lines.cpp

#include <SDL.h>
#include <GLFW/glfw3.h>
#include <imgui/imgui.h>
#include <imgui/imgui_impl_glfw.h>
#include <vector>
#include<time.h>
// Model
struct Line {
	ImVec2 from, to;
	unsigned int color;
};

struct Image {
	std::vector<Line> lines;
};

//UI
int main(int, char**) {
	srand(time(0));
	// Setup window
	glfwSetErrorCallback(error_callback);
	if (!glfwInit()) {
		exit(1);
	}

	auto w=1280, h=720;
	auto window = glfwCreateWindow(w, h, "ImGui OpenGL2 example", NULL, NULL);
	glfwMakeContextCurrent(window);

	// Setup ImGui binding
	ImGui_ImplGlfw_Init(window, true);

	ImVec4 clear_color = ImColor(114, 144, 154);
	auto myColor = ImColor(rand()%255, rand()%200, rand()%200);

	auto opened = true;
	Image image;
	Line current;
	current.color = myColor;
	auto adding_line = false;

	// Main loop
	while (!glfwWindowShouldClose(window)) {
		glfwPollEvents();
		ImGui_ImplGlfw_NewFrame();

		ImGui::SetNextWindowSize(ImVec2(w-30,h-30), ImGuiSetCond_FirstUseEver);
		if (!ImGui::Begin("Example: Custom Rendering",&opened)) {
			ImGui::End();
			return 0;
		}


		if (image.lines.size() > 0) {
			ImGui::SameLine(); 
			if (ImGui::Button("Undo")) {
				image.lines.pop_back();
			}
		}
		ImGui::Text("Your Color");
		ImGui::SameLine();         
		ImGui::PushStyleColor(ImGuiCol_Button, myColor);
        ImGui::PushStyleColor(ImGuiCol_ButtonHovered, myColor);
        ImGui::PushStyleColor(ImGuiCol_ButtonActive, myColor);
		ImGui::Button("------");
        ImGui::PopStyleColor(3);


		ImGui::Text("Left-click and drag to add lines");
		ImGui::SameLine(); 

		ImGui::Text("Right-click to undo");

		auto draw_list = ImGui::GetWindowDrawList();

		auto canvas_pos = ImGui::GetCursorScreenPos(); // ImDrawList API uses screen coordinates!
		auto canvas_size = ImVec2(ImMax(50.0f,ImGui::GetWindowContentRegionMax().x-ImGui::GetCursorPos().x),
								  ImMax(50.0f,ImGui::GetWindowContentRegionMax().y-ImGui::GetCursorPos().y));    // Resize canvas what's available
		draw_list->AddRect(canvas_pos, ImVec2(canvas_pos.x + canvas_size.x, canvas_pos.y + canvas_size.y), 0xFFFFFFFF);
		auto adding_preview = false;
		ImGui::InvisibleButton("canvas", canvas_size);

		if (ImGui::IsItemHovered()) {
			auto mouse_pos_in_canvas = ImVec2(ImGui::GetIO().MousePos.x - canvas_pos.x, ImGui::GetIO().MousePos.y - canvas_pos.y);
			if (!adding_line && ImGui::GetIO().MouseClicked[0]) {
				current.from = mouse_pos_in_canvas;
				adding_line = true;
			}
			if (adding_line) {
				adding_preview = true;
				current.to = mouse_pos_in_canvas;
				image.lines.push_back(current); // will be removed if preview_mode before next frame
				if (!ImGui::GetIO().MouseDown[0]) {
					adding_line = adding_preview = false;
				}
			}
			if (ImGui::GetIO().MouseClicked[1] && !image.lines.empty()) {
				adding_line = false;
				image.lines.pop_back();
			}
		}
		draw_list->PushClipRect(ImVec4(canvas_pos.x,
									   canvas_pos.y,
									   canvas_pos.x+canvas_size.x,
									   canvas_pos.y+canvas_size.y)); // clip lines within the canvas (if we resize it, etc.)

		for (auto l : image.lines) {

			draw_list->AddLine(canvas_pos + l.from, 
							   canvas_pos + l.to, l.color);
		}

		draw_list->PopClipRect();
		if (adding_preview) {
			image.lines.pop_back();
		}

		ImGui::End();

		// Rendering
		glViewport(0, 0, static_cast<int>(ImGui::GetIO().DisplaySize.x),
						 static_cast<int>(ImGui::GetIO().DisplaySize.y));
		glClearColor(clear_color.x, clear_color.y, clear_color.z, clear_color.w);
		glClear(GL_COLOR_BUFFER_BIT);
		ImGui::Render();
		glfwSwapBuffers(window);
	}

	// Cleanup
	ImGui_ImplGlfw_Shutdown();
	glfwTerminate();

	return 0;
}