Nothing is drawn when using gles 2.0\egl\native activity on Android

Question

Basically I have a very simple shader drawing a very primitive square on GLES 2.0, the shaders compile and load correctly, and I'm having no glErrors. I can get the glClearColor to display by using glClear, but nothing else is drawn. I've linked with GLESv2 and included both <GLES2/gl2.h> and <GLES2/gl2ext.h>

This is my EGL initialization code:

//...
const EGLint attribs[] = {
    EGL_RENDERABLE_TYPE,
    EGL_OPENGL_ES2_BIT,
    EGL_SURFACE_TYPE, 
    EGL_WINDOW_BIT,
    EGL_BLUE_SIZE, 8,
    EGL_GREEN_SIZE, 8,
    EGL_RED_SIZE, 8,
    EGL_ALPHA_SIZE, 8, 
    EGL_DEPTH_SIZE, 0,
    EGL_STENCIL_SIZE, 0,
    EGL_NONE
};
EGLDisplay display = eglGetDisplay(EGL_DEFAULT_DISPLAY);
EGLint w, h, format;
EGLint numConfigs;
EGLConfig config;
EGLSurface surface;
EGLContext context;

eglInitialize(display, 0, 0);
eglChooseConfig(display, attribs, &config, 1, &numConfigs)
eglGetConfigAttrib(display, config, EGL_NATIVE_VISUAL_ID, &format)
ANativeWindow_setBuffersGeometry(engine->app->window, 320, 200, format);
surface = eglCreateWindowSurface(display, config, engine->app->window, NULL);
context = eglCreateContext(display, config, NULL, NULL);
eglMakeCurrent(display, surface, surface, context)
//...

This is my render loop, including egl swap buffers:

void RenderSimpleScene(int iWidth, int iHeight, float mouseX, float mouseY, int mouseButtons) {
    const static auto kStartTime = std::chrono::system_clock::now();
    const auto elapsedMillis = std::chrono::duration_cast<std::chrono::milliseconds>(std::chrono::system_clock::now() - kStartTime).count();
    static bool sInitialized = false;
    static GLuint programId = -1;
    static GLuint vboId = -1;
    // I have tried different z-values
    std::vector<GLfloat> verticesData = {
        -1, -1, 0.5, 1,
        1, -1, 0.5, 1,
        1,  1, 0.5, 1,
        //
        -1, -1, 0.5, 1,
        1,  1, 0.5, 1,
        -1,  1, 0.5, 1
    };
    const auto bytesPerCoord = sizeof(GLfloat);
    const auto coordsPerVertex = 4;
    const auto bytesPerVertex = bytesPerCoord * coordsPerVertex;
    const auto numVerts = verticesData.size() / coordsPerVertex;

    if (!sInitialized) {
        sInitialized = true;
        GetGLErrors();
        const std::string vertexSource =
            "#version 100\n"
            "attribute vec4 aVertex;\n"
            "void main() {\n"
            "  gl_Position = aVertex;\n"
            "}\n";
        const std::string fragmentSource =
            "#version 100\n"
            "precision mediump float;\n"
            "void main() {\n"
            "  vec4 color = vec4(1.0, 0.0, 0.0, 1.0);\n"
            "    gl_FragColor = color;}\n";
        programId = glCreateProgram();
        const auto CompileFunctor = [](const std::string& iShaderSrc, const GLenum iShaderType) -> GLuint {
            const auto id = glCreateShader(iShaderType);
            const GLchar* srcAdr = iShaderSrc.c_str();
            glShaderSource(id, 1, &srcAdr, 0);
            glCompileShader(id);
            GLint status = 0;
            glGetShaderiv(id, GL_COMPILE_STATUS, &status);
            ASSERT(status == GL_TRUE);
            GetGLErrors();
            return id;
        };
        const auto vertexId = CompileFunctor(vertexSource, GL_VERTEX_SHADER);
        const auto fragmentId = CompileFunctor(fragmentSource, GL_FRAGMENT_SHADER);
        glAttachShader(programId, vertexId);
        glAttachShader(programId, fragmentId);
        glLinkProgram(programId);
        GLint linked = -1;
        glGetProgramiv(programId, GL_LINK_STATUS, &linked);
        ASSERT(linked == GL_TRUE);
        GetGLErrors();

        // VBO
        glGenBuffers(1, (GLuint*)&vboId);
        glBindBuffer(GL_ARRAY_BUFFER, vboId);
        const auto numBytes = sizeof(verticesData.front())*verticesData.size();
        glBufferData(GL_ARRAY_BUFFER, numBytes, verticesData.data(), GL_STATIC_DRAW);
        GetGLErrors();
    }

    // Prepare GL
    // The background color varies as clear color changes
    glClearColor(0, 0.5, (std::sin(elapsedMillis*0.001f) + 1) * 0.33, 1);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glDisable(GL_DEPTH_TEST);
    glDisable(GL_CULL_FACE);
    glDisable(GL_BLEND);
    glDisable(GL_SCISSOR_TEST);
    glDisable(GL_STENCIL_TEST);
    glViewport(0, 0, iWidth, iHeight);
    GetGLErrors();

    // Activate program
    glUseProgram(programId);

    // VBO
    glBindBuffer(GL_ARRAY_BUFFER, vboId);
    ASSERT(glGetAttribLocation(programId, "aVertex") != -1);
    const auto vertexLocation = glGetAttribLocation(programId, "aVertex");
    glVertexAttribPointer(vertexLocation, coordsPerVertex, GL_FLOAT, GL_FALSE, 0, nullptr);
    glEnableVertexAttribArray(vertexLocation);
    GetGLErrors();

    // Render
    glDrawArrays(GL_TRIANGLES, 0, numVerts);
    GetGLErrors();

    // Clean up
    glDisableVertexAttribArray(vertexLocation);
    glUseProgram(0);
    GetGLErrors();

    eglSwapBuffers(engine->display, engine->surface);
}

Answer 1

Remade my initialization code to this, and it worked: Changing the eglCreateContext(...) was what I missed: const int attrib_list[] = { EGL_CONTEXT_CLIENT_VERSION, 2, EGL_NONE }; context = eglCreateContext(display, selectedConfig, EGL_NO_CONTEXT, attrib_list);

const EGLint attribs[] = {
        EGL_RENDERABLE_TYPE,
        EGL_OPENGL_ES2_BIT,
        EGL_SURFACE_TYPE, 
        EGL_WINDOW_BIT,
        EGL_BLUE_SIZE, 8,
        EGL_GREEN_SIZE, 8,
        EGL_RED_SIZE, 8,
        EGL_ALPHA_SIZE, 8, 
        EGL_DEPTH_SIZE, 24,
        EGL_STENCIL_SIZE, 8,
        EGL_NONE
    };

    EGLDisplay display = eglGetDisplay(EGL_DEFAULT_DISPLAY);
    eglInitialize(display, 0, 0);

    // Get num configs
    EGLint numConfigs;
    eglChooseConfig(display, attribs, nullptr, 1, &numConfigs);
    auto configs = std::vector<EGLConfig>(numConfigs);
    EGLint fetchedConfigs = 0;
    eglChooseConfig(display, attribs, configs.data(), numConfigs, &fetchedConfigs);
    for (const auto& config : configs) {
        EGLint depth = 0;
        EGLint stencil = 0;
        EGLint r = 0;
        EGLint g = 0;
        EGLint b = 0;
        EGLint a = 0;
        eglGetConfigAttrib(display, config, EGL_DEPTH_SIZE, &depth);
        eglGetConfigAttrib(display, config, EGL_STENCIL_SIZE, &stencil);
        eglGetConfigAttrib(display, config, EGL_RED_SIZE, &r);
        eglGetConfigAttrib(display, config, EGL_GREEN_SIZE, &g);
        eglGetConfigAttrib(display, config, EGL_BLUE_SIZE, &b);
        eglGetConfigAttrib(display, config, EGL_ALPHA_SIZE, &a);
    }
    const auto& selectedConfig = configs.front();
    EGLSurface surface;
    EGLContext context;
    EGLint format = -1;
    eglGetConfigAttrib(display, selectedConfig, EGL_NATIVE_VISUAL_ID, &format);
    ANativeWindow_setBuffersGeometry(engine->app->window, 320, 200, format);
    surface = eglCreateWindowSurface(display, selectedConfig, engine->app->window, NULL);
    // ------ This was the crucial part ---
    const int attrib_list[] = { EGL_CONTEXT_CLIENT_VERSION, 2, EGL_NONE };
    context = eglCreateContext(display, selectedConfig, EGL_NO_CONTEXT, attrib_list);
    // -----------------------------------
    eglMakeCurrent(display, surface, surface, context);