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當使用相同的緩沖區進行計算着色和渲染時，為什么glDrawElements會干擾glGetBufferSubData

[英]Why glDrawElements is interfering with glGetBufferSubData when using the same buffer for Compute Shading and rendering

 原文  2015-01-09 13:36:33  6   1    c++/ qt/ opengl

問題描述

我有一個渲染點流的程序。 用於獲取給定幀的點的方法具有很強的時間一致性，因此，在渲染循環中，首先，我使用Compute Shader壓縮流，去除不需要的點。 其次，我將新點添加到集合中。 最后，我使用glDrawElements渲染它。

這就是執行該過程的代碼（它使用Qt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;
}

======編輯======

我寫了一個自動化測試來檢查整個過程。 此測試壓縮點位置數組和另一個屬性數組，以致刪除奇數索引中的數據。 當我在禁用地址空間隨機化的gdb下運行代碼時，它可以完美地通過。 但是，在沒有gdb或gdb中啟用了addres空間隨機化的情況下運行時，除非我在render()方法中注釋glDrawArrays ，否則返回的數組都為零。

======編輯結束====== 

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 );
        }
    }

下一個函數讀取壓縮結果，測試中的window將其用於設置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;
}

======編輯======

那么，僅當啟用“地址空間隨機化”時，才可能出現此錯誤的原因是什么？ 我被困住了，已經幾天試圖解決這個問題了。 有任何想法嗎？

======編輯結束======

1 個解決方案

解決方案1
 0 已采納  2015-01-12 18:55:44

最后，問題是調用glVertexAttribPointer時發生了一個錯誤的隱式類型轉換。

void glVertexAttribPointer(GLuint index, GLint size, GLenum type, GLboolean normalized, GLsizei stride, const GLvoid * pointer)接收作為最后一個參數一個const GLvoid * pointer 。 在我的代碼中，我將變量unsigned int bufferOffset的地址作為最后一個參數傳遞。 那應該導致OpenGL內部混亂。

要修復代碼，我們只需要在render()函數的switch子句中更改glVertexAttribPointer調用即可： 

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, ( void * ) 0 );
            m_openGL->glEnableVertexAttribArray( QGL::Position );

            m_outputBuffers[ ATTRIB0 ]->bind();
            m_openGL->glVertexAttribPointer( QGL::Normal, 3, GL_FLOAT, GL_FALSE, 0, ( void * ) 0 );
            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, ( void * ) 0 );
            m_openGL->glEnableVertexAttribArray( QGL::Position );

            m_outputBuffers[ ATTRIB0 ]->bind();
            m_openGL->glVertexAttribPointer( QGL::Color, 3, GL_FLOAT, GL_FALSE, 0, ( void * ) 0 );
            m_openGL->glEnableVertexAttribArray( QGL::Color );

            break;
        }
        case Attributes::COLORS_AND_NORMALS:
        {
            throw logic_error( "Colors and normals not supported yet." );
            break;
        }
    }

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