How to use QOpenGLFrameBufferObject for shadow mapping?

Question

I am trying to render a scene (bike on a platform shown below) with shadow using Qt's OpenGL classes.

Screenshot of bike model without shadows

I realize that we would need to make two passes for this.

The first pass for rendering a depth-map from the light's point of view and the second for rendering the scene from the camera's point of view where the fragment shader uses the depth-map as a texture for determining whether fragments are inside the shadow or not.

For capturing the depth map, I am rendering the scene from the light's point of view into a framebuffer as follows

uint ShadowRenderWindow::renderToShadowMap()
{
    if(!m_shadowFBO)
    {
        m_shadowFBO = new QOpenGLFramebufferObject(1024,
1024, QOpenGLFramebufferObject::Depth);
        m_shadowFBO->bind();
        glDrawBuffer(GL_NONE);
        glReadBuffer(GL_NONE);
        m_shadowFBO->release();
    }

    m_shadowFBO->bind();

    glViewport(0, 0, m_shadowFBO->width(), m_shadowFBO->height());
    glEnable(GL_DEPTH_TEST);
    glClear(GL_DEPTH_BUFFER_BIT);

    m_lightViewMatrix.setToIdentity();
    m_lightViewMatrix.lookAt(
      m_lightPositionMatrix.map( QVector3D(0,0,0) ),
      m_sceneBounds.center(),
      m_lightPositionMatrix.map( QVector3D(0,1,0) ).normalized() );

    for(int i=0; i<m_models.size()-1; i++)
    {
        ObjModel *model = m_models.at(i);
        model->setRenderMode(ObjModel::ShadowMode);
        model->render(m_projectionMatrix, m_lightViewMatrix);
    }

    m_shadowFBO->release();

    return m_shadowFBO->texture();
}

Each model is rendered using

m_shader->bind();
model->m_vertexBuffer->bind();
model->m_indexBuffer->bind();

const QMatrix4x4 lightViewProjectionMatrix = projectionMatrix *
lightViewMatrix * model->m_matrix;

m_shader->enableAttributeArray("qt_Vertex");
m_shader->setAttributeBuffer("qt_Vertex", GL_FLOAT, 0, 3, 0);

m_shader->setUniformValue("qt_LightViewProjectionMatrix",
lightViewProjectionMatrix);

Q_FOREACH(ObjModel::Part part, model->m_parts)
{
    const uint offset = part.start * sizeof(uint);
    glDrawElements(part.type, part.length,
         GL_UNSIGNED_INT, (void*)offset);
}

model->m_indexBuffer->release();
model->m_vertexBuffer->release();
m_shader->release();

with vertex shader

attribute vec3 qt_Vertex;
uniform mat4 qt_LightViewProjectionMatrix;
const float c_one = 1.0;

void main(void)
{
    gl_Position = qt_LightViewProjectionMatrix * vec4(qt_Vertex, c_one);
}

and fragment shader

void main(void)
{
    gl_FragDepth = gl_FragCoord.z;
}

And then for using the shadow texture in the second pass, I do this in the fragment shader of the second pass.

float evaluateShadow(in vec4 shadowPos)
{
    vec3 shadowCoords = shadowPos.xyz / shadowPos.w;
    shadowCoords = shadowCoords * 0.5 + 0.5;

    float closestDepth = texture2D(qt_ShadowMap, shadowCoords.xy).r;
    float currentDepth = shadowPos.z;
    float shadow = (currentDepth > closestDepth) ? 1.0 : 0.5;

    return shadow;
}

void main(void)
{
    vec4 lmColor = evaluateLightMaterialColor(v_Normal);
    if(qt_ShadowEnabled == true)
    {
        float shadow = evaluateShadow(v_ShadowPosition);
        gl_FragColor = vec4(lmColor.xyz * shadow, qt_Material.opacity);
    }
    else
        gl_FragColor = lmColor;
}

But all of the above seems to make no difference. I am unable to see any shadow in the second pass.

Can someone please point to me where I am going wrong please?

Prashanth N Udupa Prashanth N Udupa · Accepted Answer · 2016-02-02T05:46:24

Since using QOpenGLFrameBufferObject was out of question, I had to create the buffer by myself using gl function calls as follows.

// Refer http://learnopengl.com/#!Advanced-Lighting/Shadows/Shadow-Mapping
if(m_shadowMapFBO != 0)
    return;

// Create a texture for storing the depth map
glGenTextures(1, &m_shadowMapTex);
glBindTexture(GL_TEXTURE_2D, m_shadowMapTex);
glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT,
             SHADOW_WIDTH, SHADOW_HEIGHT, 0, GL_DEPTH_COMPONENT, GL_FLOAT, NULL);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_BORDER);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_BORDER);
GLfloat borderColor[] = { 1.0, 1.0, 1.0, 1.0 };
glTexParameterfv(GL_TEXTURE_2D, GL_TEXTURE_BORDER_COLOR, borderColor);

// Create a frame-buffer and associate the texture with it.
glGenFramebuffers(1, &m_shadowMapFBO);
glBindFramebuffer(GL_FRAMEBUFFER, m_shadowMapFBO);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D, m_shadowMapTex, 0);

// Let OpenGL know that we are not interested in colors for this buffer
glDrawBuffer(GL_NONE);
glReadBuffer(GL_NONE);

// Cleanup for now.
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glBindTexture(GL_TEXTURE_2D, 0);

Then I rendered the scene by binding the shadow texture. The fragment shader code had to be updated a bit to determine whether a fragment lies within a shadow or outside of it.

const float qt_ZNear=0.1;
const float qt_ZFar=1000.0;

float linearizeDepth(float depth)
{
    float z = depth * 2.0 - 1.0; // Back to NDC
    return (2.0 * qt_ZNear * qt_ZFar) / (qt_ZFar + qt_ZNear - z * (qt_ZFar - qt_ZNear));
}
float evaluateShadow(in vec4 shadowPos)
{
    vec3 shadowCoords = shadowPos.xyz / shadowPos.w;
    shadowCoords = shadowCoords * c_half + c_half;

    if(shadowCoords.z > c_one)
        return c_one;

    float closestDepth = linearizeDepth( texture2D(qt_ShadowMap, shadowCoords.xy).r );
    float currentDepth = shadowPos.z;
    float shadow = (currentDepth < closestDepth) ? c_one : c_half;

    return shadow;
}

With that done, I was now able to render the bike with shadows

The complete code can be downloaded from here

How to use QOpenGLFrameBufferObject for shadow mapping?

2 Answers