renderer: add support for the GL_RED image format

Author: illwieckz

src/engine/renderer/tr_backend.cpp

@@ -739,12 +739,11 @@ static GLint GL_ToSRGB( GLint internalFormat, bool isSRGB )
 	{
 		switch ( format )
 		{
-#if 0 // Not used in the code base.
 			/* EXT_texture_sRGB_R8 extension.
 			See: https://github.com/KhronosGroup/OpenGL-Registry/blob/main/extensions/EXT/EXT_texture_sRGB_R8.txt */
 			case GL_RED:
+				ASSERT( glConfig2.textureSrgbR8Available );
 				return GL_SR8_EXT;
-#endif
 			case GL_RGB:
 				return GL_SRGB;
 			case GL_RGBA:

src/engine/renderer/tr_image.cpp

@@ -176,6 +176,7 @@ class ListImagesCmd : public Cmd::StaticCmd
 			{ GL_RGBA32UI, { "RGBA32UI", 16 } },
 			{ GL_ALPHA16F_ARB, { "A16F", 2 } },
 			{ GL_ALPHA32F_ARB, { "A32F", 4 } },
+			{ GL_RED, { "R8", 1 } },
 			{ GL_R16F, { "R16F", 2 } },
 			{ GL_R32F, { "R32F", 4 } },
 			{ GL_LUMINANCE_ALPHA16F_ARB, { "LA16F", 4 } },
@@ -933,6 +934,18 @@ void R_UploadImage( const char *name, const byte **dataArray, int numLayers, int
 		format = GL_DEPTH_STENCIL;
 		internalFormat = GL_DEPTH24_STENCIL8;
 	}
+	else if ( image->bits & IF_RED )
+	{
+		if( isSRGB && !glConfig2.textureSrgbR8Available )
+		{
+			Log::Warn("red image '%s' cannot be loaded as sRGB", image->name );
+			internalFormat = GL_RGBA8;
+		}
+		else
+		{
+			internalFormat = GL_RED;
+		}
+	}
 	else if ( image->bits & ( IF_RGBA16F | IF_RGBA32F | IF_TWOCOMP16F | IF_TWOCOMP32F | IF_ONECOMP16F | IF_ONECOMP32F ) )
 	{
 		if( !glConfig2.textureFloatAvailable ) {
@@ -2433,6 +2446,11 @@ static void R_CreateFogImage()
 	imageParams.filterType = filterType_t::FT_DEFAULT;
 	imageParams.wrapType = wrapTypeEnum_t::WT_CLAMP;
 
+	if ( glConfig2.textureSrgbR8Available )
+	{
+		imageParams.bits |= IF_RED;
+	}
+
 	tr.fogImageNaive = R_CreateImage( "_fogNaive", ( const byte ** ) &data, FOG_S, FOG_T, 1, imageParams );
 
 	imageParams.bits |= IF_SRGB;

src/engine/renderer/tr_public.h

@@ -121,6 +121,7 @@ struct glconfig2_t
 	bool gpuShader4Available;
 	bool gpuShader5Available;
 	bool textureGatherAvailable;
+	bool textureSrgbR8Available;
 	int      maxDrawBuffers;
 
 	float    maxTextureAnisotropy;

src/engine/sys/sdl_glimp.cpp

@@ -123,6 +123,8 @@ static Cvar::Cvar<bool> r_ext_texture_integer( "r_ext_texture_integer",
 	"Use GL_EXT_texture_integer if available", Cvar::NONE, true );
 static Cvar::Cvar<bool> r_ext_texture_rg( "r_ext_texture_rg",
 	"Use GL_EXT_texture_rg if available", Cvar::NONE, true );
+static Cvar::Cvar<bool> r_ext_texture_srgb_r8( "r_ext_texture_srgb_r8",
+	"Use GL_EXT_texture_sRGB_R8 if available", Cvar::NONE, true );
 static Cvar::Cvar<bool> r_khr_debug( "r_khr_debug",
 	"Use GL_KHR_debug if available", Cvar::NONE, true );
 static Cvar::Cvar<bool> r_khr_shader_subgroup( "r_khr_shader_subgroup",
@@ -2019,6 +2021,7 @@ static void GLimp_InitExtensions()
 	Cvar::Latch( r_ext_texture_float );
 	Cvar::Latch( r_ext_texture_integer );
 	Cvar::Latch( r_ext_texture_rg );
+	Cvar::Latch( r_ext_texture_srgb_r8 );
 	Cvar::Latch( r_khr_debug );
 	Cvar::Latch( r_khr_shader_subgroup );
 
@@ -2198,6 +2201,8 @@ static void GLimp_InitExtensions()
 	// made required in OpenGL 3.0
 	glConfig2.textureCompressionRGTCAvailable = LOAD_EXTENSION( ExtFlag_CORE, ARB_texture_compression_rgtc );
 
+	glConfig2.textureSrgbR8Available = LOAD_EXTENSION_WITH_TEST( ExtFlag_NONE, EXT_texture_sRGB_R8, r_ext_texture_srgb_r8.Get() );
+
 	// Texture - others
 	glConfig2.textureAnisotropyAvailable = false;
 	glConfig2.textureAnisotropy = 0.0f;