Makes first attempt to draw only new lines.

Outputs/OpenGL/ScanTarget.cpp

@@ -336,7 +336,7 @@ void ScanTarget::announce(Event event, uint16_t x, uint16_t y) {
 	// (maybe set a flag and zero out the line coordinates?)
 }
 
-template <typename T> void ScanTarget::patch_buffer(const T &array, GLuint target, uint16_t submit_pointer, uint16_t read_pointer) {
+/*template <typename T> void ScanTarget::patch_buffer(const T &array, GLuint target, uint16_t submit_pointer, uint16_t read_pointer) {
 	if(submit_pointer != read_pointer) {
 		// Bind the buffer and map it into CPU space.
 		glBindBuffer(GL_ARRAY_BUFFER, target);
@@ -361,7 +361,7 @@ template <typename T> void ScanTarget::patch_buffer(const T &array, GLuint targe
 		glFlushMappedBufferRange(GL_ARRAY_BUFFER, 0, GLsizeiptr(buffer_size));
 		glUnmapBuffer(GL_ARRAY_BUFFER);
 	}
-}
+}*/
 
 void ScanTarget::draw(bool synchronous, int output_width, int output_height) {
 	if(fence_ != nullptr) {
@@ -375,12 +375,9 @@ void ScanTarget::draw(bool synchronous, int output_width, int output_height) {
 	if(is_drawing_.test_and_set()) return;
 
 	// Grab the current read and submit pointers.
-	const auto submit_pointers = submit_pointers_.load();
+	auto submit_pointers = submit_pointers_.load();
 	const auto read_pointers = read_pointers_.load();
 
-	// Submit scans and lines; TODO: for lines, rotate in.
-	patch_buffer(line_buffer_, line_buffer_name_, submit_pointers.line, read_pointers.line);
-
 	// Submit scans; only the new ones need to be communicated.
 	size_t new_scans = (submit_pointers.scan_buffer + scan_buffer_.size() - read_pointers.scan_buffer) % scan_buffer_.size();
 	if(new_scans) {
@@ -514,30 +511,84 @@ void ScanTarget::draw(bool synchronous, int output_width, int output_height) {
 		accumulation_texture_ = std::move(new_framebuffer);
 	}
 
-	// Bind the accumulation texture.
-	accumulation_texture_->bind_framebuffer();
-	glClear(GL_STENCIL_BUFFER_BIT);
+	// Figure out how many new spans are ostensible ready; use two less than that.
+	uint16_t new_spans = (submit_pointers.line + LineBufferHeight - read_pointers.line) % LineBufferHeight;
+	if(new_spans > 2) {
+		new_spans -= 2;
 
-	// Enable stenciling and ensure spans increment the stencil buffer.
+		// Bind the accumulation framebuffer.
+		accumulation_texture_->bind_framebuffer();
+
+		// Enable blending and stenciling, and ensure spans increment the stencil buffer.
+		glEnable(GL_BLEND);
 		glEnable(GL_STENCIL_TEST);
 		glStencilFunc(GL_EQUAL, 0, GLuint(-1));
 		glStencilOp(GL_KEEP, GL_KEEP, GL_INCR);
 
-	// Output all lines except the one currently being worked on.
+		// Prepare to output lines.
 		glBindVertexArray(line_vertex_array_);
 		output_shader_->bind();
-	glEnable(GL_BLEND);
-	glDrawArraysInstanced(GL_TRIANGLE_STRIP, 0, 4, GLsizei(line_buffer_.size() - 2));
+
+		// Prepare to upload data that will consitute lines.
+		glBindBuffer(GL_ARRAY_BUFFER, line_buffer_name_);
+
+		// Divide spans by which frame they're in.
+		uint16_t start_line = read_pointers.line;
+		submit_pointers.line = (read_pointers.line + new_spans) % LineBufferHeight;
+		while(new_spans) {
+			uint16_t end_line = start_line+1;
+
+			// Find the limit of spans to draw in this cycle.
+			size_t spans = 1;
+			while(end_line != submit_pointers.line && !line_metadata_buffer_[end_line].is_first_in_frame) {
+				end_line = (end_line + 1) % LineBufferHeight;
+				++spans;
+			}
+
+			// Upload and draw.
+			const auto buffer_size = spans * sizeof(Line);
+			if(!end_line || end_line > start_line) {
+				glBufferSubData(GL_ARRAY_BUFFER, 0, GLsizeiptr(buffer_size), &line_buffer_[start_line]);
+			} else {
+				uint8_t *destination = static_cast<uint8_t *>(
+					glMapBufferRange(GL_ARRAY_BUFFER, 0, GLsizeiptr(buffer_size), GL_MAP_WRITE_BIT | GL_MAP_FLUSH_EXPLICIT_BIT)
+				);
+				assert(destination);
+
+				const size_t buffer_length = line_buffer_.size() * sizeof(Line);
+				const size_t start_position = start_line * sizeof(Line);
+				memcpy(&destination[0], &line_buffer_[start_line], buffer_length - start_position);
+				memcpy(&destination[buffer_length - start_position], &line_buffer_[0], end_line * sizeof(Line));
+
+				glFlushMappedBufferRange(GL_ARRAY_BUFFER, 0, GLsizeiptr(buffer_size));
+				glUnmapBuffer(GL_ARRAY_BUFFER);
+			}
+
+			glDrawArraysInstanced(GL_TRIANGLE_STRIP, 0, 4, GLsizei(spans));
+
+			// If this is end-of-frame, clear any untouched pixels and flush the stencil buffer
+			if(line_metadata_buffer_[end_line].is_first_in_frame) {
+				full_display_rectangle_.draw(0.0, 0.0, 0.0);
+				glClear(GL_STENCIL_BUFFER_BIT);
+
+				// Rebind the program for span output.
+				glBindVertexArray(line_vertex_array_);
+				output_shader_->bind();
+			}
+
+			start_line = end_line;
+			new_spans -= spans;
+		}
 
 		// Clear untouched parts of the display. (TODO: at vertical sync, probably)
-	full_display_rectangle_.draw(0.0, 0.0, 0.0);
 		glDisable(GL_STENCIL_TEST);
+		glDisable(GL_BLEND);
+	}
 
 	// Copy the accumulatiion texture to the target (TODO: don't assume framebuffer 0).
 	glBindFramebuffer(GL_FRAMEBUFFER, 0);
 	glViewport(0, 0, (GLsizei)output_width, (GLsizei)output_height);
 
-	glDisable(GL_BLEND);
 	glClear(GL_COLOR_BUFFER_BIT);
 	accumulation_texture_->bind_texture();
 	accumulation_texture_->draw(float(output_width) / float(output_height), 4.0f / 255.0f);

Outputs/OpenGL/ScanTarget.hpp

@@ -24,6 +24,11 @@ namespace Outputs {
 namespace Display {
 namespace OpenGL {
 
+/*!
+	Provides a ScanTarget that uses OpenGL to render its output;
+	this uses various internal buffers so that the only geometry
+	drawn to the target framebuffer is a quad.
+*/
 class ScanTarget: public Outputs::Display::ScanTarget {
 	public:
 		ScanTarget();