問題描述

將立方體貼圖投影到 2D 紋理 (Project cubemap to 2D texture)

我想調試我的渲染到立方體貼圖函數，將整個東西投影到一個 2D 紋理，就像這樣：

在紋理著色器的渲染中，我只有可用的 UV 紋理坐標（範圍從 (0,0) 到 (1,1)）。如何在一次繪製調用中將立方體貼圖投影到屏幕上？

參考解法

方法 1:

You can do this by rendering 6 quads and using 3D texture coords (s,t,p) pointing to each vertex of the cube so 8 variations of ( +/‑1,+/‑1,+/‑1 ).

The UV 2D coords (s,t) like 4 variations of (0/1,0/1) are not usable for whole CUBE_MAP only for its individual sides.

Look for txr_skybox in here

• Normal mapping gone horribly wrong

on how CUBE_MAP is used in fragment shader.

PS in OpenGL the texture coords are called s,t,p,q instead of u,v,w,...

Here related QA:

• rendering cube map layout, understanding glTexCoord3f parameters

方法 2:

My answer is essentially the same as the one accepted one, but I have used this very technique to debug my depth‑cubemap (used for shadowcasting) in my current project, so I thought I would include a working sample of the fragment shader code I used.

Unfolding cubemap

This is supposed to be rendered to a rectangle on top of the screen with aspect ratio 3/4 directly on the screen and with s,t going from (0,0) in the lower‑left corner to (1,1) at the upper‑right corner.

Note that in this case, the cubemap I use is inverted, that is objects to the +(x,y,z) side of the cubemap origen is rendered to ‑(x,y,z), and the direction I choose as up for the top/bottom quads are completely arbitrary; so to get this example to work you may need to change some signs or swap s and t some times, also note that I here only read one channel, as it is debth map:

Fragment shader code for a quad‑map as the one in the question:

//Should work in most other versions
#version 400 core

uniform samplerCube dynamic_texture;

out vec4 out_color;
in vec2 ST;

void main()
{
    //In this example i use a debthmap with only 1 channel, but the projection should work with a colored cubemap to, just replace this with a vec3 or vec4    
    float debth=0;
    vec2 localST=ST;




    //Scale Tex coordinates such that each quad has local coordinates from 0,0 to 1,1
    localST.t = mod(localST.t*3,1);
    localST.s = mod(localST.s*4,1);


    //Due to the way my debth‑cubemap is rendered, objects to the ‑x,y,z side is projected to the positive x,y,z side


    //Inside where tob/bottom is to be drawn?
    if (ST.s*4>1 && ST.s*4<2)
    {
        //Bottom (‑y) quad
        if (ST.t*3.f < 1)
        {
            vec3 dir=vec3(localST.s*2‑1,1,localST.t*2‑1);//Get lower y texture, which is projected to the +y part of my cubemap

            debth = texture( dynamic_texture, dir ).r;
        }
        //top (+y) quad
        else if (ST.t*3.f > 2)
        {
            vec3 dir=vec3(localST.s*2‑1,‑1,‑localST.t*2+1);//Due to the (arbitrary) way I choose as up in my debth‑viewmatrix, i her emultiply the latter coordinate with ‑1

            debth = texture( dynamic_texture, dir ).r;
        }
        else//Front (‑z) quad
        {
            vec3 dir=vec3(localST.s*2‑1,‑localST.t*2+1,1);

            debth = texture( dynamic_texture, dir ).r;

        }


    }
    //If not, only these ranges should be drawn
    else if (ST.t*3.f > 1 && ST.t*3 < 2)
    {
        if (ST.x*4.f < 1)//left (‑x) quad
        {
            vec3 dir=vec3(‑1,‑localST.t*2+1,localST.s*2‑1);

            debth = texture( dynamic_texture, dir ).r;

        }
        else if (ST.x*4.f < 3)//right (+x) quad (front was done above)
        {
            vec3 dir=vec3(1,‑localST.t*2+1,‑localST.s*2+1);

            debth = texture( dynamic_texture, dir ).r;

        }
        else //back (+z) quad 
        {
            vec3 dir=vec3(‑localST.s*2+1,‑localST.t*2+1,‑1);

            debth = texture( dynamic_texture, dir ).r;

        }


    }
    else//Tob/bottom, but outside where we need to put something
    {
        discard;//No need to add fancy semi transparant borders for quads, this is just for debugging purpose after all
    }

    out_color = vec4(vec3(debth),1);
}

Here is a screenshot of this technique used to render my depth‑map in the lower‑right corner of the screen (rendering with a point‑light source placed at the very center of an empty room with no other objects than the walls and the player character):

Equirectangular projection

I must, however, say that I prefer using an equirectangular projection for debugging cubemaps, as it doesn't have any holes in it; and, luckily, these are even easier to make than unfolded cubemaps, just use a fragment shader like this (still with s,t going from (0,0) to (1,1) from lower‑left to upper‑right corner), but this time with aspect ratio 1/2:

//Should work in most other versions
#version 400 core

uniform samplerCube dynamic_texture;

out vec4 out_color;
in vec2 ST;

void main()
{
    float phi=ST.s*3.1415*2;
    float theta=(‑ST.t+0.5)*3.1415;


    vec3 dir = vec3(cos(phi)*cos(theta),sin(theta),sin(phi)*cos(theta));

    //In this example i use a debthmap with only 1 channel, but the projection should work with a colored cubemap to
    float debth = texture( dynamic_texture, dir ).r;
    out_color = vec4(vec3(debth),1);
}

Here is a screenshot where an equirectangular projection is used to display my depth‑map in the lower‑right corner:

(by Bruno Sena、Spektre、Nikolaj)

參考文件

1. Project cubemap to 2D texture (CC BY‑SA 2.5/3.0/4.0)