Particle appearances in the executable

An FXR effect file states what a particle looks like as properties of typed actions; the executable turns them into drawn quads. This page records what the code of Sekiro 1.06 does for one appearance, the billboard, as read from its decompiled functions. The result is the state the appearance keeps per particle; the vertex data of a draw and the constants of the effect programs are not read.

The classes

The executable’s run-time type information names these classes for effects (namespace GXFFX): the appearances GXFfxAppearanceBillboardEx, GXFfxAppearanceMultiTextureBillboardEx, GXFfxAppearanceTracer, GXFfxAppearanceModel, GXFfxLensFlareAppearance, GXVfxStandardParticleAppearance, GXVfxSparkParticleAppearance, GXVfxParticleAppearanceLightShaft, the area appearances of the force fields (GXForceField*AreaAppearance, GXCollisionFieldAreaAppearance) and of water interaction, and the draw classes GXFfxDrawPass, GXFfxDrawContext, GXFfxManager, GXFfxSceneCtrl and GXFfxGraphicsResourceManager. Observed[the type descriptors of the image dumped from the running game]{games=“sekiro”} The names of the Vfx appearances agree with the GPU particle programs of the effect shader bundle. Inferred

The billboard appearance has a table of 12 virtual methods. Observed · Sekiro Its fourth entry (index 3) builds the state of the particles made in a call and its fifth (index 4) advances it by a time step. Observed · the functions’ stores and loads The sixth (index 5) widens a box by the extent a particle can reach, from the offsets, the width and the height. Observed

The state of a billboard particle

A billboard particle has a record of 24 bytes: two floats (a width multiplier and a height multiplier), three floats (the rotation about x, y and z, in radians) and a 32-bit colour. Observed · Sekiro The record is made when the particle is made:

Bytes	Value
0, 4	the two scale multipliers, each a property of another property block of the appearance times a random factor between 1 and the scaleVariationX or scaleVariationY field; with uniformScale set the second uses the first’s factor, and with a further flag set the height’s property is the width’s
8, 12, 16	rotationX, rotationY and rotationZ in degrees times pi / 180; the x rotation is negated
20	the colour: the product of a vec4 property of the other block and color2, each channel clamped to 0 to 1, times 255, rounded, packed as red, green, blue, alpha from the lowest byte to the highest

Observed[the method’s stores, the constants read from the image: 0.0174533 for the degree factor, 255 for the packing, 1.0 for the lower bound of the scale factor]{games=“sekiro”} The property indices are those of the action’s first list in the order the typed struct gives (rotations 11 to 13, colours from 7, angular speeds 14 to 19), and the scale and colour properties of the other block are its indices 1, 2 and 4. Observed The other block is probably the particle modifier’s, whose scale and colour multiply the appearance’s own. Inferred

Each step adds, to the three rotations, the angular speed of the axis times its multiplier times pi / 180 times the particle’s time step (angularSpeedX and angularSpeedMultiplierX and so on, indices 14 to 19); the x speed is negated like the x rotation. Observed · the update method A particle’s colour above 1 is therefore lost in this record: light brighter than the stored colour comes from somewhere else. Inferred

The random numbers of the particle are two draws of a shared xorshift generator with a four-word state (shifts 11, 8 and 19), each turned into a float as the bits of 1.0 plus the draw modulo 0x800001, minus 1: a number from 0 to 1 inclusive. Observed · Sekiro · the generator’s update steps and the mantissa construction in the method

What reaches the GPU

The vertex that the running engine submits for a billboard particle is recorded in the effect draws: one vertex of 112 bytes for each particle of the plain textured draw, whose colour at byte 12 is the packed colour of the record above, with the sprite’s texture coordinates and a 3 by 4 matrix holding its axes (scaled by the particle’s size) and its position. Observed · Sekiro The record of the particle is therefore turned into the vertex by the draw methods, which keep the colour as it is and fold size, rotation and position into the matrix rows. Inferred

Which expressions fill the second colour, the texture coordinates and the constant buffers?

The eighth table entry (about 4,000 lines of vector arithmetic) fills the vertex array of a draw: it writes one 112-byte vertex for each particle in the order the capture shows: the position words are 0, the dword at byte 12 is the packed colour (the particle record’s colour times a per-channel factor, clamped and repacked), the four floats at byte 16 are a vec4 property of the appearance, evaluated through its list of modifiers, times four stored floats of the appearance’s draw configuration, byte 56 holds another evaluated property divided by a constant and byte 60 the mean of the particle’s two scale factors times a constant, bytes 32 to 47 are built from integer cell coordinates (a floor) scaled by a step, which reads as a flipbook cell, and the last 48 bytes are the three matrix rows. Observed · Sekiro The draw configuration the method reads (a structure of about 0x340 bytes, copied by a separate function into a larger draw-parameter structure) holds the integers and floats that go to the effect programs, among them values at configuration bytes 0x4c, 0x74, 0x90 and 0x224, and a second function builds, from a configuration object of about 0x270 bytes, a parameter structure of about 0x3b0 bytes whose float and integer fields are copies and products of the configuration’s (for example its byte 0x170 times a factor is the field at structure byte 0x1e8, the offset where the pixel programs’ common buffer keeps the soft-sprite scale), but whose fields do not follow the order of the common buffers’ variables, so it is not their image. Observed[the two functions’ loads and stores, compared with the buffers’ variable offsets] Which field of the effect file’s actions each configuration byte comes from, and which draw-parameter field becomes which variable of the pixel and domain programs’ common buffers, is not read by code: the function that writes the common buffers was not identified. Unknown · Sekiro

What the captured draws say about the fields is on the effect shader programs: a draw’s textures tie it to the files that could have drawn it, and over a Sekiro frame and three Elden Ring frames the 160-byte (Sekiro) and 148-byte (Elden Ring) multi-texture vertex holds the layers’ colours (COLOR1 to COLOR3) as bytes inside the ranges of the file’s layer1Color to layer3Color, the second colour (COLOR4 there, COLOR1 at byte 16 in the 112-byte vertex) as floats inside the range of the file’s color3 in every draw read (61 of 61 and 59 of 59), while color2‘s range holds them in 34 and 40, so the second colour is the third colour property of the appearance and not the second. Observed · Sekiro, Elden Ring The texture coordinates are the flipbook cell’s corner and size (from frameIndex, columns), the next cell’s corner, and per-layer offset and scale vectors that the domain program turns into the layers’ coordinates. Observed · Sekiro, Elden Ring For the constant buffers the integer words, premultiplyAlpha, blendMode, the lighting flag and the two multipliers reach the pixel constants as that page’s table shows. The expression in the executable that computes each is still not read. Unknown · Sekiro

The appearances are made by factories that a single draw-system object registers in its constructor (one caller, an object of which the running engine keeps one); the constructor also creates four GPU buffers of 0x2b0, 0x2c0, 0x3c0 and 0x50 bytes, whose sizes are the candidates for the common buffers of the effect programs. Inferred · Sekiro · the constructor’s decompiled stores A billboard appearance holds its node at byte 8 and is made only for a node whose data holds the action type 0x25b at byte 0x240. Inferred The list of live effects, the class of an effect instance and the link from an appearance up to the effect it belongs to are not read. Unknown · Sekiro