Difference between revisions of "Materials (Metroid Prime)"

Revision as of 10:08, 13 March 2015

The format for materials is seen in both the CMDL and MREA formats and is identical in both. This particular material format appears in both Metroid Prime and Metroid Prime 2 with minor differences.

This file format is almost completely documented There are a couple unknown values left. Also, basically everything that was introduced in Echoes (two general settings and a vertex attribute) is unknown.

GX Overview

The materials system in Metroid Prime is heavily dependent on the GameCube and Wii's graphics system, GX. GX is a fixed-function graphics pipeline, similar to old versions of OpenGL. In GX, rendering is done through a series of steps called Texture EnVironment Stages, or or TEV stages for short.

Prior to TEV stages, per-vertex calculations are performed. Lighting calculations are performed and passed to GX as a rasterized vertex color; texture coordinate generation (texgen) is also performed and made available to TEV.

How TEV stages actually work: Each TEV stage takes in four color (RGB) inputs and four alpha inputs, from one of eight sources. Then these four input colors are combined into one output color, and the output is saved into one of four registers, which can subsequently be used as input in the next TEV stages. The final stage must always save its output into the "previous TEV stage" register; that register is used as the final pixel color that gets displayed onscreen.

Material Set Format

Materials come as part of a set; often there will only be one set per file, but many CMDLs can have more than one. The set begins with a short header before the actual material data begins.

Material Format

General Settings

Each material begins with a flags value, followed by a list of texture indices:

Flags

These are the known flag settings:

Vertex Attribute Flags

These flags are generally toggled in pairs, with each pair corresponding to a vertex attribute; if a pair is set, then vertices using this material will have the corresponding attribute. This is vital for reading geometry.

An important note is that while the leftmost byte is not used in Prime 1, in Echoes there's occasionally a value there that toggles an extra vertex attribute preceding position. This needs research to determine what the new attribute actually is. Also, GX supports up to 8 texture coords per vertex, but the game doesn't seem to allow you to assign more than 7 (though this could do with some double-checking).

These are the possible attributes:

Konst Colors

These values are only present when flag 0x8 is enabled. These allow you to set Konstant values, which can subsequently be used as inputs in TEV stages. The maximum number of Konst values you can set on one material is 4. The colors themselves are simply 32-bit RGBA values.

Blend Mode

The two blend factors set the blending mode used. The most common values you'll see are 0/1, which is used on opaque materials; transparent materials will usually have either 1/1, for additive blending, or 5/4, for alpha blending. Here's the full range of possible settings for each value:

Color Channels

The color channels data consists of a count value indicating the number of color channels, and then one set of flags that controls the COLOR0 channel. The flags correspond to arguments passed to GX_SetChanCtrl(). These are the settings:

There's only one set of flags, which operates on COLOR0. COLOR1 is reserved for dynamic shadowmaps, and its settings can't be modified. Here's how the other color channel settings work.

If shadowmaps are enabled:

• No processing will be done if the count value is 0. Otherwise, the count value is ignored; the game will always set up both color channels.
• COLOR1A1's ambient color is black, and its material color is white.
• COLOR1 will use the diffuse function GX_DF_CLAMP and the attenuation function GX_AF_SPOT. It will have lighting disabled entirely if there are no lights in range.
• COLOR0 will use the flags from the material, with the light mask merged in. It will also have lighting disabled if there are no lights in range.
• If there are no lights in range, COLOR0A0 will have its material color set to white. Otherwise, its material color will be set to match its ambient color.

If shadowmaps are disabled:

• COLOR1 is disabled if the count value is not 2. If it is 2, then its ambient color will be black and its material color will be white.
• COLOR0 is disabled if the count value is 0. Otherwise, it's set up the same way as with shadowmaps.

Finally, an important note is that the formatting of the color channel info is bugged. There is always a color channel count and a single set of flags; 8 bytes total. However, for some reason the game is expecting one set of flags per color channel. To reach the TEV stages, you need to advance (4 * count) bytes past the count value, rather than 4. A likely explanation is that there was one set of flags per channel earlier in development, and the material loading code wasn't fully updated when this was changed. It's not normally an issue because the count is always 1 in every file the game uses, but beware for experimentation/custom materials.

TEV Stages

There'll be one of these structures per TEV stage:

After looping through each TEV stage, there will be one of these structures per stage:

Color Input Flags

These flags set the four color inputs that are used by the TEV stage. Each color is allocated 5 bits, even though only 4 are actually used; the lower bits correspond to the lower input numbers (eg. the bottom 5 bits refer to the first input).

There are 16 possible color sources:

Alpha Input Flags

Similar to the color input flags, these set the four alpha inputs used by the TEV stage. Each value is allocated 5 bits, although only 3 are used. The main difference with alpha is that there are only 8 possible sources instead of 16:

Color Combine Flags

These flags specify how the operation that combines the four input colors into one output color is performed.

This is the combiner function:

tevrigid = (d (tevop) ((1.0-c)*a + c*b) + tevbias) * tevscale;

The values set in the color operation flags correspond to the parameters passed to GX_SetTevColorOp(). These are the settings:

Note that the vast majority of materials in the game, if not all of them, don't really bother with any of this; they enable clamping, set an output register, and leave everything else at their defaults of 0.

Alpha Combine Flags

This is exactly the same as the color combine flags; the only difference is it operates on alpha instead of color.

Texgen

After the TEV stages comes a sequence of flags determining how texgen is executed. Texgen is the process of taking input values and using them to generate texture coordinates (or UV coordinates), which can then be used by the TEV stages. It's important to note that any vertex attribute can be used as a texgen input; in fact, it's rather common for materials to use the vertex normal as an input to simulate reflections. That means the number of texture coords present on each vertex is not the same as the number of texture coords available to TEV.

The flags correspond to arguments passed to GX_SetTexCoordGen2(). These are the settings:

UV Animations

The UV animations section immediately follows the texgen flags. Its purpose is to generate texture matrices and post-transform matrices, which are then used to transform UV coordinates. This is commonly used to animate textures via simple UV scrolls, but it's also often used to simulate reflective surfaces that move with the camera. Materials can have multiple UV animations; in that case, each animation generates a separate texture/post-transform matrix and are loaded into GX sequentially. Texgen is used to set which matrices are used by which UV coordinates (if any).

The section starts with this short header:

The structure of the animations themselves is somewhat simple. Each animation has a 32-bit mode setting, followed by a number of float parameters. The number and usage of these float parameters varies depending on what mode is set. There are 8 possible modes.

For all of the following modes, s refers to seconds mod 900.

Mode 0: Inverse ModelView Matrix (No Translation)

This mode is commonly used with vertex normals to simulate reflective surfaces using spheremaps that move with the camera. It takes no parameters, and will generate both a texture matrix and a post-transform matrix. Translation is ignored.

The texture matrix is calculated like this. Note that the multiplication of the view matrix by the model matrix should be modified slightly rather than being a straight multiplication to ignore translation on the model matrix. (The game uses a function called MultiplyIgnoreTranslation for this.)

texmtx = inverse(ViewMatrix) * ModelMatrix;
texmtx[0][3] = texmtx[1][3] = texmtx[2][3] = 0;

The post-transform matrix is a constant value. [1][1] and [1][2] may need to be swapped around for correct playback.

0.5, 0.0, 0.0, 0.5,
0.0, 0.0, 0.5, 0.5,
0.0, 0.0, 0.0, 1.0

Mode 1: Inverse ModelView Matrix

This mode is nearly identical to mode 0; the only difference is that translation is left as-is. The multiplication of the view matrix by the model matrix actually is a straight multiplication in this mode, and the translation values on the texture matrix aren't set to 0.

Mode 2: UV Scroll

This mode is used to scroll both U and V at the same time. It will only generate a texture matrix. It has four float parameters: offsetA, offsetB, scaleA, and scaleB.

uOffset = (s * scaleA) + offsetA;
vOffset = (s * scaleB) + offsetB;

Mode 3: Rotation

This mode rotates the texture. It will only generate a texture matrix. It has two float parameters: offset and scale.

float angle = (s * scale) + offset;
float acos = cos(angle);
float asin = sin(angle);
float translateX = (1.0 - (acos - asin)) * 0.5;
float translateY = (1.0 - (asin + acos)) * 0.5;

The resulting texture matrix is laid out as:

acos, -asin, 0.0, translateX,
asin,  acos, 0.0, translateY,
 0.0,   0.0, 1.0,        0.0

Mode 4/5: Horizontal/Vertical Filmstrip

These modes can be used to create a filmstrip-like effect. The texture steps a set distance each animation frame, rather than the scroll being smoothly interpolated every game frame. The same calculation is done for both modes, with the only difference being whether the calculated offset is applied to the U or V coordinate. It will only generate a texture matrix. There are four float parameters: scale, numFrames, step, and offset.

The animation is made up of a number of pseudo-frames, where step controls the amount that the texture scrolls by each frame, and numFrames sets how many frames are iterated through before resetting back to 0. scale roughly controls animation playback speed, and offset modifies the time input value.

float value = step * scale * (offset + s);
float uv_offset = (float)(short)(float)(numFrames * fmod(value, 1.0f)) * step;

Mode 6: Model Matrix

This mode is similar to modes 0 and 1 in that it simulates reflective surfaces, but it only takes the model matrix into account, which means camera movement doesn't affect the reflection. It takes no parameters and it generates both a texture matrix and a post-transform matrix.

The texture matrix is simply the model matrix, with the translation values set to 0. The post-transform matrix is set to the following:

0.5, 0.0, 0.0, ModelMatrix[0][3] * 0.50000001,
0.0, 0.0, 0.5, ModelMatrix[1][3] * 0.50000001,
0.0, 0.0, 0.0, 1.0

Mode 7: Mode-Who-Must-Not-Be-Named

Mode 7 takes two parameters and generates both a texture matrix and a post-transform matrix. We don't know what to name it.

The texture matrix is generated the same way as in mode 0; translation is ignored.

texmtx = inverse(ViewMatrix) * ModelMatrix;
texmtx[0][3] = texmtx[1][3] = texmtx[2][3] = 0;

The post-transform matrix is where it gets a little complicated. A little math is required to calculate some values:

float xy = ((ViewMatrix[0][3] + ViewMatrix[1][3]) * 0.025f * ParamB;
xy = (xy - (int) xy); // This truncates the integer portion of the value, leaving only the fractional part (mantissa).

float z = ViewMatrix[2][3] * 0.05f * ParamB;
z = (z - (int) z);

float halfA = ParamA * 0.5f;

The post-transform matrix is then constructed as:

halfA, 0.0f,  0.0f,  xy,
 0.0f, 0.0f, halfA,   z,
 0.0f, 0.0f,  0.0f, 1.0f