Entry 340

The frame-mode deflicker: read circuit 1 a line lower, merged at ALP 127

2026-10-01renderengine

Filescrates/piney-gs/src/lib.rscrates/piney-draw/src/lib.rscrates/piney-mpeg/src/movie.rscrates/piney-game/src/main.rsdocs/engine/overview.mdREADME.md

While working out SetDisplayOffset's units (339), a second pair of DISPLAY values turned up at ccSystem +0xe10 / +0xe20. It is the game's deflicker. In frame mode, the mode the game runs in outside the movies, the GS shows the frame buffer through both read circuits, one a line lower, merged half and half.

The game

SetScreenModeMain (INF SLUS_202.67:0x0010ad40), when the field flag (+0xbd6) is 0, after sceGsSetDefDBuff:

0x0010b4e8  both buffers' PMODE (+0xbe0, +0xc08): bit 0 (EN1) set
0x0010b51c  their ALP byte (+0xbe1, +0xc09) = 127
0x0010b524  +0xe10 = buffer 0's DISPLAY (+0xbf8), +0xe20 = buffer 1's (+0xc20)
0x0010b534  each copy's DY + 1

libgraph's PMODE is 0x66 (sceGsSetDefDispEnv 0x0010dea0): EN2, CRTMD 1, MMOD 1 (the merge's alpha is ALP), AMOD 1, SLBG 0 (circuit 2 is the background). Each frame SwapDoubleBuffer (0x0010ac40) writes, in frame mode, the shown buffer's DISPFB (+0xbf0 + 40 n) to DISPFB1 (0x12000070) and its copy (+0xe10 + 16 n) to DISPLAY1 (0x12000080). sceGsSwapDBuff then puts the buffer's own environment, with DISPFB2 and DISPLAY2. SetDisplayOffset keeps the copies a line lower (339). So both circuits read the same buffer, circuit 1 a line lower, and a screen line comes out as ALP / 255 x the buffer's line above + (1 - ALP / 255) x its own, about half each. On an interlaced TV that softens one-line detail that would flicker between the fields. Field mode (the movies' SetScreenMode(640, 448, 2)) skips all of it and sets sceGsSetHalfOffset instead.

PCSX2's "Anti-Blur" option, on by default, is meant to undo blurs of this kind, so PCSX2 players see the sharp picture. That is from the option's description; it was not tried here.

The port

Frame::field_mode (piney-draw) says how a frame is shown. It is false for the game's frames and true for piney_mpeg::movie::draw. Gs::render remembers it. piney_gs::Pcrtc carries the display offset and deflicker to Presenter::present. The shader's merged() mixes the moved picture with itself one row (1/448) up, at 127/255, before the sRGB conversion: the GS merges the stored values. Above the top row the line above is black, as circuit 1 there has nothing to show.

The port's picture is progressive, so there is no flicker to fight, and the present is kept sharp (its sharp bilinear). So the merge is off by default, like PCSX2's anti-blur. --deflicker and the console's deflicker on|off switch it (README "Play").

present_merges_the_line_above_in_frame_mode draws a one-row white line at row 200. Sharp, rows 199-201 read 0, 255, 0. With the deflicker they read 0, 128, 127, 0. A field-mode frame keeps 255, 0.