OpenGL渲染三角形而不是四边形[闭合]

我目前正在尝试将纹理渲染到全屏 QWindow

我在屏幕的左上角得到了一个红色三角形,而不是一个四边形。
我看不出我在代码上犯了什么错误,所以我请你检查一下我的代码。
glTexImage2D 看起来很好,没有错误 glGetError .

OpenGL上下文和QWindow的初始化:

int i;
for( i=0;i<QApplication::screens().size();i++ )
{
    if(QApplication::screens().at(i)->name().compare(szScreenName)==0)
        break;
}
if(i==QApplication::screens().size())
    i--;

setGeometry(QApplication::screens().at(i)->availableGeometry());
showFullScreen();

setSurfaceType(QWindow::OpenGLSurface);
QSurfaceFormat fmt;
fmt.setSwapBehavior(QSurfaceFormat::SingleBuffer);
fmt.setRenderableType(QSurfaceFormat::OpenGLES);
setFormat(fmt);
_pContext = new QOpenGLContext(this);
_pContext->setFormat(fmt);
//_pContext->setShareContext(QOpenGLContext::globalShareContext());
_pContext->create();

QObject::connect( this, &DLP::postRenderSignal, this, &DLP::render );

_pContext->makeCurrent(this);
initializeOpenGLFunctions();

//_pTexManager = new TextureManager(QOpenGLContext::globalShareContext()->functions());
_pTexManager = new TextureManager(this);

char* szVertexShader =                          "#version 110\n"
                                                "attribute vec4 a_Position;\n"
                                                "attribute vec2 a_TexCoord;\n"
                                                "varying   vec2 v_TexCoord;\n"
                                                "void main()\n"
                                                "{\n"
                                                "    gl_Position = a_Position;\n"
                                                "    v_TexCoord = a_TexCoord;\n"
                                                "}\n";
char* szFragmentShader =                        "#version 110\n"
                                                "precision mediump float;\n"
                                                "varying vec2 v_TexCoord;\n"
                                                "uniform sampler2D u_Texture;\n"
                                                "void main()\n"
                                                "{\n"
                                                "    gl_FragColor = vec4(1.0,0.0,0.0,1.0);//texture2D( u_Texture, v_TexCoord );\n"
                                                "}\n";

//_nProgram = linkShader(QOpenGLContext::globalShareContext()->functions(), szVertexShader, szFragmentShader);
_nProgram = linkShader(this, szVertexShader, szFragmentShader);

if(_nProgram!=0)
{
    _nShaderSamplerLoc = glGetUniformLocation(_nProgram,"u_Texture");
    _nShaderTextureLoc = glGetAttribLocation(_nProgram,"a_TexCoord");
    _nShaderPositionLoc = glGetAttribLocation(_nProgram,"a_Position");
}

glGenBuffers(2, _pBuffers );

static const unsigned short pIndexData[] = { 0, 1, 2, 3 };
glBindBuffer(GL_ARRAY_BUFFER, _pBuffers[1]);
glBufferData(GL_ARRAY_BUFFER, sizeof(pIndexData), pIndexData, GL_STATIC_DRAW);

static const float pPosBuffer[20] = {
        -1,   1,   0,    0, 0,
         1,   1,   0,    1, 0,
        -1,  -1,   0,    0, 1,
         1   -1,   0,    1, 1,
};
glBindBuffer(GL_ARRAY_BUFFER, _pBuffers[0]);
glBufferData(GL_ARRAY_BUFFER, 20*4, pPosBuffer, GL_STATIC_DRAW);

渲染代码(DLP::Render):

if (!isExposed() || !_pContext)
    return;

_pContext->makeCurrent(this);

glClearColor(0.0, 0.0, 0.0, 1.0);
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT|GL_STENCIL_BUFFER_BIT);
glViewport(0,0,m_nWidth,m_nHeight);

glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

glUseProgram(_nProgram);

glBindBuffer(GL_ARRAY_BUFFER, _pBuffers[0]);
// Load the vertex position
glVertexAttribPointer(_nShaderPositionLoc, 3, GL_FLOAT, false, 5 * 4, (void*)0);
glEnableVertexAttribArray(_nShaderPositionLoc);
// Load the texture coordinate
//glVertexAttribPointer(_nShaderTextureLoc, 2, GL_FLOAT, false, 5 * 4, (void*)(3 * 4));
//glEnableVertexAttribArray(_nShaderTextureLoc);

glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, _pTexManager->getTexture(key));
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glUniform1i(_nShaderSamplerLoc, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, _pBuffers[1]);
glDrawElements(GL_TRIANGLE_STRIP, 4, GL_UNSIGNED_SHORT, 0);
glDisableVertexAttribArray(_nShaderPositionLoc);
//glDisableVertexAttribArray(_nShaderTextureLoc);

glFinish();

_pContext->swapBuffers(this);