imgui/examples/imgui_impl_opengl2.cpp

// ImGui Renderer for: OpenGL2 (legacy OpenGL, fixed pipeline)
// This needs to be used along with a Platform Binding (e.g. GLFW, SDL, Win32, custom..)

// Implemented features:
//  [X] User texture binding. Use 'GLUint' OpenGL texture identifier as void*/ImTextureID. Read the FAQ about ImTextureID in imgui.cpp.
//  [X] Multi-viewport rendering (when ImGuiConfigFlags_ViewportsEnable is enabled).

// **DO NOT USE THIS CODE IF YOUR CODE/ENGINE IS USING MODERN OPENGL (SHADERS, VBO, VAO, etc.)**
// **Prefer using the code in imgui_impl_opengl3.cpp**
// This code is mostly provided as a reference to learn how ImGui integration works, because it is shorter to read.
// If your code is using GL3+ context or any semi modern OpenGL calls, using this is likely to make everything more
// complicated, will require your code to reset every single OpenGL attributes to their initial state, and might
// confuse your GPU driver. 
// The GL2 code is unable to reset attributes or even call e.g. "glUseProgram(0)" because they don't exist in that API.

// CHANGELOG 
// (minor and older changes stripped away, please see git history for details)
//  2018-XX-XX: Platform: Added support for multiple windows via the ImGuiPlatformIO interface.
//  2018-06-08: Misc: Extracted imgui_impl_opengl2.cpp/.h away from the old combined GLFW/SDL+OpenGL2 examples.
//  2018-06-08: OpenGL: Use draw_data->DisplayPos and draw_data->DisplaySize to setup projection matrix and clipping rectangle.
//  2018-02-16: Misc: Obsoleted the io.RenderDrawListsFn callback and exposed ImGui_ImplGlfwGL2_RenderDrawData() in the .h file so you can call it yourself.
//  2017-09-01: OpenGL: Save and restore current polygon mode.
//  2016-09-10: OpenGL: Uploading font texture as RGBA32 to increase compatibility with users shaders (not ideal).
//  2016-09-05: OpenGL: Fixed save and restore of current scissor rectangle.

#include "imgui.h"
#include "imgui_impl_opengl2.h"

// Include OpenGL header (without an OpenGL loader) requires a bit of fiddling
#if defined(_WIN32) && !defined(APIENTRY)
#define APIENTRY __stdcall                  // It is customary to use APIENTRY for OpenGL function pointer declarations on all platforms.  Additionally, the Windows OpenGL header needs APIENTRY.
#endif
#if defined(_WIN32) && !defined(WINGDIAPI)
#define WINGDIAPI __declspec(dllimport)     // Some Windows OpenGL headers need this
#endif
#if defined(__APPLE__)
#include <OpenGL/gl.h>
#else
#include <GL/gl.h>
#endif

// OpenGL Data
static GLuint       g_FontTexture = 0;

// Forward Declarations
static void ImGui_ImplOpenGL2_InitPlatformInterface();
static void ImGui_ImplOpenGL2_ShutdownPlatformInterface();

// Functions
bool    ImGui_ImplOpenGL2_Init()
{
    // Setup back-end capabilities flags
    ImGuiIO& io = ImGui::GetIO();
    io.BackendFlags |= ImGuiBackendFlags_RendererHasViewports;    // We can create multi-viewports on the Renderer side (optional)
    if (io.ConfigFlags & ImGuiConfigFlags_ViewportsEnable)
        ImGui_ImplOpenGL2_InitPlatformInterface();
    return true;
}

void    ImGui_ImplOpenGL2_Shutdown()
{
    ImGui_ImplOpenGL2_ShutdownPlatformInterface();
    ImGui_ImplOpenGL2_DestroyDeviceObjects();
}

void    ImGui_ImplOpenGL2_NewFrame()
{
    if (!g_FontTexture)
        ImGui_ImplOpenGL2_CreateDeviceObjects();
}

// OpenGL2 Render function.
// (this used to be set in io.RenderDrawListsFn and called by ImGui::Render(), but you can now call this directly from your main loop)
// Note that this implementation is little overcomplicated because we are saving/setting up/restoring every OpenGL state explicitly, in order to be able to run within any OpenGL engine that doesn't do so. 
void ImGui_ImplOpenGL2_RenderDrawData(ImDrawData* draw_data)
{
    // Avoid rendering when minimized, scale coordinates for retina displays (screen coordinates != framebuffer coordinates)
    ImGuiIO& io = ImGui::GetIO();
    int fb_width = (int)(draw_data->DisplaySize.x * io.DisplayFramebufferScale.x);
    int fb_height = (int)(draw_data->DisplaySize.y * io.DisplayFramebufferScale.y);
    if (fb_width == 0 || fb_height == 0)
        return;
    draw_data->ScaleClipRects(io.DisplayFramebufferScale);

    // We are using the OpenGL fixed pipeline to make the example code simpler to read!
    // Setup render state: alpha-blending enabled, no face culling, no depth testing, scissor enabled, vertex/texcoord/color pointers, polygon fill.
    GLint last_texture; glGetIntegerv(GL_TEXTURE_BINDING_2D, &last_texture);
    GLint last_polygon_mode[2]; glGetIntegerv(GL_POLYGON_MODE, last_polygon_mode);
    GLint last_viewport[4]; glGetIntegerv(GL_VIEWPORT, last_viewport);
    GLint last_scissor_box[4]; glGetIntegerv(GL_SCISSOR_BOX, last_scissor_box); 
    glPushAttrib(GL_ENABLE_BIT | GL_COLOR_BUFFER_BIT | GL_TRANSFORM_BIT);
    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glDisable(GL_CULL_FACE);
    glDisable(GL_DEPTH_TEST);
    glEnable(GL_SCISSOR_TEST);
    glEnableClientState(GL_VERTEX_ARRAY);
    glEnableClientState(GL_TEXTURE_COORD_ARRAY);
    glEnableClientState(GL_COLOR_ARRAY);
    glEnable(GL_TEXTURE_2D);
    glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
    //glUseProgram(0); // You may want this if using this code in an OpenGL 3+ context where shaders may be bound

    // Setup viewport, orthographic projection matrix
    // Our visible imgui space lies from draw_data->DisplayPos (top left) to draw_data->DisplayPos+data_data->DisplaySize (bottom right). DisplayMin is (0,0) for single viewport apps.
    glViewport(0, 0, (GLsizei)fb_width, (GLsizei)fb_height);
    glMatrixMode(GL_PROJECTION);
    glPushMatrix();
    glLoadIdentity();
    glOrtho(draw_data->DisplayPos.x, draw_data->DisplayPos.x + draw_data->DisplaySize.x, draw_data->DisplayPos.y + draw_data->DisplaySize.y, draw_data->DisplayPos.y, -1.0f, +1.0f);
    glMatrixMode(GL_MODELVIEW);
    glPushMatrix();
    glLoadIdentity();

    // Render command lists
    ImVec2 pos = draw_data->DisplayPos;
    for (int n = 0; n < draw_data->CmdListsCount; n++)
    {
        const ImDrawList* cmd_list = draw_data->CmdLists[n];
        const ImDrawVert* vtx_buffer = cmd_list->VtxBuffer.Data;
        const ImDrawIdx* idx_buffer = cmd_list->IdxBuffer.Data;
        glVertexPointer(2, GL_FLOAT, sizeof(ImDrawVert), (const GLvoid*)((const char*)vtx_buffer + IM_OFFSETOF(ImDrawVert, pos)));
        glTexCoordPointer(2, GL_FLOAT, sizeof(ImDrawVert), (const GLvoid*)((const char*)vtx_buffer + IM_OFFSETOF(ImDrawVert, uv)));
        glColorPointer(4, GL_UNSIGNED_BYTE, sizeof(ImDrawVert), (const GLvoid*)((const char*)vtx_buffer + IM_OFFSETOF(ImDrawVert, col)));

        for (int cmd_i = 0; cmd_i < cmd_list->CmdBuffer.Size; cmd_i++)
        {
            const ImDrawCmd* pcmd = &cmd_list->CmdBuffer[cmd_i];
            if (pcmd->UserCallback)
            {
                // User callback (registered via ImDrawList::AddCallback)
                pcmd->UserCallback(cmd_list, pcmd);
            }
            else
            {
                ImVec4 clip_rect = ImVec4(pcmd->ClipRect.x - pos.x, pcmd->ClipRect.y - pos.y, pcmd->ClipRect.z - pos.x, pcmd->ClipRect.w - pos.y);
                if (clip_rect.x < fb_width && clip_rect.y < fb_height && clip_rect.z >= 0.0f && clip_rect.w >= 0.0f)
                {
                    // Apply scissor/clipping rectangle
                    glScissor((int)clip_rect.x, (int)(fb_height - clip_rect.w), (int)(clip_rect.z - clip_rect.x), (int)(clip_rect.w - clip_rect.y));

                    // Bind texture, Draw
                    glBindTexture(GL_TEXTURE_2D, (GLuint)(intptr_t)pcmd->TextureId);
                    glDrawElements(GL_TRIANGLES, (GLsizei)pcmd->ElemCount, sizeof(ImDrawIdx) == 2 ? GL_UNSIGNED_SHORT : GL_UNSIGNED_INT, idx_buffer);
                }
            }
            idx_buffer += pcmd->ElemCount;
        }
    }

    // Restore modified state
    glDisableClientState(GL_COLOR_ARRAY);
    glDisableClientState(GL_TEXTURE_COORD_ARRAY);
    glDisableClientState(GL_VERTEX_ARRAY);
    glBindTexture(GL_TEXTURE_2D, (GLuint)last_texture);
    glMatrixMode(GL_MODELVIEW);
    glPopMatrix();
    glMatrixMode(GL_PROJECTION);
    glPopMatrix();
    glPopAttrib();
    glPolygonMode(GL_FRONT, (GLenum)last_polygon_mode[0]); glPolygonMode(GL_BACK, (GLenum)last_polygon_mode[1]);
    glViewport(last_viewport[0], last_viewport[1], (GLsizei)last_viewport[2], (GLsizei)last_viewport[3]);
    glScissor(last_scissor_box[0], last_scissor_box[1], (GLsizei)last_scissor_box[2], (GLsizei)last_scissor_box[3]);
}

bool ImGui_ImplOpenGL2_CreateFontsTexture()
{
    // Build texture atlas
    ImGuiIO& io = ImGui::GetIO();
    unsigned char* pixels;
    int width, height;
    io.Fonts->GetTexDataAsRGBA32(&pixels, &width, &height);   // Load as RGBA 32-bits (75% of the memory is wasted, but default font is so small) because it is more likely to be compatible with user's existing shaders. If your ImTextureId represent a higher-level concept than just a GL texture id, consider calling GetTexDataAsAlpha8() instead to save on GPU memory.

    // Upload texture to graphics system
    GLint last_texture;
    glGetIntegerv(GL_TEXTURE_BINDING_2D, &last_texture);
    glGenTextures(1, &g_FontTexture);
    glBindTexture(GL_TEXTURE_2D, g_FontTexture);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glPixelStorei(GL_UNPACK_ROW_LENGTH, 0);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, pixels);

    // Store our identifier
    io.Fonts->TexID = (void *)(intptr_t)g_FontTexture;

    // Restore state
    glBindTexture(GL_TEXTURE_2D, last_texture);

    return true;
}

void ImGui_ImplOpenGL2_DestroyFontsTexture()
{
    if (g_FontTexture)
    {
        ImGuiIO& io = ImGui::GetIO();
        glDeleteTextures(1, &g_FontTexture);
        io.Fonts->TexID = 0;
        g_FontTexture = 0;
    }
}

bool    ImGui_ImplOpenGL2_CreateDeviceObjects()
{
    return ImGui_ImplOpenGL2_CreateFontsTexture();
}

void    ImGui_ImplOpenGL2_DestroyDeviceObjects()
{
    ImGui_ImplOpenGL2_DestroyFontsTexture();
}

//--------------------------------------------------------------------------------------------------------
// MULTI-VIEWPORT / PLATFORM INTERFACE SUPPORT
// This is an _advanced_ and _optional_ feature, allowing the back-end to create and handle multiple viewports simultaneously.
// If you are new to dear imgui or creating a new binding for dear imgui, it is recommended that you completely ignore this section first..
//--------------------------------------------------------------------------------------------------------

static void ImGui_ImplOpenGL2_RenderWindow(ImGuiViewport* viewport, void*)
{
    if (!(viewport->Flags & ImGuiViewportFlags_NoRendererClear))
    {
        ImVec4 clear_color = ImVec4(0.0f, 0.0f, 0.0f, 1.0f);
        glClearColor(clear_color.x, clear_color.y, clear_color.z, clear_color.w);
        glClear(GL_COLOR_BUFFER_BIT);
    }
    ImGui_ImplOpenGL2_RenderDrawData(viewport->DrawData);
}

static void ImGui_ImplOpenGL2_InitPlatformInterface()
{
    ImGuiPlatformIO& platform_io = ImGui::GetPlatformIO();
    platform_io.Renderer_RenderWindow = ImGui_ImplOpenGL2_RenderWindow;
}

static void ImGui_ImplOpenGL2_ShutdownPlatformInterface()
{
    ImGui::DestroyPlatformWindows();
}