为什么 glDrawElements 在使用相同的缓冲区进行计算着色和渲染时会干扰 glGetBufferSubData

我有一个渲染点流的程序。用于获取给定帧的点的方法具有很强的时间一致性，因此，在渲染循环中，首先，我使用计算着色器来压缩流，删除不需要的点。其次，我向集合添加新点。最后，我使用 glDrawElements 渲染它。

这是执行该过程的代码（它使用Qt来操作OpenGL）：

template< typename Vec3 >
unsigned int CompactionRenderingState< Vec3 >::render()
{
    // Compact stream.
    m_nElements = compact();
    // Sends new points to GPU.
    QOpenGLBuffer* buffer = m_outputBuffers[ POS ];
    buffer->bind();
    buffer->write( m_nElements * BYTES_PER_VERTEX, ( void * ) &RenderingState::m_positions[ 0 ],
                   RenderingState::m_positions.size() * BYTES_PER_VERTEX );
    buffer = m_outputBuffers[ ATTRIB0 ];
    buffer->bind();
    buffer->write( m_nElements * BYTES_PER_VERTEX, ( void * ) &RenderingState::m_colors[ 0 ],
                   RenderingState::m_colors.size() * BYTES_PER_VERTEX );
    m_nElements += RenderingState::m_positions.size();
    // Draws the resulting points.
    m_arrayObj->bind();
    unsigned int bufferOffset = 0;
    switch( RenderingState::m_attribs )
    {
        case Attributes::NORMALS:
        {
            RenderingState::m_painter->setStandardEffect( QGL::LitMaterial );
            m_outputBuffers[ POS ]->bind();
            m_openGL->glVertexAttribPointer( QGL::Position, 3, GL_FLOAT, GL_FALSE, 0, &bufferOffset );
            m_openGL->glEnableVertexAttribArray( QGL::Position );
            m_outputBuffers[ ATTRIB0 ]->bind();
            m_openGL->glVertexAttribPointer( QGL::Normal, 3, GL_FLOAT, GL_FALSE, 0, &bufferOffset );
            m_openGL->glEnableVertexAttribArray( QGL::Normal );
            break;
        }
        case Attributes::COLORS:
        {
            m_renderingProgram->bind();
            m_outputBuffers[ POS ]->bind();
            m_openGL->glVertexAttribPointer( QGL::Position, 3, GL_FLOAT, GL_FALSE, 0, &bufferOffset );
            m_openGL->glEnableVertexAttribArray( QGL::Position );
            m_outputBuffers[ ATTRIB0 ]->bind();
            m_openGL->glVertexAttribPointer( QGL::Color, 3, GL_FLOAT, GL_FALSE, 0, &bufferOffset );
            m_openGL->glEnableVertexAttribArray( QGL::Color );
            break;
        }
        case Attributes::COLORS_AND_NORMALS:
        {
            throw logic_error( "Colors and normals not supported yet." );
            break;
        }
    }
    m_openGL->glMemoryBarrier( GL_VERTEX_ATTRIB_ARRAY_BARRIER_BIT );
    m_openGL->glDrawArrays( GL_POINTS, 0, m_nElements );
    m_openGL->glDisableVertexAttribArray( QGL::Position );
    m_openGL->glDisableVertexAttribArray( QGL::Normal );
    m_openGL->glDisableVertexAttribArray( QGL::Color );
    m_openGL->glBindBuffer( GL_ARRAY_BUFFER, 0 );
    m_renderingProgram->release();
    m_arrayObj->release();
    // Swaps buffers for the next frame.
    for( int i = 0; i < N_BUFFER_TYPES; ++i )
    {
        std::swap( m_inputBuffers[ i ], m_outputBuffers[ i ] );
    }
}
template< typename Vec3 >
unsigned int CompactionRenderingState< Vec3 >::compact()
{
    // Makes the compaction of the unused points.
    unsigned int nElements = m_compactionFlags.size();
    unsigned int nBlocks = ( unsigned int ) ceil( ( float ) nElements / BLOCK_SIZE );
    nElements = m_scan.doScan( m_compactionFlags );
    m_openGL->glBindBufferBase( GL_SHADER_STORAGE_BUFFER, Scan::N_BUFFER_TYPES + POS, m_inputBuffers[ POS ]->bufferId() );
    m_openGL->glBindBufferBase( GL_SHADER_STORAGE_BUFFER, Scan::N_BUFFER_TYPES + ATTRIB0,
                                m_inputBuffers[ ATTRIB0 ]->bufferId() );
    m_openGL->glBindBufferBase( GL_SHADER_STORAGE_BUFFER, Scan::N_BUFFER_TYPES + N_BUFFER_TYPES + POS,
                                m_outputBuffers[ POS ]->bufferId() );
    m_openGL->glBindBufferBase( GL_SHADER_STORAGE_BUFFER, Scan::N_BUFFER_TYPES + N_BUFFER_TYPES + ATTRIB0,
                                m_outputBuffers[ ATTRIB0 ]->bufferId() );
    m_compactionProgram->bind();
    m_compactionProgram->enableAttributeArray( "flags" );
    m_compactionProgram->enableAttributeArray( "prefixes" );
    m_compactionProgram->enableAttributeArray( "inputVertices" );
    m_compactionProgram->enableAttributeArray( "inputAttrib0" );
    m_compactionProgram->enableAttributeArray( "outputVertices" );
    m_compactionProgram->enableAttributeArray( "outputAttrib0" );
    m_openGL->glDispatchCompute( nBlocks, 1, 1 );
    m_openGL->glMemoryBarrier( GL_SHADER_STORAGE_BARRIER_BIT );
    m_compactionProgram->disableAttributeArray( "flags" );
    m_compactionProgram->disableAttributeArray( "prefixes" );
    m_compactionProgram->disableAttributeArray( "inputVertices" );
    m_compactionProgram->disableAttributeArray( "inputAttrib0" );
    m_compactionProgram->disableAttributeArray( "outputVertices" );
    m_compactionProgram->disableAttributeArray( "outputAttrib0" );
    m_openGL->glBindBuffer( GL_SHADER_STORAGE_BUFFER, 0 );
    return nElements;
}

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我已经编写了一个自动测试来检查整个过程。此测试压缩一个点位置数组和另一个属性，以便删除奇数索引中的数据。当我在禁用地址空间随机化的情况下在 gdb 下运行代码时，它会完美地通过。但是，当在没有 gdb 或在 gdb 中启用地址空间随机化的情况下运行时，返回的数组全为零，除非我在方法 render() 中glDrawArrays注释。

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TEST_F( CompactionTest, Compaction )
    {
        QGuiApplication app( g_argc, g_argv );
        QSurfaceFormat format;
        format.setVersion( 4, 3 );
        format.setRenderableType( QSurfaceFormat::OpenGL );
        format.setSwapBehavior( QSurfaceFormat::DoubleBuffer );
        format.setSamples( 16 );
        unsigned int nElements = 3000;
        vector< unsigned int  > flags( nElements );
        vector< vec3 > pos( nElements );
        vector< vec3 > attrib0( nElements );
        for( int i = 0; i < nElements; ++i )
        {
            flags[ i ] = i % 2;
            pos[ i ] = vec3( i, i, i );
            attrib0[ i ] = vec3( i + nElements, i + nElements, i + nElements );
        }
        CompactionQGLView window( flags, pos, attrib0, format );
        window.resize(640, 480);
        window.show();
        app.exec();
        pos = window.m_compactedPos;
        attrib0 = window.m_compactedAttrib0;
        ASSERT_EQ( pos.size(), nElements * 0.5 );
        ASSERT_EQ( attrib0.size(), nElements * 0.5 );
        float expected = 1.;
        for( int i = 0; i < pos.size(); ++i, expected += 2 )
        {
            vec3 expectedVec( expected, expected, expected );
            cout << "Pos: " << pos[ i ] << ". Expected: " << expectedVec << endl;
            ASSERT_EQ( pos[ i ], expectedVec );
            expectedVec = vec3( expected + nElements, expected + nElements, expected + nElements );
            cout << "Attrib0: " << attrib0[ i ] << ". Expected: " << expectedVec << endl << endl;
            ASSERT_EQ( attrib0[ i ], expectedVec );
        }
    }

下一个函数读取压缩结果，并由测试中的window用于设置m_compactedPos和m_compactedAttrib0。

template< typename Vec3 >
vector< vector< Vec3 > > CompactionRenderingState< Vec3 >::getResultCPU()
{
    m_openGL->glMemoryBarrier( GL_SHADER_STORAGE_BARRIER_BIT );
    unsigned int resultSize = sizeof( Vec3 ) * m_nElements;
    Vec3* result = ( Vec3* ) malloc( resultSize );
    vector< vector< Vec3 > > results;
    for( int i = 0; i < N_BUFFER_TYPES; ++i )
    {
        if( m_inputBuffers[ i ] != NULL )
        {
            m_openGL->glBindBuffer( GL_SHADER_STORAGE_BUFFER, m_inputBuffers[ i ]->bufferId() );
            m_openGL->glGetBufferSubData( GL_SHADER_STORAGE_BUFFER, 0, resultSize, ( void * ) result );
            vector< Vec3 > tempVec( m_nElements );
            std::copy( result, result + m_nElements, tempVec.begin() );
            results.push_back( tempVec );
        }
    }
    free( result );
    return results;
}

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那么，仅在启用地址空间随机化时发生此错误的可能原因是什么？我被困住了，已经有几天试图弄清楚这一点。有什么想法吗？

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