LinearEyeDepth的作用、原理

作用
LinearEyeDepth的使用方法相对简单,只需传入深度纹理中的深度值(范围0~1)即可计算出实际的深度值。

原理
先来看看深度纹理中记录的深度值是怎么来的。在观察空间中,顶点z经过投影矩阵得到齐次坐标z’,z’除以齐次分量得到[-1,1]范围的坐标z’’,z’’*0.5+0.5得到[0,1]范围的坐标z’’’,z’’'的z分量既是深度纹理中储存的值。
现在,我们只需要把深度纹理中的深度值进行反向计算,就可以得到实际的深度值。LinearEyeDepth会帮我们完成这个反向计算。

### LinearEyeDepth in Computer Graphics Implementation and Usage In computer graphics, `LinearEyeDepth` refers to a technique that converts non-linear depth values into linear space relative to the viewer's eye position. This conversion is crucial because standard depth buffers store depth information using perspective-corrected (non-linear) values which can lead to inaccuracies when performing certain calculations such as shadow mapping or post-processing effects. The process involves transforming normalized device coordinates (NDC) back into view-space positions where distances are represented more uniformly across different depths within the scene. For this purpose, one typically uses the inverse of the projection matrix applied during rendering[^1]. To implement `LinearEyeDepth`, consider following code snippet demonstrating how OpenGL shaders handle this transformation: ```glsl // Vertex Shader Code #version 330 core layout(location = 0) in vec3 vertexPosition; uniform mat4 modelViewProjectionMatrix; out float fragmentDepth; void main() { gl_Position = modelViewProjectionMatrix * vec4(vertexPosition, 1.0); fragmentDepth = gl_Position.z / gl_Position.w; // Store raw clip-space z/w value } // Fragment Shader Code #version 330 core in float fragmentDepth; out vec4 fragColor; uniform sampler2D textureSampler; uniform float nearPlane; uniform float farPlane; float getLinearDepth(float depthValue) { float n = nearPlane; float f = farPlane; // Convert from NDC [-1, 1] range to [0, 1] float ndc_depth = depthValue * 2.0 - 1.0; // Calculate linear depth based on camera frustum parameters return (2.0 * n) / (f + n - ndc_depth * (f - n)); } void main() { float linearDepth = getLinearDepth(fragmentDepth); fragColor = vec4(linearDepth, linearDepth, linearDepth, 1.0); } ``` This shader program first stores the untransformed \( \frac{z}{w} \) component (`fragmentDepth`) calculated at each vertex stage before passing it along to fragments for further processing. Within the fragment shader, function `getLinearDepth()` performs necessary computations converting given depth value into its corresponding linear equivalent suitable for various graphical operations requiring accurate distance measurements between objects seen through viewport window[^2].
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