
+                    if (gb->frame_skip_state == GB_FRAMESKIP_SECOND_FRAME_RENDERED) {
+                        gb->frame_skip_state = GB_FRAMESKIP_LCD_TURNED_ON;
+                    }
                 }
                 else if (!(value & 0x80) && (gb->io_registers[GB_IO_LCDC] & 0x80)) {
                     /* Sync after turning off LCD */