SDL2: Use of SDL_Renderer messes up font rendering

[THUNDERGROOVE]

Given the modified sdl_opengl_example:

// ImGui - standalone example application for SDL2 + OpenGL

#include <imgui.h>
#include "imgui_impl_sdl.h"
#include <stdio.h>
#include <SDL2/SDL.h>
#include <SDL2/SDL_opengl.h>

int main(int, char**)
{
    if (SDL_Init(SDL_INIT_EVERYTHING) != 0)
        {
            printf("Error: %s\n", SDL_GetError());
            return -1;
        }

    SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
    SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 24);
    SDL_GL_SetAttribute(SDL_GL_STENCIL_SIZE, 8);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 2);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 2);
    SDL_DisplayMode current;
    SDL_GetCurrentDisplayMode(0, &current);
    SDL_Window *window = SDL_CreateWindow("ImGui SDL2+OpenGL example", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 1280, 720, SDL_WINDOW_OPENGL|SDL_WINDOW_RESIZABLE);
    SDL_GLContext glcontext = SDL_GL_CreateContext(window);

    SDL_Renderer *renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED);

    ImGui_ImplSdl_Init(window);

    ImGuiIO& io = ImGui::GetIO();
    io.Fonts->AddFontDefault();

    bool show_test_window = true;
    bool show_another_window = false;
    ImVec4 clear_color = ImColor(114, 144, 154);

    // Main loop
    bool done = false;
    while (!done)
        {
            SDL_Event event;
            while (SDL_PollEvent(&event))
                {
                    ImGui_ImplSdl_ProcessEvent(&event);
                    if (event.type == SDL_QUIT)
                        done = true;
                }
            ImGui_ImplSdl_NewFrame(window);

            {
                static float f = 0.0f;
                ImGui::Text("Hello, world!");
                ImGui::SliderFloat("float", &f, 0.0f, 1.0f);
                ImGui::ColorEdit3("clear color", (float*)&clear_color);
                if (ImGui::Button("Test Window")) show_test_window ^= 1;
                if (ImGui::Button("Another Window")) show_another_window ^= 1;
                ImGui::Text("Application average %.3f ms/frame (%.1f FPS)", 1000.0f / ImGui::GetIO().Framerate, ImGui::GetIO().Framerate);
            }

            if (show_another_window)
                {
                    ImGui::SetNextWindowSize(ImVec2(200,100), ImGuiSetCond_FirstUseEver);
                    ImGui::Begin("Another Window", &show_another_window);
                    ImGui::Text("Hello");
                    ImGui::End();
                }

            if (show_test_window)
                {
                    ImGui::SetNextWindowPos(ImVec2(650, 20), ImGuiSetCond_FirstUseEver);
                    ImGui::ShowTestWindow(&show_test_window);
                }

            SDL_Rect rect;
            rect.x = 100;
            rect.y = 100;
            rect.w = 50;
            rect.h = 50;

            glViewport(0, 0, (int)ImGui::GetIO().DisplaySize.x, (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();
            SDL_RenderDrawRect(renderer, &rect);
            SDL_RenderPresent(renderer);
            // SDL_GL_SwapWindow(window);  No longer needed.  SDL_RenderPresent does it for us
        }
    ImGui_ImplSdl_Shutdown();
    SDL_GL_DeleteContext(glcontext);  
    SDL_DestroyWindow(window);
    SDL_Quit();

    return 0;
}

The each glyph of text is rendered as a solid white rectangle.

Even if we stop calling SDL_RenderDrawRect in our loop the fonts continue to render incorrectly.

I wish I had more information/knowledge on the subject but I don't have a whole lot of experience with OpenGL.

Thanks.

[ocornut]

It looks like SDLRenderer is not compatible with using OpenGL2 calls directly.
Perhaps using the OpenGL3 version (which is unmerged yet) would fix it.
#356

[ocornut]

I have now merged the aforementioned example in trunk, see updates in #356
It should work better.

Howver note that SDL_Renderer support different backends and doesn't guarantee OpenGL unlessyou force the underlying render driver when calling SDL_CreateRenderer(). See http://hg.libsdl.org/SDL/file/31b7adf67756/src/render/SDL_sysrender.h

[paulsapps]

I'm sure that mixing GL and SDL_RenderPresent actually isn't supported

[ocornut]

We have a new imgui_impl_sdlrenderer.cpp backend!

FYI, although I personally recommend using native-backends, SDL 2.0.18 (tentative scheduled in 52 days) will add support for a new SDL_RenderGeometry() function which allowed a SDL_Renderer backend to be created.

This is now available on master (currently requires building SDL from latest sources)
#3926 (comment)

(Note the backend currently doesn't support multi-viewports.)