bool Texture::isValidImageIndex(const ImageIndex &index) const
{
    auto iter = mImageDescs.find(index);
    if (iter == mImageDescs.end())
    {
	return false;
    }

    const ImageDesc &desc = iter->second;

    // width & height check necessary?

    return (!index.hasLayer() || static_cast<size_t>(index.layerIndex) < desc.size.depth);
}