screenSpace: make it work when GL_EXT_gpu_shader4 is missing

Sadly I do not have the artistic talent to render a new coordinate
system for the ASCII art square so I had to delete it :-(  --slipher

Co-Authored-By: slipher <[email protected]>

Author: illwieckz

src/engine/renderer/glsl_source/screenSpace_vp.glsl

@@ -34,8 +34,16 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 /* screenSpace_vp.glsl */
 
+#if defined(HAVE_EXT_gpu_shader4)
 const vec2 vertices[3] = vec2[3] ( vec2( -1.0f, -1.0f ), vec2( 3.0f, -1.0f ), vec2( -1.0f, 3.0f ) );
 
 void main() {
 	gl_Position = vec4( vertices[gl_VertexID], 0.0f, 1.0f );
 }
+#else
+IN vec3 attr_Position;
+
+void main() {
+	gl_Position = vec4( attr_Position, 1.0f );
+}
+#endif

src/engine/renderer/tr_surface.cpp

@@ -531,13 +531,21 @@ void Tess_AddCubeWithNormals( const vec3_t position, const vec3_t minSize, const
 void Tess_InstantScreenSpaceQuad() {
 	GLIMP_LOGCOMMENT( "--- Tess_InstantScreenSpaceQuad ---" );
 
-	tr.skipVBO = true;
-
-	Tess_Begin( Tess_StageIteratorDummy, nullptr, true, -1, 0 );
-	rb_surfaceTable[Util::ordinal( *( tr.genericTriangle->surface ) )]( tr.genericTriangle->surface );
-	Tess_DrawElements();
-
-	tr.skipVBO = false;
+	if ( glConfig2.gpuShader4Available )
+	{
+		tr.skipVBO = true;
+		Tess_Begin( Tess_StageIteratorDummy, nullptr, true, -1, 0 );
+		rb_surfaceTable[Util::ordinal( *( tr.genericTriangle->surface ) )]( tr.genericTriangle->surface );
+		Tess_DrawElements();
+		tr.skipVBO = false;
+	}
+	else
+	{
+		Tess_Begin( Tess_StageIteratorDummy, nullptr, true, -1, 0 );
+		rb_surfaceTable[Util::ordinal( *( tr.genericQuad->surface ) )]( tr.genericQuad->surface );
+		GL_VertexAttribsState( ATTR_POSITION );
+		Tess_DrawElements();
+	}
 
 	GL_CheckErrors();
 
@@ -550,12 +558,15 @@ void Tess_InstantQuad( u_ModelViewProjectionMatrix &shader, const float x, const
 
 	Tess_Begin( Tess_StageIteratorDummy, nullptr, true, -1, 0 );
 
+	/* We don't use x, y, width, height directly to make it compatible
+	with R_InitGenericVBOs() in tr_vbo.cpp.
+	See: https://github.com/DaemonEngine/Daemon/pull/1739 */
 	matrix_t modelViewMatrix;
 	MatrixCopy( matrixIdentity, modelViewMatrix );
-	modelViewMatrix[12] = x;
-	modelViewMatrix[13] = y;
-	modelViewMatrix[0] = width;
-	modelViewMatrix[5] = height;
+	modelViewMatrix[12] = 0.5f * width + x;
+	modelViewMatrix[13] = 0.5f * height + y;
+	modelViewMatrix[0] = 0.5f * width;
+	modelViewMatrix[5] = 0.5f * height;
 	GL_LoadModelViewMatrix( modelViewMatrix );
 	shader.SetUniform_ModelViewProjectionMatrix(
 		glState.modelViewProjectionMatrix[ glState.stackIndex ] );

src/engine/renderer/tr_vbo.cpp

@@ -556,24 +556,22 @@ void R_BindNullIBO()
 }
 
 static void R_InitGenericVBOs() {
+	/* Those values are chosen to specify a full-screen rectangle for
+	screen-space shaders without a projection matrix. The values here
+	don't influence the triangle but this is a fallback for when the
+	gl_VertexID-based triangle can't be used (when GL_EXT_gpu_shader4
+	is missing).
+	See: https://github.com/DaemonEngine/Daemon/pull/1739 */
 	// Min and max coordinates of the quad
-	static const vec3_t min = { 0.0f, 0.0f, 0.0f };
+	static const vec3_t min = { -1.0f, -1.0f, 0.0f };
 	static const vec3_t max = { 1.0f, 1.0f, 0.0f };
 	{
 		/*
 			Quad is a static mesh with 4 vertices and 2 triangles
 
-			 z
-			 ^
-			 |     1------2
-			 |   y |      |
-			 |  /  |      |
-			 | /   |      |
-			 |/    0------3
-			 0---------->x
 			   Verts:
-			   0: 0.0 0.0 0.0
-			   1: 0.0 1.0 0.0
+			   0: -1.0 0.0 0.0
+			   1: -1.0 1.0 0.0
 			   2: 1.0 1.0 0.0
 			   3: 1.0 0.0 0.0
 			   Surfs: