diff --git a/source/glerminal-private.h b/source/glerminal-private.h
index d7765ca..75a7479 100644
--- a/source/glerminal-private.h
+++ b/source/glerminal-private.h
@@ -82,7 +82,7 @@ namespace glerminal
 		unsigned int m_sprites[CELL_SIZE * CELL_SIZE * (1 << (8 * sizeof(*m_cells)))];
 		glerminal_main_cb m_main;
 
-#ifdef _DEBUG
+#ifdef GLERMINAL_OPENGL_DEBUG_CONTEXT
 		mutable std::ofstream m_log;
 		void log(GLenum type, GLuint id, GLenum severity, const char* message) const;
 #endif
diff --git a/source/glerminal.cpp b/source/glerminal.cpp
index dd33ae4..cccfff2 100644
--- a/source/glerminal.cpp
+++ b/source/glerminal.cpp
@@ -25,7 +25,7 @@ namespace
 		-1,  0 // top left
 	};
 
-	constexpr char* VERTEX_SHADER_SOURCE =
+	constexpr char VERTEX_SHADER_SOURCE[] =
 		"#version 450 core\n"
 		"layout (location = 0) in vec2 position;\n"
 		"layout (location = 1) in vec2 offset;\n"
@@ -36,7 +36,6 @@ namespace
 		"	float scales[];\n"
 		"} lss;\n"
 		"out VS_OUT {\n"
-		"	flat vec2 offset;\n"
 		"	flat int sprite;\n"
 		"	flat int layer;\n"
 		"	vec2 texcoord;\n"
@@ -44,21 +43,22 @@ namespace
 		"void main()\n"
 		"{\n"
 		"	const int layer = int(floor(gl_InstanceID / " GRID_SIZE_UNIFORM_NAME ".y));\n"
+		"	vec2 scaled_offset = 2 * offset * " GRID_SIZE_UNIFORM_NAME ".zw;\n"
 		"	vs_out.sprite = sprite;\n"
-		"	vs_out.offset = offset * " GRID_SIZE_UNIFORM_NAME ".zw;\n"
 		"	vs_out.layer = layer;\n"
 		"	vs_out.texcoord = vec2(position.x + 1, -position.y);\n"
 		"	vec2 cell_position = vec2(lss.scales[layer] + (gl_InstanceID % int(" GRID_SIZE_UNIFORM_NAME ".y)) - " GRID_SIZE_UNIFORM_NAME ".x * floor((gl_InstanceID % int(" GRID_SIZE_UNIFORM_NAME ".y)) * " GRID_SIZE_UNIFORM_NAME ".z), -floor((gl_InstanceID % int(" GRID_SIZE_UNIFORM_NAME ".y)) * " GRID_SIZE_UNIFORM_NAME ".z));\n"
 		"	vec2 temp = ((position + vec2(-0.5, 0.5)) * lss.scales[layer] + cell_position + vec2(0.5, -0.5)) * " GRID_SIZE_UNIFORM_NAME ".zw * 2 + vec2(-1, 1);\n"
-		"	gl_Position = vec4(temp.x, -temp.y, 0, 1);\n"
+		"	gl_Position = vec4(scaled_offset.x + temp.x, scaled_offset.y - temp.y, 0, 1);\n"
 		"}";
 
-	constexpr char* GEOMETRY_SHADER_SOURCE =
+	constexpr const char* VERTEX_SHADER_SOURCE_PTR = VERTEX_SHADER_SOURCE;
+
+	constexpr char GEOMETRY_SHADER_SOURCE[] =
 		"#version 450 core\n"
 		"layout (triangles) in;\n"
 		"layout (triangle_strip, max_vertices = 3) out;\n"
 		"in VS_OUT {\n"
-		"   flat vec2 offset;\n"
 		"	flat int sprite;\n"
 		"	flat int layer;\n"
 		"	vec2 texcoord;\n"
@@ -68,24 +68,26 @@ namespace
 		"void main()\n"
 		"{\n"
 		"	gl_Layer = gs_in[0].layer;\n"
-		"	gl_Position = vec4(gl_in[0].gl_Position.xy + gs_in[0].offset * 2, 0, 1);\n"
+		"	gl_Position = gl_in[0].gl_Position;\n"
 		"	sprite = gs_in[0].sprite;\n"
 		"	texcoord = gs_in[0].texcoord;\n"
 		"	EmitVertex();\n"
 		"	gl_Layer = gs_in[1].layer;\n"
-		"	gl_Position = vec4(gl_in[1].gl_Position.xy + gs_in[1].offset * 2, 0, 1);\n"
+		"	gl_Position = gl_in[1].gl_Position;\n"
 		"	sprite = gs_in[1].sprite;\n"
 		"	texcoord = gs_in[1].texcoord;\n"
 		"	EmitVertex();\n"
 		"	gl_Layer = gs_in[2].layer;\n"
-		"	gl_Position = vec4(gl_in[2].gl_Position.xy + gs_in[2].offset * 2, 0, 1);\n"
+		"	gl_Position = gl_in[2].gl_Position;\n"
 		"	sprite = gs_in[2].sprite;\n"
 		"	texcoord = gs_in[2].texcoord;\n"
 		"	EmitVertex();\n"
 		"	EndPrimitive();\n"
 		"}";
 
-	constexpr char* FRAGMENT_SHADER_SOURCE =
+	constexpr const char* GEOMETRY_SHADER_SOURCE_PTR = GEOMETRY_SHADER_SOURCE;
+
+	constexpr char FRAGMENT_SHADER_SOURCE[] =
 		"#version 450 core\n"
 		"in vec2 texcoord;\n"
 		"flat in int sprite;\n"
@@ -96,7 +98,9 @@ namespace
 		"	FragColor = texture(" SPRITES_UNIFORM_NAME ", vec3(texcoord, sprite));\n"
 		"}";
 
-	constexpr char* SCREEN_VERTEX_SHADER_SOURCE =
+	constexpr const char* FRAGMENT_SHADER_SOURCE_PTR = FRAGMENT_SHADER_SOURCE;
+
+	constexpr char SCREEN_VERTEX_SHADER_SOURCE[] =
 		"#version 450 core\n"
 		"layout (location = 0) in vec2 position;\n"
 		"out vec2 texcoord;\n"
@@ -106,6 +110,8 @@ namespace
 		"	texcoord = vec2(position.x + 1, -position.y);\n"
 		"}";
 
+	constexpr const char* SCREEN_VERTEX_SHADER_SOURCE_PTR = SCREEN_VERTEX_SHADER_SOURCE;
+
 	constexpr char* SCREEN_FRAGMENT_SHADER_SOURCE =
 		"#version 450 core\n"
 		"in vec2 texcoord;\n"
@@ -126,6 +132,8 @@ namespace
 		"	}\n"
 		"	FragColor = vec4(current_color, 1);\n"
 		"}";
+
+	constexpr const char* SCREEN_FRAGMENT_SHADER_SOURCE_PTR = SCREEN_FRAGMENT_SHADER_SOURCE;
 }
 
 namespace glerminal
@@ -444,15 +452,15 @@ namespace glerminal
 		// -- setup shader program --
 		// compile
 		const unsigned int vertex_shader = glCreateShader(GL_VERTEX_SHADER);
-		glShaderSource(vertex_shader, 1, &VERTEX_SHADER_SOURCE, nullptr);
+		glShaderSource(vertex_shader, 1, &VERTEX_SHADER_SOURCE_PTR, nullptr);
 		glCompileShader(vertex_shader);
 
 		const unsigned int geometry_shader = glCreateShader(GL_GEOMETRY_SHADER);
-		glShaderSource(geometry_shader, 1, &GEOMETRY_SHADER_SOURCE, nullptr);
+		glShaderSource(geometry_shader, 1, &GEOMETRY_SHADER_SOURCE_PTR, nullptr);
 		glCompileShader(geometry_shader);
 
 		const unsigned int fragment_shader = glCreateShader(GL_FRAGMENT_SHADER);
-		glShaderSource(fragment_shader, 1, &FRAGMENT_SHADER_SOURCE, nullptr);
+		glShaderSource(fragment_shader, 1, &FRAGMENT_SHADER_SOURCE_PTR, nullptr);
 		glCompileShader(fragment_shader);
 
 		constexpr int INFO_LOG_SIZE = 512;
@@ -523,11 +531,11 @@ namespace glerminal
 
 		// compile
 		const unsigned int screen_vertex_shader = glCreateShader(GL_VERTEX_SHADER);
-		glShaderSource(screen_vertex_shader, 1, &SCREEN_VERTEX_SHADER_SOURCE, nullptr);
+		glShaderSource(screen_vertex_shader, 1, &SCREEN_VERTEX_SHADER_SOURCE_PTR, nullptr);
 		glCompileShader(screen_vertex_shader);
 
 		const unsigned int screen_fragment_shader = glCreateShader(GL_FRAGMENT_SHADER);
-		glShaderSource(screen_fragment_shader, 1, &SCREEN_FRAGMENT_SHADER_SOURCE, nullptr);
+		glShaderSource(screen_fragment_shader, 1, &SCREEN_FRAGMENT_SHADER_SOURCE_PTR, nullptr);
 		glCompileShader(screen_fragment_shader);
 
 		// verify compile