From d5dceb68748e93d1949136104e8b8a17a21bf84f Mon Sep 17 00:00:00 2001
From: Christoph Oelckers <coelckers@zdoom.fake>
Date: Sat, 21 Jun 2014 12:52:19 +0200
Subject: [PATCH] - changed alpha texture handling to avoid using the
 deprecated GL_ALPHA8 texture format unless we have a compatibility context of
 an older GL version.

---
 src/gl/system/gl_interface.cpp   |  1 -
 src/gl/system/gl_interface.h     |  5 +++++
 src/gl/textures/gl_bitmap.cpp    | 26 ++++++++++++++++++++------
 src/gl/textures/gl_hwtexture.cpp | 27 +++++++++++++++++++++++++--
 4 files changed, 50 insertions(+), 9 deletions(-)

diff --git a/src/gl/system/gl_interface.cpp b/src/gl/system/gl_interface.cpp
index d9816e120..7d979f921 100644
--- a/src/gl/system/gl_interface.cpp
+++ b/src/gl/system/gl_interface.cpp
@@ -153,7 +153,6 @@ void gl_LoadExtensions()
 	{
 		gl.flags|=RFL_FRAMEBUFFER;
 	}
-
 }
 
 //==========================================================================
diff --git a/src/gl/system/gl_interface.h b/src/gl/system/gl_interface.h
index d280242e1..d5346fa7d 100644
--- a/src/gl/system/gl_interface.h
+++ b/src/gl/system/gl_interface.h
@@ -41,6 +41,11 @@ struct RenderContext
 	{
 		return glslversion >= 1.3f;
 	}
+
+	bool hasCompatibility()	// will return false, once transition to a core profile is possible and a core profile is used.
+	{
+		return true;
+	}
 };
 
 extern RenderContext gl;
diff --git a/src/gl/textures/gl_bitmap.cpp b/src/gl/textures/gl_bitmap.cpp
index 83a92e0fe..c37e7a90e 100644
--- a/src/gl/textures/gl_bitmap.cpp
+++ b/src/gl/textures/gl_bitmap.cpp
@@ -42,6 +42,7 @@
 #include "gl/renderer/gl_lightdata.h"
 #include "gl/textures/gl_translate.h"
 #include "gl/textures/gl_bitmap.h"
+#include "gl/system/gl_interface.h"
 
 //===========================================================================
 // 
@@ -54,7 +55,7 @@ void iCopyColors(unsigned char * pout, const unsigned char * pin, bool alphatex,
 {
 	int i;
 
-	if (!alphatex)
+	if (!alphatex || gl.version >= 3.3f)	// GL 3.3+ uses a GL_R8 texture for alpha textures so the channels can remain as they are.
 	{
 		for(i=0;i<count;i++)
 		{
@@ -139,12 +140,25 @@ void FGLBitmap::CopyPixelData(int originx, int originy, const BYTE * patch, int
 		// alpha map with 0==transparent and 1==opaque
 		if (alphatex) 
 		{
-			for(int i=0;i<256;i++) 
+			if (gl.version < 3.3f)
 			{
-				if (palette[i].a != 0)
-					penew[i]=PalEntry(i, 255,255,255);
-				else
-					penew[i]=PalEntry(0,255,255,255);	// If the palette contains transparent colors keep them.
+				for (int i = 0; i<256; i++)
+				{
+					if (palette[i].a != 0)
+						penew[i] = PalEntry(i, 255, 255, 255);
+					else
+						penew[i] = PalEntry(0, 255, 255, 255);	// If the palette contains transparent colors keep them.
+				}
+			}
+			else	// on GL 3.3+ we use a GL_R8 texture so the layout is different.
+			{
+				for (int i = 0; i<256; i++)
+				{
+					if (palette[i].a != 0)
+						penew[i] = PalEntry(255, i, 255, 255);
+					else
+						penew[i] = PalEntry(255, 0, 255, 255);	// If the palette contains transparent colors keep them.
+				}
 			}
 		}
 		else if (translation > 0)
diff --git a/src/gl/textures/gl_hwtexture.cpp b/src/gl/textures/gl_hwtexture.cpp
index 7aa8b610c..a2a83e5db 100644
--- a/src/gl/textures/gl_hwtexture.cpp
+++ b/src/gl/textures/gl_hwtexture.cpp
@@ -190,8 +190,26 @@ void FHardwareTexture::LoadImage(unsigned char * buffer,int w, int h, unsigned i
 	bool deletebuffer=false;
 	bool use_mipmapping = TexFilter[gl_texture_filter].mipmapping;
 
-	if (alphatexture) texformat=GL_ALPHA8;
-	else if (forcenocompression) texformat = GL_RGBA8;
+	if (alphatexture)
+	{
+		// thanks to deprecation and delayed introduction of a suitable replacement feature this has become a bit messy...
+		if (gl.version >= 3.3f)
+		{
+			texformat = GL_R8;
+		}
+		else if (gl.version <= 3.0f || gl.hasCompatibility())
+		{
+			texformat = GL_ALPHA8;
+		}
+		else
+		{
+			texformat = GL_RGBA8;
+		}
+	}
+	else if (forcenocompression)
+	{
+		texformat = GL_RGBA8;
+	}
 	if (glTexID==0) glGenTextures(1,&glTexID);
 	glBindTexture(GL_TEXTURE_2D, glTexID);
 	lastbound[texunit]=glTexID;
@@ -238,6 +256,11 @@ void FHardwareTexture::LoadImage(unsigned char * buffer,int w, int h, unsigned i
 
 	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, wrapparam==GL_CLAMP? GL_CLAMP_TO_EDGE : GL_REPEAT);
 	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, wrapparam==GL_CLAMP? GL_CLAMP_TO_EDGE : GL_REPEAT);
+	if (alphatexture && gl.version >= 3.3f)
+	{
+		static const GLint swizzleMask[] = {GL_ONE, GL_ONE, GL_ONE, GL_RED};
+		glTexParameteriv(GL_TEXTURE_2D, GL_TEXTURE_SWIZZLE_RGBA, swizzleMask);
+	}
 	clampmode = wrapparam==GL_CLAMP? GLT_CLAMPX|GLT_CLAMPY : 0;
 
 	if (forcenofiltering)