Fix vertically flipped output on the OpenGL backend

code/bgfxbackend.cpp

@@ -126,7 +126,7 @@ static void Build_Convert_Table(void)
 /// <summary>
 /// Submits one textured rectangle covering the given destination.
 /// </summary>
-static void Submit_Quad(bgfx::ViewId view, bgfx::TextureHandle texture, float x, float y, float width, float height, unsigned int samplerflags)
+static void Submit_Quad(bgfx::ViewId view, bgfx::TextureHandle texture, float x, float y, float width, float height, unsigned int samplerflags, bool flipv = false)
 {
 	bgfx::TransientVertexBuffer buffer;
 
@@ -139,12 +139,15 @@ static void Submit_Quad(bgfx::ViewId view, bgfx::TextureHandle texture, float x,
 	BackendVertex * vertex = (BackendVertex *)buffer.data;
 	const unsigned int white = 0xFFFFFFFF;
 
-	vertex[0] = { x, y, 0.0f, 0.0f, white };
-	vertex[1] = { x + width, y, 1.0f, 0.0f, white };
-	vertex[2] = { x + width, y + height, 1.0f, 1.0f, white };
-	vertex[3] = { x, y, 0.0f, 0.0f, white };
-	vertex[4] = { x + width, y + height, 1.0f, 1.0f, white };
-	vertex[5] = { x, y + height, 0.0f, 1.0f, white };
+	const float vtop = flipv ? 1.0f : 0.0f;
+	const float vbottom = flipv ? 0.0f : 1.0f;
+
+	vertex[0] = { x, y, 0.0f, vtop, white };
+	vertex[1] = { x + width, y, 1.0f, vtop, white };
+	vertex[2] = { x + width, y + height, 1.0f, vbottom, white };
+	vertex[3] = { x, y, 0.0f, vtop, white };
+	vertex[4] = { x + width, y + height, 1.0f, vbottom, white };
+	vertex[5] = { x, y + height, 0.0f, vbottom, white };
 
 	bgfx::setVertexBuffer(0, &buffer);
 	bgfx::setTexture(0, _TextureSampler, texture, samplerflags);
@@ -450,6 +453,7 @@ void Backend_Present(void const * pixels, int pitch, int destx, int desty, int d
 
 	bgfx::TextureHandle source = _FrameTexture;
 	unsigned int samplerflags = BGFX_SAMPLER_U_CLAMP | BGFX_SAMPLER_V_CLAMP;
+	bool from_prescale = false;
 
 	if (mode == BACKEND_SCALE_NEAREST) {
 		samplerflags |= BGFX_SAMPLER_POINT;
@@ -475,6 +479,7 @@ void Backend_Present(void const * pixels, int pitch, int destx, int desty, int d
 				Set_View_Transform(VIEW_PRESCALE, _PrescaleWidth, _PrescaleHeight);
 				Submit_Quad(VIEW_PRESCALE, _FrameTexture, 0.0f, 0.0f, (float)_PrescaleWidth, (float)_PrescaleHeight, BGFX_SAMPLER_U_CLAMP | BGFX_SAMPLER_V_CLAMP | BGFX_SAMPLER_POINT);
 				source = bgfx::getTexture(_PrescaleTarget);
+				from_prescale = true;
 			}
 		}
 	}
@@ -484,7 +489,9 @@ void Backend_Present(void const * pixels, int pitch, int destx, int desty, int d
 	bgfx::setViewFrameBuffer(VIEW_PRESENT, BGFX_INVALID_HANDLE);
 	bgfx::setViewClear(VIEW_PRESENT, BGFX_CLEAR_COLOR, 0x000000FF);
 	Set_View_Transform(VIEW_PRESENT, _WindowWidth, _WindowHeight);
-	Submit_Quad(VIEW_PRESENT, source, (float)destx, (float)desty, (float)destwidth, (float)destheight, samplerflags);
+
+	bool flipv = from_prescale && bgfx::getCaps()->originBottomLeft;
+	Submit_Quad(VIEW_PRESENT, source, (float)destx, (float)desty, (float)destwidth, (float)destheight, samplerflags, flipv);
 
 	bgfx::frame();
 }

manual/changes/opengl-prescale-flip.md

@@ -0,0 +1,14 @@
+---
+title: Keep the pixel-art scaled frame upright on OpenGL
+category: fix
+release: 0.2.0
+targets: []
+credit: [EJ]
+---
+
+The picture is no longer vertically flipped during fullscreen or scaled windowed play on
+OpenGL. The pixel-art filter magnifies the frame through an intermediate render target
+whenever the window is larger than the frame by a non-integer amount, and on backends
+that store render targets bottom-up that target is now sampled flipped on the way to the
+screen. Other backends, filters, and window sizes already presented upright and are
+unchanged.