GSdx-HW: Revamp buffer size calculation

Added a more robust buffer size calculation mechanism for custom
resolutions. Improves performance in higher resolutions for games
which don't need a big buffer. There's a great boost in performance
at GS limited scenarios.

I don't even feel there's a need for the large framebuffer option right
now, For future - I plan on making the large framebuffer enabled version
as the default as the overhead is there only at situations when it's
necessary. Until then keeping the original code just to be on the safe
side in case any issue pops up.

plugins/GSdx/GSRendererHW.cpp

@@ -23,9 +23,9 @@
 #include "GSRendererHW.h"
 
 GSRendererHW::GSRendererHW(GSTextureCache* tc)
-	: m_width(1280)
-	, m_height(1024)
-	, m_custom_width(1280)
+	: m_width(native_buffer.x)
+	, m_height(native_buffer.y)
+	, m_custom_width(1024)
 	, m_custom_height(1024)
 	, m_reset(false)
 	, m_upscale_multiplier(1)
@@ -117,13 +117,44 @@ void GSRendererHW::SetScaling()
 	{
 		if (m_height == m_custom_height)
 		{
-			float ratio = ceil(static_cast<float>(m_height) / crtc_size.y);
-			float buffer_scale_offset = (m_large_framebuffer) ? ratio : 0.5f;
-			ratio = round(ratio + buffer_scale_offset);
+			const float scaling_ratio = ceil(static_cast<float>(m_custom_height) / crtc_size.y);
+			// Avoid using a scissor value which is too high, developers can even leave the scissor to max (2047)
+			// at some cases when they don't want to limit the rendering size. Our assumption is that developers
+			// set the scissor to the actual data in the buffer. Let's use the scissoring value only at such cases
+			const int scissor_height = std::min(640u, (m_context->SCISSOR.SCAY1 - m_context->SCISSOR.SCAY0) + 1);
+			const int single_buffer_size = std::max(GetDisplayRect().height(), scissor_height);
+
+			// We have two contexts of framebuffer height -
+			// One for lower memory consumption and low accuracy
+			// Another one for higher memory consumption at necessary scenarios for higher accuracy
+			std::array<int, 2> framebuffer_height;
+			// When Large Framebuffer is disabled - Let's only consider the height of the display rectangle
+			// as the base (This is wrong implementation when we consider it theoretically as CRTC has no relation
+			// to the Framebuffer size)
+			framebuffer_height[0] = static_cast<int>(round((crtc_size.y * scaling_ratio)));
+			// When Large Framebuffer is enabled - We also consider for potential scissor sizes which are around
+			// the size of the actual image data stored. (Helps ICO to properly scale to right size by help of the
+			// scissoring values) Display rectangle has a height of 256 but scissor has a height of 512 which seems to
+			// be the real buffer size.
+			framebuffer_height[1] = static_cast<int>(round(single_buffer_size * scaling_ratio));
 
 			m_tc->RemovePartial();
-			m_width = max(m_width, 1280);
-			m_height = max(static_cast<int>(crtc_size.y * ratio) , 1024);
+			m_width = std::max(m_width, native_buffer.x);
+			m_height = std::max(framebuffer_height[m_large_framebuffer], native_buffer.y);
+
+			std::string overhead = to_string(framebuffer_height[1] - framebuffer_height[0]);
+			std::string message = "(Custom resolution) Framebuffer size set to " + to_string(crtc_size.x) + "x" + to_string(crtc_size.y);
+			message += " (" + to_string(m_width) + "x" + to_string(m_height) + ")\n";
+
+			if (m_large_framebuffer)
+			{
+				message += "Additional " + overhead + " pixels overhead by enabling Large Framebuffer\n";
+			}
+			else
+			{
+				message += "Saved " + overhead + " pixels overhead by disabling Large Framebuffer\n";
+			}
+			printf("%s", message);
 		}
 	}
 

plugins/GSdx/GSRendererHW.h

@@ -155,6 +155,7 @@ protected:
 	bool m_channel_shuffle;
 
 	GSVector2i m_lod; // Min & Max level of detail
+	const GSVector2i native_buffer = GSVector2i(1280, 1024);
 
 public:
 	GSRendererHW(GSTextureCache* tc);