Implement EFB Peeks for compressed z16 formats

This fixes an issue in RS3 where engine lens flares would shine
though ships during cutscenes

Source/Core/VideoCommon/RenderBase.cpp

@@ -230,18 +230,24 @@ u32 Renderer::AccessEFB(EFBAccessType type, u32 x, u32 y, u32 poke_data)
     if (!g_ActiveConfig.backend_info.bSupportsReversedDepthRange)
       depth = 1.0f - depth;
 
-    u32 ret = 0;
+    // Convert to 24bit depth
+    u32 z24depth = std::clamp<u32>(static_cast<u32>(depth * 16777216.0f), 0, 0xFFFFFF);
+
     if (bpmem.zcontrol.pixel_format == PixelFormat::RGB565_Z16)
     {
-      // if Z is in 16 bit format you must return a 16 bit integer
-      ret = std::clamp<u32>(static_cast<u32>(depth * 65536.0f), 0, 0xFFFF);
-    }
-    else
-    {
-      ret = std::clamp<u32>(static_cast<u32>(depth * 16777216.0f), 0, 0xFFFFFF);
+      // When in RGB565_Z16 mode, EFB Z peeks return a 16bit value, which is presumably a
+      // resolved sample from the MSAA buffer.
+      // Dolphin doesn't currently emulate the 3 sample MSAA mode (and potentially never will)
+      // it just transparently upgrades the framebuffer to 24bit depth and color and whatever
+      // level of MSAA and higher Internal Resolution the user has configured.
+
+      // This is mostly transparent, unless the game does an EFB read.
+      // But we can simply convert the 24bit depth on the fly to the 16bit depth the game expects.
+
+      return CompressZ16(z24depth, bpmem.zcontrol.zformat);
     }
 
-    return ret;
+    return z24depth;
   }
 }
 

Source/Core/VideoCommon/VideoCommon.h

@@ -4,8 +4,13 @@
 
 #pragma once
 
+#include <algorithm>
+
+#include "Common/BitUtils.h"
 #include "Common/CommonTypes.h"
 
+#include "VideoCommon/BPMemory.h"
+
 // These are accurate (disregarding AA modes).
 constexpr u32 EFB_WIDTH = 640;
 constexpr u32 EFB_HEIGHT = 528;
@@ -59,3 +64,71 @@ inline u32 Z24ToZ16ToZ24(u32 src)
 {
   return (src & 0xFFFF00) | (src >> 16);
 }
+
+inline u32 CompressZ16(u32 z24depth, DepthFormat format)
+{
+  // Flipper offers a number of choices for 16bit Z formats that adjust
+  // where the bulk of the precision lies.
+
+  if (format == DepthFormat::ZLINEAR)
+  {
+    // This is just a linear depth buffer with 16 bits of precision
+    return z24depth >> 8;
+  }
+
+  // ZNEAR/ZMID/ZFAR are custom floating point formats with 2/3/4 bits of exponent
+  // The exponent is simply the number of leading ones that have been removed
+  // The first zero bit is skipped and not stored. The mantissa contains the next 14/13/12 bits
+  // If exponent is at the MAX (3, 7, or 12) then the next bit might still be a one, and can't
+  // be skipped, so the mantissa simply contains the next 14/13/12 bits
+
+  u32 leading_ones = Common::CountLeadingZeros((~z24depth) << 8);
+  bool next_bit_is_one = false;  // AKA: Did we clamp leading_ones?
+  u32 exp_bits;
+
+  switch (format)
+  {
+  case DepthFormat::ZNEAR:
+    exp_bits = 2;
+    if (leading_ones >= 3u)
+    {
+      leading_ones = 3u;
+      next_bit_is_one = true;
+    }
+    break;
+  case DepthFormat::ZMID:
+    exp_bits = 3;
+    if (leading_ones >= 7u)
+    {
+      leading_ones = 7u;
+      next_bit_is_one = true;
+    }
+    break;
+  case DepthFormat::ZFAR:
+    exp_bits = 4;
+    if (leading_ones >= 12u)
+    {
+      // The hardware implementation only uses values 0 to 12 in the exponent
+      leading_ones = 12u;
+      next_bit_is_one = true;
+    }
+    break;
+  default:
+    return z24depth >> 8;
+  }
+
+  u32 mantissa_bits = 16 - exp_bits;
+
+  // Calculate which bits we need to extract from z24depth for our mantissa
+  u32 top = std::max<u32>(24 - leading_ones, mantissa_bits);
+  if (!next_bit_is_one)
+  {
+    top -= 1;  // We know the next bit is zero, so we don't need to include it.
+  }
+  u32 bottom = top - mantissa_bits;
+
+  u32 exponent = leading_ones << mantissa_bits;  // Upper bits contain exponent
+  u32 mantissa = Common::ExtractBits(z24depth, bottom, top - 1);
+
+  return exponent | mantissa;
+}