Abstract: A real-time soft shadow mapping algorithm based on backprojection is presented to generate real-time shadows.The algorithm uses a single shadow map per light source，and treats the shadow map as a simple discretized representation of the scene.Also，a visibility factor is introduced to calculate the percentage of light seen by a single pixel in the scene and some other solutions are proposed to accelerate the efficiency of rendering，such as fragment shader with GPU，a Hierarchical Shadow Map（HSM），adaptive precision，and so on.The experimentation shows that the algorithm is well suited to render both complex and dynamic scenes.It naturally handles occluder fusion in particular，and maps well to programmable graphics hardware.

Key words: shadow maps, soft shadow, projection, visibility factor, penumbra

摘要： 基于阴影映射算法，提出一种利用反向投影实现的实时软阴影的新算法。算法对每个光源都产生对应的阴影图，使用阴影图作为对场景的离散化表示，引入可见因子来计算场景点的亮度信息，并采用GPU片元着色、层次阴影图、自适应精度等方法加速阴影渲染。实验表明，算法非常适合于实时渲染复杂、动态的场景，可以很好地处理遮挡物的融合，并且很容易在可编程图形硬件上实现。

关键词: 阴影图, 软阴影, 投影, 可见因子, 半影