CN106408639A - Curvature flow-based screen space fluid rendering method - Google Patents
Curvature flow-based screen space fluid rendering method Download PDFInfo
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- CN106408639A CN106408639A CN201610789716.3A CN201610789716A CN106408639A CN 106408639 A CN106408639 A CN 106408639A CN 201610789716 A CN201610789716 A CN 201610789716A CN 106408639 A CN106408639 A CN 106408639A
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- point
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- G06—COMPUTING; CALCULATING OR COUNTING
- G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
- G06T15/00—3D [Three Dimensional] image rendering
- G06T15/005—General purpose rendering architectures
Abstract
The invention relates to a curvature flow-based screen space fluid rendering method comprising the following steps: in step S1, a smooth particle fluid model of a three dimensional object in a screen space is obtained, the model comprises positions and speed of particles, a depth image of particles of fluid surfaces is generated, and a nearest surface of the three dimensional object is determined; a method for determining the nearest surface of the three dimensional object comprises the following steps: if an object intersects with the same straight line in a direction of an optical axis at two or more intersection points, an intersection point which is a minimum distance away from a camera is used as a point on the nearest surface; in step S2, the nearest surface is smoothed via curvature flow filtering operation; in step S3, the number of fluids between each point and the nearest surface is calculated as thickness of the point; in step S4, noise is added to the nearest surface; in step S5, color and transparency of each point are weakened according to the thickness of the point and the speed of the particles, and a fluid rendering graph can be obtained. Compared with technologies of the prior art, the curvature flow-based screen space fluid rendering method is advantaged by high speed, and all processing can be directly finished via graphic hardware.
Description
Technical field
The present invention relates to a kind of image rendering method, especially relate to a kind of render based on the screen space fluid of curvature flow
Method.
Background technology
For interactively application program, such as play, the fluid simulation method based on particle such as smooth particle fluid force
Learn (Smoothed Particle Hydrodynamics, SPH) generally preference Euler's fluid meter and show form.This is due to fluid
Arbitrarily can flow under the scene not needing to define fluid grid, under generally this scene, internal memory and the consumption calculating are very
Expensive.SPH simulation is a kind of powerful instrument of simulation fluid, however it is necessary that advanced technology is realizing the real wash with watercolours of result
Dye.
Scene geometric figure can be collided with particle as other strict objects, particle method is also more easily
It is integrated in existing physical system, become grid without by scene geometric figure voxel.Its shortcoming is to extract a table
Face becomes more difficult to carry out rendering.In existing method, most of all inapplicable for the method for rendered particle fluid
Real-time use in game.Generally, the surface of fluid is to create in world space, directly as a network, or
As the surface of implicit expression, then carry out polygonization using marching cube or a similar algorithm.After this, relax and
Optimize operation to be used in whole network to increase the smoothness on surface, smoothness is to calculate and memory-intensive.
Implicit surfaces polygonization method also suffers from the problem in frame grid discretization workpiece in frame continuity, because net
Lattice are static, and do not need to move with fluid.This phenomenon is outstanding when real-time rendering length uses the grid of low resolution
It is obvious.Zhang[1]One rendering intent based on point was proposed, it is to calculate on GPU completely that light ball intersects, and uses
One two stage rendering intent.This method removes the requirement of grid discretization.
If fluid is mobile, then render each frame and only need to only one or at most several viewpoint, one more
Many internal memories and the effective method of calculating need only to construction surface representated by visible particle to camera in visible space.
Content of the invention
The purpose of the present invention is exactly to provide to overcome the defect that above-mentioned prior art exists that a kind of speed is fast, effect is good
The screen space fluid rendering intent based on curvature flow.
The purpose of the present invention can be achieved through the following technical solutions：
A kind of screen space fluid rendering intent based on curvature flow, comprises the following steps：
S1, obtains the smooth particle fluid model of three-dimensional body in screen space, and model includes position and the speed of particle,
Generate the depth map of the particle of flow surface, determine the nearest surface of three-dimensional body, the nearest surface of three-dimensional body determines method
It is specially：If object has two or more intersection points with the same straight line of camera light direction of principal axis, take nearest from video camera
Intersection point, as the point on nearest surface；
S2, by curvature flow filter, smooths nearest surface；
S3, calculates the amount of fluid between each point and nearest surface, as the thickness of this point；
S4, adds noise for nearest surface；
S5, the thickness according to point and the speed of particle, weaken color a little and transparency, obtain fluid and render figure.
In described step S1, particle renders are become spheroid, calculate on each spheroid from the point that video camera is nearest.
In described step S2, nearest surface is smoothed using average curvature.
In described step S4, noise is Berlin noise.
In described step S5, the thickness of point is bigger it is seen that degree is lower, and the color of point and reduced transparency are more.
Compared with prior art, the present invention has fireballing advantage, directly can complete all process on graphic hardware；
By smooth surface, it is to avoid fluid produces speckle or jelly effect.
Brief description
Fig. 1 is the graph of a relation between flow surface of the present invention and camera angle；
Fig. 2 is the schematic diagram that particle is seen globulate；
Fig. 3 (a), (b) are the spherical schematic diagram of particle at two visual angles；
Fig. 4 (a), (b) are respectively and carry out smooth effect with screen space curvature flow to NVIDIA using bilateral the obscuring of Gauss
Fruit is schemed；
Fig. 5 (a), (b) are the contrast effect figure before and after being added noise to a pipeline；
Fig. 6 is to render sand fluid design sketch using the present embodiment method；
Fig. 7 is the flow chart of the inventive method.
Specific embodiment
The present invention is described in detail with specific embodiment below in conjunction with the accompanying drawings.The present embodiment is with technical solution of the present invention
Premised on implemented, give detailed embodiment and specific operating process, but protection scope of the present invention be not limited to
Following embodiments.
Embodiment
As shown in fig. 7, a kind of screen space fluid rendering intent based on curvature flow, comprise the following steps：
S1, obtains the smooth particle fluid model of three-dimensional body in screen space, and model includes position and the speed of particle,
Generate the depth map of the particle of flow surface, determine the nearest surface of three-dimensional body, the nearest surface of three-dimensional body determines method
It is specially：If object has two or more intersection points with the same straight line of camera light direction of principal axis, take nearest from video camera
Intersection point, as the point on nearest surface；
S2, by curvature flow filter, smooths nearest surface；
S3, calculates the amount of fluid between each point and nearest surface, as the thickness of this point；
S4, adds noise for nearest surface；
S5, the thickness according to point and the speed of particle, weaken color a little and transparency, obtain fluid and render figure.
The following is this method to illustrate.
The first step, the depth map of the particle of generation flow surface.
Assume that a kind of smoothed particle method particle simulation exists.Input data comprises the particle pi of n out-of-order
(i=1 ..., position n).Optionally, particle has density and the speed from simulation of correlation.Before being rendered, video camera
The surface of the fluid foremost paid has set.
As shown in figure 1, screen space fluid renders only generating apart from the nearest surface of video camera.By by particle renders
Become spheroid, retain the nearest value in each pixel using hardware depth test, as shown in Figure 2 and Figure 3.
In order to obtain the surface of the fluid obtaining from the visual angle of observer, using the point of the replacement of depth in fragment shader
Particle renders are become spheroid by block texture.This avoid the use of complex geometry.Depth value will be used in following smoothing step
Arrive.Render process can be tended to abandon some particles, because they do not constitute any part on surface, by from simulation
During to density add a threshold value, can be easily carried out.So that transitional processes more smooth out, low-density grain
Son individually can be rendered as spraying.
Second step, smoothing method.
Although the first step is to be spheroid by particle renders, particle looks like the result that spheroid is not intended to, because
For a untrue, appearance as fruit jelly so can be led to.One smooth and even curface, is similar to the position needs of particle
Realized by the surface in smooth screen space.
Existing method is using a Gaussian Blur or the such as bilateral Gauss filter of its variant.But direct Gauss
Obscure and will obscure contour edge, lead to the steady statue of equal depth when using big kernel function.Bilateral filter is permissible
Protect edge information, but it is unseparated.Efficiently realize one using the core that a width changes on graphic hardware to obscure
Equally highly difficult.
Alternative as a kind of Gaussian smoothing, this problem can be treated from another kind of different mode：For one kind
The suddenly change of curvature between the particle at place can be smoothed, form a surface smoothly and continuously.Consider a kind of this mode
It is to minimize curvature.This is similar to the surface tension of the rational fluid of the formation of water droplet in nature and water whirlpool.This
Individual process is called curvature flow.
The surface of curvature flow depends on the symbol of the average curvature on size and surface along normal direction speed.Surface can only
Moved on the z direction of visual plane, be continuous it is still desirable to pass through mobile and curvature yet with vision point
Proportional z value, to realize the ideal effect that surface smooths, so defines
Wherein, t is a smooth time step, and H is average curvature.Start to this step, this method is called screen
Curvature of space stream.Average curvature is defined as the deviation of the unit vector on a surface,
By transposition Mapping and Converting, one of depth buffer is worth one of mapped meeting visual space Vx and Vy point P,
Fx and Fy be after x and y direction focal length,
Normal is to take advantage of to calculate by the difference taking the derivative in x and y direction for the P,
Wherein,Ignore the derivative term of the P depending on visual position Wx and Wy, because so
Simplify many calculating, and difference is negligible when its contribution very little.Unit vector is defined as follows：
Wherein
Substituted in the formula of average curvature, therefore H can be exported.The value of the z part of derivative always 0, because z
It is a function with regard to x and y, therefore all without change when they all remain constant.Thus obtain
Wherein,
The simple Euler's integral of of equation (1) is used to change the value of z in each iteration.The local derviation of z is to use to have
Limit difference calculates.
Because the surface profile in screen space is possibly discontinuous.In order to prevent that the different masses on surface are blended in one
Rise, on the edge of these borders and screen, partial derivative is forced to 0 to prevent any smooth generation.
The quantity of iteration is that smoothness as needed carrys out selection.Iterationses are more, and surface is more smooth, but this
The individual cost that will lead to the incremental calculating time.
3rd step, calculated thickness.
One object determines its visible level according to the number of the fluid before it.It is thus desirable to calculating in video camera
The quantity of the fluid and the nearest opaque article of each pixel between, this is named as thickness.When rendering, thickness is used
To weaken color and the transparency of fluid.
Particle is considered the spheroid having the fluid of fixed dimension in world space.The process rendering is with the process of depth
It is the same, difference is fragment shading, it exports the thickness of particle rather than depth.Extra mixing is used as fluid
Quantity on screen be each position accumulation.It is possible during depth test, therefore only before screen set
Particle is just rendered,
Wherein, d is depth kernel function, xiAnd yiIt is the mapping position of particle, x and y is screen coordinate, σiIt is the rule of mapping
Mould.
Strictly, only when particle is not covered with, the measurement of thickness is correct, but this is a conjunction in SPH
Reason it is assumed that because particle between repulsion.
4th step, avoid too manually smoothing, and add noise.
Although this method helps conceal the essence based on particle of fluid, this result still seems too people
Work smooths.Surface details and foam are important visual elements in real fluid.One is directly improved this defect
Method is to upset surface using a noise texture, then adds small-scale details, just as Johanson realizes.
However, the noise generating the fixation in world space or visual-field space can make noise show must seem to be absorbed in place.This
Challenge is to convey to produce noise by fluid advection, but only than simulation based on the fluid of particle have less scale and
Higher frequency.Therefore, this method proposes each mapping by the octave rank of noise is distributed to the index value based on it
Particle come using Berlin noise, therefore, the type of noise of a determination is kept together with other particles.By using volume
Outer mixing, this will lead to Berlin noise, and octave rank moves relative to each other with stream.
For each particle, a point block texture is rendered using gaussian kernel.The value obtaining and a base under the surface
In an index decreased of depth, therefore particle contribution when they are from merging is less,
Wherein, p is the visual space position of this pixel, and d is the sample depth from case depth texture, x and y is -1
And change between 1.Noise texture noise is change to prevent pattern from becoming obvious between each particle.
Then this noise core sum up for each particle on screen, in order to obtain can obtaining at each pixel
To be used for sheltering from heat or light to a noise figure,
When fluid is very violence, fluid should become more multilated, when speed viThere is great variety
When, realized by marked fluid particle,
|vi(t)-vi(t-1) | ＞ τ (12)
Wherein τ is a threshold value.For these particles, noise amplitude will be higher.Soon after, under particle calmness
Come, change into normal state.
5th step, render.
In the final step, all of intermediate result all will be synthesized last figure by rendering a full frame four-tuple
Piece.The optical property of fluid is based on Fresnel equation, has a reflection and the part reflecting and a Feng Shi bloom, calculates defeated
Go out color
Cout=a (1-F (n v))+bF (n v)+ks(n·h)α(13)
Wherein F is Fresnel equation, and a is the fluid color of refraction, and b is the scene color of reflection, ksIt is Specular Highlight with α
Constant, n is surface normal, and h is the half-angle between video camera and optical fiber, v be video camera vector.When rendering fluid,
Depth test is possible, and the depth being returned by fragment shader replicates from case depth.For gradual change fluid
Surface, the normal n of visual space uses the finite difference of case depth d (x, y) to calculate, as shown in equation (4).Simply make
Calculate discovery with the finite difference in a direction and will lead to the manual of edge.When discontinuity is detected, by comparing
Depth, to the difference of threshold value, selects minimum absolute finite difference.In addition, Noise texture N (x, y) is to be used to upset find,
By the partial derivative of Noise texture being added to the surface details that little picture ripple to be added on the normal of calculating to fluid.Additionally, one
The color of individual gray scale can add one surfactant foam effect of simulation by the series of noise.
Thickness T (x, y) is used to weaken the refraction color of fluid a,
A=lerp (Cfluid,S(x+βnx,y+βny),e-T(x,y)) (14)
Fluid is thicker, and it is more that it weakens background colour.The region of fluid is crossed background scene and is shown.When gradual change fluid
Wait, a slightly different index decreased is used to each color channel, such color will be interesting with one kind with thickness
Mode change.For transparency, the scene not having fluid is to be rendered by background texture S (x, y) at first.It is used to take
The texture coordinate of sample background scene texture be based on normal to a surface n multilated be given fluid behind refractor unreal
Feel.β with thickness linear increment,
β=T (x, y) γ (15)
Wherein, γ is to rely on the constant of a certain fluid, determines the degree that background is refracted.The color b quilt of reflection
The cube texture of the environment based on the direction of the launch for the sampling, come determine, is to calculate from surface normal and line of sight
's.
Because realize curvature to minimize using PDE is that comparison is stiff, therefore using a kind of extra synthetic schemes, surely
Qualitative question can propose to lead to system to be shaken.For this reason, in high-resolution, take substantial amounts of iteration at one
Stability is kept on little time step.One compromise can be used, and sacrifices some performance qualities, in a relatively low resolution
Do the step after fluid renders and processes under rate simultaneously.This scaling is difficult, because the representing of edge.In a fluid
In vivo, depth is linear insertion, but edge be come as special circumstances processed.These should not seem sharp or
Person is uneven, if it appear that fuzzy or smooth also more more receptive.For this reason, we be mixed with last
Graduated colors, calculate rather than at normal or depth value at a low resolution.Thus there is the effect of smoothed profile.
It is inserted with the result that altofrequency also just will lost due to sampling.Because flatness, half or a quarter
Resolution fluid can seem good than full resolution.
The method of the present invention is realized comprising using several critical points such as fragment shader and middle post-processing object, according to above-mentioned
Step, for existing test video and be analyzed, this test set have chosen NVIDIA trade mark, in the scene such as pipeline, corridor
Fluid carried out partial simulation.All tests are realized all on PC computer, and the major parameter of this PC computer is：Centre
Reason device Intel (R) Core (TM) 2Quad CPU Q8400@2.66GHz, internal memory 4GB.
Shown in some comparative simulation effects such as Fig. 4 (a), 4 (b), Fig. 5 (a), 5 (b).
Claims (5)
1. a kind of screen space fluid rendering intent based on curvature flow is it is characterised in that comprise the following steps：
S1, obtains the smooth particle fluid model of three-dimensional body in screen space, and model includes position and the speed of particle, generates
The depth map of the particle of flow surface, determines the nearest surface of three-dimensional body, and the nearest surface of three-dimensional body determines that method is concrete
For：If object has two or more intersection points with the same straight line of camera light direction of principal axis, take from the nearest intersection point of video camera,
As the point on nearest surface；
S2, by curvature flow filter, smooths nearest surface；
S3, calculates the amount of fluid between each point and nearest surface, as the thickness of this point；
S4, adds noise for nearest surface；
S5, the thickness according to point and the speed of particle, weaken color a little and transparency, obtain fluid and render figure.
2. a kind of screen space fluid rendering intent based on curvature flow according to claim 1 is it is characterised in that described
Step S1 in, particle renders are become spheroid, calculate on each spheroid from the point that video camera is nearest.
3. a kind of screen space fluid rendering intent based on curvature flow according to claim 1 is it is characterised in that described
Step S2 in, nearest surface is smoothed using average curvature.
4. a kind of screen space fluid rendering intent based on curvature flow according to claim 1 is it is characterised in that described
Step S4 in, noise be Berlin noise.
5. a kind of screen space fluid rendering intent based on curvature flow according to claim 1 is it is characterised in that described
Step S5 in, the thickness of point is bigger it is seen that degree is lower, and the color of point and reduced transparency are more.
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| CN110827391A (en) * | 2019-11-12 | 2020-02-21 | 腾讯科技（深圳）有限公司 | Image rendering method, device and equipment and storage medium |
| CN111986303A (en) * | 2020-09-09 | 2020-11-24 | 网易（杭州）网络有限公司 | Fluid rendering method and device, storage medium and terminal equipment |
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