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Last post: 15 days ago
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|GBAtek is an amazing piece of documentation, but it can be improved upon
This is a general pile of findings. I claim no ownership on those, they come from several individuals.
* transfer time is 8 cycles for a command, 4 cycles per response word (basically 1 cycle per byte) (see ROMCTRL bit27 for cycle duration)
* plus start delay and 0x200-block delay at the start of each 0x200 block
* bit28 allows skipping incoming bytes automatically during delays
* DRQ bit (bit23) is set once a response word has been transferred
* reading from 0x04100010 clears the DRQ bit, and:
** if the transfer is finished: clears the busy bit and triggers IRQ if specified
** if there are more words to transfer: begins transferring the next word
* The main memory display FIFO is a simple circular buffer that holds 16 pixels. The video controller reads from it regardless of whether you fill it. It doesn't get 'empty' or 'full'.
* Writing to the upper halfword of 0x04000068 increments the FIFO write pointer by two (writes to the lower halfword leave it unchanged). The write pointer simply wraps to 0 when reaching the end of the buffer. It is also reset upon VBlank.
* 8-bit writes to 0x04000068 don't work well. TODO: figure out what's happening. eventually.
* Colors are converted early from 5-bit to 6-bit, as such: 6bit = 5bit*2
* Color special effects (brightness, blending) are applied to the 6-bit color components
* In some cases, the MSb of color values is used as LSb for the green component. TODO: find out where this applies. Confirmed to apply to the standard BG palette.
* Bitmap sprite blending follows the same rules as non-bitmap semitransparent sprites, with EVA=alpha+1 and EVB=16-EVA. Except: bitmap sprites with alpha=0 are always hidden.
* 3D layer blending follows rules similar to those of semitransparent sprites (only requires second target bits set in BLDCNT, overrides BLDCNT color effect selection where it applies).
* 3D layer blending uses 5-bit alpha values (from the 3D graphics), such as: EVA=alpha+1 and EVB=32-EVA.
* When the 3D alpha is less than 16, the final color components are incremented by one. (seems to be some hardware glitch??)
* 3D layer pixels with alpha=0 are always hidden (not rendered). They're preserved when capturing the 3D output alone, though.
* Shadow polygons can use textures. In that case, decal blending is applied.
* The stencil buffer can hold two scanlines. It's cleared only when the current scanline contains shadow mask polygons, before rendering a group of shadow mask polygons.
* Stencil buffer bits are set only where the shadow mask is drawn but fails the depth test.
* Visible shadow polygons (polyID>0) are only drawn where stencil buffer bits are set and where the destination pixel's polygonID is different from that of the shadow, regardless of whether that pixel was translucent.
* Toon highlight mode uses the following formula: (GBAtek is wrong)
v=vertex t=texture s=tooncolor=toontable[Rv]
R = ((Rt+1)*(Rv+1)-1)/64+Rs ;truncated to MAX=63
G = ((Gt+1)*(Rv+1)-1)/64+Gs ;truncated to MAX=63
B = ((Bt+1)*(Rv+1)-1)/64+Bs ;truncated to MAX=63
A = ((At+1)*(Av+1)-1)/64
* Translucent pixels are only drawn where the destination pixel has a different polygonID OR where the destination pixel was opaque.
* for each separate polygon, W values are 'normalized', they're collectively shifted left or right by 4 until they all fit within 16 bits (if they fit within 12 bits or less, they can be shifted left to use the 16-bit range better)
* conversion for Z values:
** Z-buffering: zbuf = (((Z * 0x4000) / W) + 0x3FFF) * 0x200 (using original W)
** W-buffering: zbuf = W (but it appears to use normalized W)
* conversion for clear depth:
** clearZ = (val * 0x200) + 0x1FF
* There are special depth-test rules for polygon borders. TODO: work it out.
** it seems to only apply to wireframe polygons
** when Z values are equal, left edges have priority over right edges, and top edges have priority over bottom edges (TODO: check wireframe vs normal)
** 'less or equal' depth test has no margin
** (dunno about other orders but they should use the regular rules. Y-sorting gets in the way)
* Cases where 'less than' depth test becomes 'less or equal':
** wireframe polygon borders as mentioned above
** apparently, polygon borders in some other cases too
** when rendering frontfacing polygon pixels over existing opaque backfacing polygon pixels
* 'equal' depth test mode has a margin of 0xFF in either direction. That is, for example, incoming Z range of 0x100-0x2FE is considered equal to an existing Z-buffer value of 0x1FF.
* PUSH/POP/STORE/RESTORE to the modelview matrix always apply to the vector matrix too, even in matrix mode 1.
* "NORMAL/VEC_TEST require matrix mode 2" <- wrong. They work the same regardless of the matrix mode.
* edge marking
Posted by GBAtek
-> polygon ID rule for screen edges confirmed
-> at screen edges, the aforementioned depth test uses CLEAR_DEPTH (when testing against a pixel that would be offscreen), even when using a clear bitmap
** seems to be calculated from edge slopes
** topmost two pixels are retained, antialiasing blends them together including alpha (except color isn't blended when the pixel below has alpha=0)
** during rendering, if an incoming pixel fails the depth test with the topmost pixel, it is checked against the pixel below
* ARM9 DMA start mode 3 is similar to the GBA's 'video capture' DMA, although GBAtek doesn't make it obvious. It is triggered at the start of each scanline from 2 to 193. The enable bit is automatically cleared on scanline 194.
* TODO: find when 'main memory display' DMA starts. Probably 8 pixels (48 cycles) in advance from the actual display. -- DMA starts ~32 cycles after the start of the scanline. Actual display starts ~48 cycles after the start of the scanline.
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