在 Open ES iPhone 应用程序中在 VBO 周围使用 VAO 导致调用 glDrawElements 时出现 EXC_BAD_ACCESS

问题描述

我正在尝试将我的代码提升到一个新的水平.遵循 Apple 的一些最佳实践，我正在尝试围绕我的顶点缓冲区对象 (VBO) 实现顶点数组对象.我这样设置我的 VBO 和 VAO:

I'm trying to take my code to the next level. Following some best practices from Apple, I'm trying to implement Vertex Array Objects around my Vertex Buffer Objects (VBO). I setup my VBOs and VAOs like this:

- (void)setupVBOs {
    glBindBuffer(GL_ARRAY_BUFFER, 0);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
    glBindVertexArrayOES(0);
    {
        glGenVertexArraysOES(1, &directArrayObject);
        glBindVertexArrayOES(directArrayObject);

    //    GLuint texCoordBuffer;
        glGenBuffers(1, &texCoordBuffer);
        glBindBuffer(GL_ARRAY_BUFFER, texCoordBuffer);
        glBufferData(GL_ARRAY_BUFFER, sizeof(DirectVertices), DirectVertices, GL_STATIC_DRAW);

        glVertexAttribPointer(directPositionSlot, 2, GL_FLOAT, GL_FALSE, sizeof(DirectVertex), (GLvoid*)offsetof(DirectVertex, position));
        glEnableVertexAttribArray(directPositionSlot);
        glVertexAttribPointer(texCoordSlot, 2, GL_UNSIGNED_BYTE, GL_FALSE, sizeof(DirectVertex), (GLvoid*)offsetof(DirectVertex, texCoord));
        glEnableVertexAttribArray(texCoordSlot);

        glGenVertexArraysOES(1, &arrayObject);
        glBindVertexArrayOES(arrayObject);

    //    GLuint vertexBuffer;
        glGenBuffers(1, &vertexBuffer);
        glBindBuffer(GL_ARRAY_BUFFER, vertexBuffer);
        glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices, GL_STATIC_DRAW);

        glVertexAttribPointer(positionSlot, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), 0);
        glEnableVertexAttribArray(positionSlot);
        glVertexAttribPointer(colorSlot, 4, GL_UNSIGNED_BYTE, GL_FALSE, sizeof(Vertex), (GLvoid*)offsetof(Vertex, Color));
        glEnableVertexAttribArray(colorSlot);

    //    GLuint indexBuffer;
        glGenBuffers(1, &indexBuffer);
        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBuffer);
        glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(Indices), Indices, GL_STATIC_DRAW);
    }
    glBindBuffer(GL_ARRAY_BUFFER, 0);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
    glBindVertexArrayOES(0);
}

我从 http://www.opengl.org/discussion_boards/ubbthreads.php?ubb=showflat&Number=287977 然后像这样使用它:

which I took from http://www.opengl.org/discussion_boards/ubbthreads.php?ubb=showflat&Number=287977 and then use it like this:

- (void) render:(CADisplayLink*)displayLink {

    glClearColor(0, 104.0/255.0, 55.0/255.0, 1.0);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glViewport(0, 0, backingWidth, backingHeight);

    [directProgram use];
    glBindVertexArrayOES(directArrayObject);
    glDisable(GL_DEPTH_TEST);

    glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_2D, videoFrameTexture);

//  // Update uniform values
    glUniform1i(videoFrameUniform, 0);

    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

    [program use];
    glBindVertexArrayOES(arrayObject);
    glDisable(GL_TEXTURE_2D);
    glEnable(GL_DEPTH_TEST);

    CC3GLMatrix *projection = [CC3GLMatrix matrix];
    float h = 4.0f * self.frame.size.height / self.frame.size.width;
    [projection populateFromFrustumLeft:-2 andRight:2 andBottom:-h/2 andTop:h/2 andNear:4 andFar:10];
    glUniformMatrix4fv(projectionUniform, 1, 0, projection.glMatrix);

    CC3GLMatrix *modelView = [CC3GLMatrix matrix];
    [modelView populateFromTranslation:CC3VectorMake(sin(CACurrentMediaTime()), 0, -7)];
    currentRotation += displayLink.duration * 90;
    [modelView rotateBy:CC3VectorMake(currentRotation, currentRotation, 0)];
    glUniformMatrix4fv(modelViewUniform, 1, 0, modelView.glMatrix);

    glDrawElements(GL_TRIANGLES, sizeof(Indices)/sizeof(Indices[0]), GL_UNSIGNED_BYTE, 0);

    glBindVertexArrayOES(0);

    BOOL success = [context presentRenderbuffer:GL_RENDERBUFFER];
    if(!success)
        NSLog(@"present failed");
}

对 glDrawArrays 的调用有效，它填充了我的纹理，但是，对 glDrawElements 的调用失败并出现 EXC_BAD_ACCESS.我的着色器程序(我使用了两个)包装在一个 GLProgram 对象中，该对象来自 http://iphonedevelopment.blogspot.com/2010/11/opengl-es-20-for-ios-chapter-4.html

The call to glDrawArrays works, and it fills my texture, however, the call to glDrawElements fails with an EXC_BAD_ACCESS. My shader programs (i use two) are wrapped in a GLProgram object that I took from http://iphonedevelopment.blogspot.com/2010/11/opengl-es-20-for-ios-chapter-4.html

推荐答案

从你的 setup 函数末尾移除 glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);.

Remove glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0); from the end of your setup function.

根据 OES_vertex_array_object 的规范a>，一个顶点数组对象封装了所有状态除了数组缓冲区绑定，所以在你绘制的时候没有元素缓冲区绑定，而你大概希望 indexBuffer 被绑定.通过在绑定离开顶点数组对象时将 indexBuffer 绑定，您可以确保在返回到该顶点数组对象时它会被重新绑定.

Per the specification for OES_vertex_array_object, a vertex array object encapsulates all state except the array buffer binding, so there is no element buffer bound at the time that you’re drawing, whereas you presumably wanted indexBuffer to be bound. By leaving indexBuffer bound at the time that you bind away from your vertex array object, you ensure that it’ll be rebound when you return to that vertex array object.

如果您想知道为什么在顶点数组对象中没有跟踪数组缓冲区绑定，这可能是因为在读取顶点数据时没有直接使用当前绑定的数组缓冲区数组——相反，每个顶点属性都有自己的缓冲区绑定，由其各自的 gl*Pointer 函数通过在函数调用时查看数组缓冲区绑定来填充.

If you’re wondering why the array buffer binding isn’t tracked in vertex array objects, this is presumably because the currently-bound array buffer isn’t used directly when reading vertex data from arrays—rather, each vertex attribute has its own buffer binding, which is filled out by its respective gl*Pointer function by looking at the array buffer binding when the function is called.

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