Transparency Joshua Barczak CMSC 435 UMBC With help

Transparency Joshua Barczak* CMSC 435 UMBC *With help from others…

Stuff That’s Transparent Fake Volumetric Effect Fur Curtains Lacy Things Civilization V (Firaxis)

Light Shafts • Typical approach: – Transparent proxy geometry – Additively blended into scene – Scroll speckle texture over it to mimic floating dust

Stuff That’s Transparent Smoke/Fire Eyebrows Facade Beard Facade Civilization V (Firaxis)

Particles • Fire/Smoke/etc – Animated point masses – Transparent, screenaligned quads “Smoke” “Fire”

Stuff that’s Transparent Fake Volumetric Effect Tips of the hair This Stuff Smoke Fire Civilization V (Firaxis)

Stuff That’s Transparent Eyebrows Fake Volumetric Effect Facades Curtains Civilization V (Firaxis)… Don’t fire me guys…

Stuff That’s Transparent Eyebrows Facades Grass Civilization V (Firaxis)…This is fair use, right?

Foliage Behrendt et al. 2005

Foliage Overgrowth (Wolfire Games)

In a raytracer • Just shoot more rays… • Given opacity ‘a’ – Shade, weight color by ‘a’ – Fire additional ray in same direction, weight its color by 1 -a

In a rasterizer… • Draw back to front • At each fragment, given opacity a: – Weight its color by a – Weight background by (1 -a) – “Over” operator • Requires sorting

Order Dependence Right Wrong Everett 2001

Triangle Sorting • This is hard… Minimum Z won’t work… Maximum Z won’t work… Average Z won’t work…

Triangle Sorting • You can’t just use a comparison sort – “In. Front” is nontransitive… • In. Front(a, b) && In. Front(b, c) – BUT !In. Front(a, c) • QSort will break… c a b

Triangle Sorting (if you must) • Directed graph of overlap relationships – Topological sort gives drawing order

Triangle Sorting • Deal with cycles somehow: – Split geometry – Or: Give up and just break them

Triangle Sorting • Order doesn’t matter if triangle AABBs don’t overlap – Amax < Bmin || – Amin > Bmax – On each axis B A

Triangle Sorting • Overlap testing – If bounding boxes overlap, then… – S’ is “in front” S if: • All vertices of S are “outside” S’ – OR • All vertices of S’ are “inside” S

Triangle Sorting • Alternative: 2 D Test • Triangles are overlapping if and only if: – Vertex covers face • Interpolate Z in plane of “covered” face – OR – Edges intersect • Compute edge Zs at intersection point

Triangle Sorting • Spatial Partitioning – Avoid N^2 overlap tests – Only test pairs that share a cell – Collision detection…

Easy cases: Particles • Sorting Particles – Sort quads by cameraspace Z

Easy cases: Restricted View • Always above treeline: – Presort leaves bottom to top – Sort trees furthest to closest

Easy cases: Restricted View • Camera doesn’t rotate – Presort from top of screen to bottom during model placement Civilization V (Firaxis)

Easy cases: Convex • Draw backfaces, then frontfaces – And you might get away with it for nonconvex

Easy cases: Non-overlapping • Object-level sort will work if there is no AABB overlap – Sort meshes by center depth

Example: Car Windows • Draw opaque parts front to back – Z prime • Draw windows back to front – For each car, backfaces, then frontfaces

Order-Independent Transparency • Methods that let us draw in any order… • Sort at pixel level – Depth peeling – A buffers • Fake it – Alpha test – Stippling This is still very much an open problem…

Depth Peeling • Order-Independent Transparency – The hard way • One pass per depth layer – Each pass “peels” off frontmost layer Everett 2001

Depth Peeling // Clear rendertarget for each pixel p { color[p] = 0; alpha[p] = 1; } For i = 0 to depth_complexity { clear zbuffer; // // Rasterize again. . . zf is fragment z cf is fragment color af is fragment alpha for each fragment f in pixel p { // kill fragments from earlier layers if( i > 0 && zf <= prev_z[p] ) discard f; // this part is just ordinary z buffering if( zf < zbuffer[p] ) { zbuffer[p] = zf; layer_color[p] = cf; layer_alpha[p] = af; } } // composite this layer into the image for pixel p { prev_z[p] = zbuffer[p]; color[p] += alpha[p]*layer_color[p]*layer_alpha[p]; alpha[p] *= (1 -layer_alpha[p]); } } • Pro: – No sorting – Fixed memory overhead – Can choose to stop after first few layers • Con: – Outrageously expensive – Scales badly with depth complexity

A-Buffer • Linked list of fragments for every pixel – Color – Alpha – Depth • Store all fragments, sort/composite when done

A-Buffer • Pro: – Simple – Order-Independent • Con: – Unbounded memory! • How much depends on depth complexity – Linked list We are not pleased!

DX 11 OIT • A-Buffer insertion from pixel shader – Per-pixel head indices – Giant array of linked list nodes • See: – Nicolas Thibieroz Holger Gruen. “OIT and indirect illumination using dx 11 linked lists. ” Figure: Barta and Kovacs 2011

Alpha Testing • Kill pixel if alpha below threshold – On/Off transparency • Pro: – Order-independent – Cheap • Con: – Bilinear artifacts – Aliasing • Which MSAA won’t fix… Civilization V (Firaxis)

Distance Fields • A better alpha test… • Create “distance field” from mask image – Search for nearby texels • Threshold based on edge distance – Distance is linearly filterable Chris Greene, 2007

Screen Door Transparency • Stippling – Drop every other pixel • May work better if blurred away afterwards • Pro: – Simple – Cheap • Con: – Only one grey level

Alpha to Coverage • Subpixel screendoor A=1 – Turn off AA samples based on alpha • N opacity levels for N AA samples A=0. 75 – Order-independent – As cheap as alpha test and better looking – Works wonders for foliage A=0. 5 A=0. 25 Fully covered pixel @ 8 x. AA

Stochastic Transparency • Kill pixel with probability proportional to alpha – ZBuffer surviving pixels – Repeat numerous times and average results Enderton et. Al, 2010

Stochastic Transparency Enderton et. Al, 2010 Basic Stochastic See paper for noise reduction tricks… Depth Peeling

$Refraction Because transparency just isn’t hard enough…$

Refraction Because transparency just isn’t hard enough…

$Refraction • Transmitted light is ‘bent’ Index of refraction (c/v) Speed of light in$

Refraction • Transmitted light is ‘bent’ Index of refraction (c/v) Speed of light in each medium Wikipedia

$Raytraced Refraction • Calculate refracted ray direction Sunshine Lolly Pops – Next time… •$

Raytraced Refraction • Calculate refracted ray direction Sunshine Lolly Pops – Next time… • Trace a new ray • Scale its color by 1 -a – For best results, use fresnel Everything is easy with raytracing…

$Rasterized Refraction Newell 2004$

Rasterized Refraction Newell 2004

$Refraction Mapping • Render background to a texture • Rasterize refractive object • Compute$

Refraction Mapping • Render background to a texture • Rasterize refractive object • Compute refracted ray… – Sample texture at projected hit location …which we do not know… Hmm…

$Refraction Mapping • …pick an arbitrary distance along the ray, and pretend that’s it…$