fix: use safer native video centering steps

Author: wizzomafizzo

menu.sv

@@ -213,8 +213,8 @@ assign LED_POWER[0]= FB ? led[2] : act_cnt2[26] ? act_cnt2[25:18] > act_cnt2[7:0
 localparam CONF_STR = {
 	"MENU;UART31250,MIDI;",
 	"-;",
-	"O[15:10],H Offset,0,+1,+2,+3,+4,+5,+6,+7,+8,+9,+10,+11,+12,+13,+14,+15,+16,+17,+18,+19,+20,+21,+22,+23,+24,+25,+26,+27,+28,+29,+30,+31,-32,-31,-30,-29,-28,-27,-26,-25,-24,-23,-22,-21,-20,-19,-18,-17,-16,-15,-14,-13,-12,-11,-10,-9,-8,-7,-6,-5,-4,-3,-2,-1;",
-	"O[19:16],V Offset,0,+1,+2,+3,+4,+5,+6,+7,-8,-7,-6,-5,-4,-3,-2,-1;",
+	"O[13:10],H Offset,0,+2,+4,+6,+8,+10,+12,+14,-16,-14,-12,-10,-8,-6,-4,-2;",
+	"O[17:14],V Offset,0,+1,+2,+3,+4,+5,+6,+7,-8,-7,-6,-5,-4,-3,-2,-1;",
 	"-;",
 	"V,v",`BUILD_DATE
 };
@@ -521,10 +521,10 @@ native_video_top native_video
 	.enable         (mode_zaparoo),
 	.active         (native_active),
 
-	// Status bit patterns match signed two's complement because CONF_STR orders
-	// each OSD enum as 0,+1..max,min..-1. $signed() makes that explicit here.
-	.h_offset       ($signed(status[15:10])),
-	.v_offset       ($signed(status[19:16]))
+	// H offset is a 4-bit signed OSD value doubled into 2-pixel steps.
+	// V offset is a normal 4-bit signed OSD value in 1-line steps.
+	.h_offset       ($signed({status[13:10], 1'b0})),
+	.v_offset       ($signed(status[17:14]))
 );
 
 // Cosine + LFSR fallback noise pattern, painted into the 320x240 active area

rtl/native_video_timing.sv

@@ -9,7 +9,7 @@ module native_video_timing
 	input  wire       reset,
 
 	// Image centering: positive = shift right/down (FP shrinks, BP grows).
-	input  wire signed [5:0] h_offset,  // -32..+31 pixels (budget H_FP=38 / H_BP=39)
+	input  wire signed [4:0] h_offset,  // -16..+14 pixels (budget H_FP=26 / H_BP=51)
 	input  wire signed [3:0] v_offset,  // -8..+7 lines   (budget V_FP=8 / V_BP=11)
 
 	output reg        hsync,
@@ -24,11 +24,11 @@ module native_video_timing
 );
 
 localparam [9:0] H_ACTIVE = 10'd320;
-// 38/32/39 keeps total blanking fixed while moving the default image 24 px
+// 26/32/51 keeps total blanking fixed while moving the default image 12 px
 // left from the earlier CRT-specific 14/32/63 porch split.
-localparam [9:0] H_FP     = 10'd38;
+localparam [9:0] H_FP     = 10'd26;
 localparam [5:0] H_SYNC   = 6'd32;
-localparam [9:0] H_BP     = 10'd39;
+localparam [9:0] H_BP     = 10'd51;
 localparam [9:0] H_TOTAL  = 10'd429;
 
 // V blanking rebalanced from 6/3/13 to 8/3/11 to give symmetric ±8 budget
@@ -41,7 +41,7 @@ localparam [8:0] V_TOTAL  = 9'd262;
 
 // Sync starts shift with the offset; two's-complement subtraction in
 // unsigned arithmetic yields the correct result at both ends of the range.
-wire [9:0] H_SYNC_START = H_ACTIVE + (H_FP - {{4{h_offset[5]}}, h_offset});
+wire [9:0] H_SYNC_START = H_ACTIVE + (H_FP - {{5{h_offset[4]}}, h_offset});
 wire [9:0] H_SYNC_END   = H_SYNC_START + H_SYNC;
 wire [8:0] V_SYNC_START = V_ACTIVE + (V_FP - {{5{v_offset[3]}}, v_offset});
 wire [8:0] V_SYNC_END   = V_SYNC_START + V_SYNC;

rtl/native_video_top.sv

@@ -32,7 +32,7 @@ module native_video_top
 	output wire        active,
 
 	// OSD image centering: 0 = default porch split.
-	input  wire signed [5:0] h_offset,
+	input  wire signed [4:0] h_offset,
 	input  wire signed [3:0] v_offset
 );