Raymarching in Shadertoy, openFrameworks and Unreal Engine

-

Raymarching is a special method of rendering 3D objects or 3D scene:
  • It's used for rendering ocean water, clouds, metaballs in Unreal Engine 4 and other engines and 3D editors.
  • It's a main technique used for rendering 3D in Shadertoy projects.

Examples

Mehod description

The "forward" method of rendering used in OpenGL by default as a creating object from vertices and faces, and then rasterizing faces to obtain raster image for showing in on the screen.
In the opposite, the "raymarching" method creates a ray the each resulted image's pixel, which goes from the camera's origin through the pixel into 3D scene. The special point moves along the ray and accumulates colors of scene while 99-100% of opacity will be accumulated.
The step of moving along the ray can be fixed or dynamically changed (just increasing for rendering clouds or depending on the distance to the objects for rigid scene rendering such as ocean).
So in raymarching we don't need to represent object as a set of triangles, and can represent it by formula, as a 3D texture or octotree.

Raymarching is a just one particular technique and  there are two similar tecnhiques:
  • raycasting - here ray don't scanned, but place of hitting the object is computed exactly,
  • raytracing - here diffusion and reflection and lighing is dealed more delicately.

Tutorials on raymarching

More Shadertoy examples

Simple volume rendering by XT95, with light scattering: https://www.shadertoy.com/view/lss3zr
Ocean with islands by frankenburgh: https://www.shadertoy.com/view/lsBSWm
Well-documented code of ocean by bteitler: https://www.shadertoy.com/view/llsXD2

Making raymarching in openFrameworks

ofxShadertoy addon by Tiago Rezende to load Shadertoy's shaders:
https://github.com/tiagosr/ofxShadertoy

Addon for raytracing on CPU using Intel Embree library:
https://github.com/cyrildiagne/ofxEmbree

ofxVolumetrix addon by Timothy Scaffidi for working with 3D textures (voxels) in OpenGL and rendering it using raycasting:
https://github.com/timscaffidi/ofxVolumetrics

Computing ray direction from OpenGL matrix:
https://stackoverflow.com/questions/2354821/raycasting-how-to-properly-apply-a-projection-matrix/52764898#52764898

Computing ray directions in openFrameworks, our article.
Formula for backward alpha blending, our article.

Making volumetric raymarcher in Unreal Engine



Comments

Post a Comment

Popular posts from this blog

Computing ray origin and direction from Model View Projection matrices for raymarching

-
When performing raymarching (as well a raycasting and raytracing) using fragment shaders in OpenGL, it's required to compute ray's origin and direction. Here we will give solution for finding it from the Model View Projection matrices in OpenGL, implemented in openFrameworks. Using it, you can combine raymarching/raycasting/raytracing with forward OpenGL rendering, so obtain hybrid rendering modes. Also, you can use it for creating VE applications based on raymarching or hybrid rendering. (Actually, we checked what this approach is corrent by creating VR rendering app in HTC Vive). The algorithm can be used on any OpenGL platform with any language supporting matrix inversion computation (in our case it's C++ with GLM library, included in openFrameworks 0.10.1). The approach is based on this hint by  GClements in this discussion on the topic: https://community.khronos.org/t/ray-origin-through-view-and-projection-matrices/7257
Read more

Forward and backward alpha blending for raymarching

-
When implementing raymatching by scanning a ray to accumulating scene's colors, containing semi-transparent colors (for example, scene with clouds), it's required to properly combine colors using alpha blending method. Here we discuss how to do it and obtain corresponding formula. (So, here we propose that objects has "uniform" ambient lighting and some transparency and all we need to accumulate their colors). The forward alpha blending formula, used "everywhere" - in OpenGL, VJ software, photo and video editors, can be formulated in the following way: Formula for "forward" alpha blending ---------------------------------- Let C is a current RGB color, c is a new RGB color and alpha is a new color's opaqueness (alpha is a float value from 0 to 1). Then updating rule for C is the following:     C = (1- alpha )* C + alpha * c    ---------------------------------- So, the more alpha , the more c affec
Read more

Forward, Deferred and Raytracing rendering in openFrameworks and web

-
Image
Currently, there are three basic approaches for rendering computer graphics scenes: forward rendering, deferred rendering, raytracing. 1. Forward rendering It's that OpenGL does, you create vertices, then make faces, and rasterize them, all passing via shaders. The rendering process is straightworward and fast, but rendering shadows, making degree-of-field effect and drawing clouds is hard here. Most of openFrameworks examples use forward rendering. 2. Deferred rendering It's the same as forward rendering, but additionally some extra data is rendered for computing shadows. The lighting pass is performed after the scene geometry is drawn. See detailed explanation here: https://learnopengl.com/Advanced-Lighting/Deferred-Shading Shadows, degree-of-field effect and some kind of reflections are possible with deferred rendering. Unreal Engine 4 and Unity currently (2019) are based on deferred rendering. openFrameworks: https://github.com/perevalovds
Read more