Rasterbars
Published
This FPGA demo effect renders four animated rasterbars. I created this effect with benjamin.computer for All You Need, a Chapterhouse prod released at Revision 2022.
For an introduction to rasterbars and other simple graphics effects, check out Racing the Beam.
The design was originally for a custom Artix-7 dev board, but this version runs on the Digilent Arty A7 or as a Verilator/SDL simulation on your computer.
Building the Demo
Find the Verilog source and build instructions in the projf-explore git repo:
https://github.com/projf/projf-explore/tree/main/demos/rasterbars/
New to FPGA graphics design? Start with Beginning FPGA Graphics.
Demo Structure
- Top module with display interfaces
- Arty (XC7): xc7/top_rasterbars.sv
- Verilator/SDL: sim/top_rasterbars.sv
- Rasterbars render module: render_rasterbars.sv (see below)
- Rasterbar module: rasterbar.sv (see below)
- Project F library modules
I’ve written a separate post on the workings of the FPGA Sine Lookup Table.
Rasterbar Rendering
Explanation to follow.
Simple Rasterbar
module rasterbar (
input wire logic clk, // clock
input wire logic start, // start control
input wire logic line, // signal new screen line
input wire logic [11:0] base_colr, // base bar colour
input wire logic [3:0] colr_steps, // bar colour steps
input wire logic [3:0] colr_lines, // lines of each colour
output logic [11:0] bar_colr, // bar draw colour
output logic drawing, // bar is drawing
output logic done // bar drawing is complete
);
// NB. We don't (yet) register colr_steps and colr_lines
logic bar_inc; // increasing (or decreasing) brightness
logic [3:0] cnt_step; // count colour steps in each bar
logic [3:0] cnt_line; // count lines of each colour
// generate bar colours
always_ff @(posedge clk) begin
if (start) begin // reset colour at start of bar
bar_colr <= base_colr;
bar_inc <= 1; // start by increasing brightness
cnt_step <= 0;
cnt_line <= 0;
drawing <= 1;
done <= 0;
end else if (line) begin // on each screen line
if (cnt_line == colr_lines-1) begin // colour complete
cnt_line <= 0;
if (cnt_step == colr_steps-1) begin // switch increase/decrease
if (bar_inc == 1) begin
bar_inc <= 0;
cnt_step <= 0;
end else begin
drawing <= 0;
done <= 1;
bar_colr <= 12'h000;
end
end else begin
bar_colr <= (bar_inc) ? bar_colr + 12'h111 : bar_colr - 12'h111;
cnt_step <= cnt_step + 1;
end
end else cnt_line <= cnt_line + 1;
end
end
endmodule
Render Multiple Rasterbars
module render_rasterbars #(
parameter CORDW=16, // signed coordinate width
parameter VCENTER=180, // vertical centre of raster render
parameter COLR_LINES=3, // lines of each colour
parameter SIN_FILE="", // sine table ROM .mem file
parameter SIN_SHIFT=0 // right-shift sine values
) (
input wire logic clk, // clock
input wire logic start, // start control
input wire logic line, // line signal
input wire logic signed [CORDW-1:0] sy, // vertical position
output logic [11:0] bar_colr, // colour to draw
output logic bar_up // bar is drawing up
);
// sine table
localparam SIN_DEPTH=64; // entires in sine ROM 0°-90°
localparam SIN_WIDTH=8; // width of sine ROM data
localparam SIN_ADDRW=$clog2(4*SIN_DEPTH); // full table -180° to +180°
logic [SIN_ADDRW-1:0] sin_id, sin_offs;
logic signed [CORDW-1:0] sin_data; // sign extend data to match coords
sine_table #(
.ROM_DEPTH(SIN_DEPTH),
.ROM_WIDTH(SIN_WIDTH),
.ROM_FILE(SIN_FILE)
) sine_table_inst (
.id(sin_id + sin_offs),
.data(sin_data)
);
// raster A
localparam BASE_COLR_A = 12'h126; // bar start colour (blue)
localparam STEPS_A = 10; // colours steps in each bar
logic [11:0] bar_colr_a;
logic bar_drawing_a;
logic signed [CORDW-1:0] bar_y_a; // screen line to draw at
logic signed [CORDW-1:0] bar_y_a_prev; // previous line for direction detection
logic bar_up_a; // bar is moving up the screen?
rasterbar raster_a (
.clk,
.start(sy == bar_y_a),
.line,
.base_colr(BASE_COLR_A),
.colr_steps(STEPS_A),
.colr_lines(COLR_LINES),
.bar_colr(bar_colr_a),
.drawing(bar_drawing_a),
/* verilator lint_off PINCONNECTEMPTY */
.done()
/* verilator lint_on PINCONNECTEMPTY */
);
// raster B
localparam BASE_COLR_B = 12'h640; // bar start colour (gold)
localparam STEPS_B = 10; // colours steps in each bar
logic [11:0] bar_colr_b;
logic bar_drawing_b;
logic signed [CORDW-1:0] bar_y_b; // screen line to draw at
logic signed [CORDW-1:0] bar_y_b_prev; // previous line for direction detection
logic bar_up_b; // bar is moving up the screen?
rasterbar raster_b (
.clk,
.start(sy == bar_y_b),
.line,
.base_colr(BASE_COLR_B),
.colr_steps(STEPS_B),
.colr_lines(COLR_LINES),
.bar_colr(bar_colr_b),
.drawing(bar_drawing_b),
/* verilator lint_off PINCONNECTEMPTY */
.done()
/* verilator lint_on PINCONNECTEMPTY */
);
// raster C
localparam BASE_COLR_C = 12'h610; // bar start colour (red)
localparam STEPS_C = 10; // colours steps in each bar
logic [11:0] bar_colr_c;
logic bar_drawing_c;
logic signed [CORDW-1:0] bar_y_c; // screen line to draw at
logic signed [CORDW-1:0] bar_y_c_prev; // previous line for direction detection
logic bar_up_c; // bar is moving up the screen?
rasterbar raster_c (
.clk,
.start(sy == bar_y_c),
.line,
.base_colr(BASE_COLR_C),
.colr_steps(STEPS_C),
.colr_lines(COLR_LINES),
.bar_colr(bar_colr_c),
.drawing(bar_drawing_c),
/* verilator lint_off PINCONNECTEMPTY */
.done()
/* verilator lint_on PINCONNECTEMPTY */
);
// raster D
localparam BASE_COLR_D = 12'h145; // bar start colour (greenish)
localparam STEPS_D = 10; // colours steps in each bar
logic [11:0] bar_colr_d;
logic bar_drawing_d;
logic signed [CORDW-1:0] bar_y_d; // screen line to draw at
logic signed [CORDW-1:0] bar_y_d_prev; // previous line for direction detection
logic bar_up_d; // bar is moving up the screen?
rasterbar raster_d (
.clk,
.start(sy == bar_y_d),
.line,
.base_colr(BASE_COLR_D),
.colr_steps(STEPS_D),
.colr_lines(COLR_LINES),
.bar_colr(bar_colr_d),
.drawing(bar_drawing_d),
/* verilator lint_off PINCONNECTEMPTY */
.done()
/* verilator lint_on PINCONNECTEMPTY */
);
// update bar positions with sine table
enum {INIT, BAR_A, BAR_B, BAR_C, BAR_D, DONE} state;
always_ff @(posedge clk) begin
case (state)
INIT: begin
state <= BAR_A;
sin_id <= sin_id + 1;
sin_offs <= 0;
end
BAR_A: begin
state <= BAR_B;
bar_y_a <= VCENTER + (sin_data >>> SIN_SHIFT);
bar_y_a_prev <= bar_y_a;
sin_offs <= 64; // offset for bar B: 64/256
end
BAR_B: begin
state <= BAR_C;
bar_y_b <= VCENTER + (sin_data >>> SIN_SHIFT);
bar_y_b_prev <= bar_y_b;
sin_offs <= 128; // offset for bar C: 128/256
end
BAR_C: begin
state <= BAR_D;
bar_y_c <= VCENTER + (sin_data >>> SIN_SHIFT);
bar_y_c_prev <= bar_y_c;
sin_offs <= 192; // offset for bar D: 192/256
end
BAR_D: begin
state <= DONE;
bar_y_d <= VCENTER + (sin_data >>> SIN_SHIFT);
bar_y_d_prev <= bar_y_d;
end
default: if (start) state <= INIT;
endcase
end
// bar rising or falling?
always_comb begin
bar_up_a = (bar_y_a < bar_y_a_prev);
bar_up_b = (bar_y_b < bar_y_b_prev);
bar_up_c = (bar_y_c < bar_y_c_prev);
bar_up_d = (bar_y_d < bar_y_d_prev);
end
always_ff @(posedge clk) begin
if (bar_drawing_a && bar_up_a) bar_colr <= bar_colr_a;
else if (bar_drawing_b && bar_up_b) bar_colr <= bar_colr_b;
else if (bar_drawing_c && bar_up_c) bar_colr <= bar_colr_c;
else if (bar_drawing_d && bar_up_d) bar_colr <= bar_colr_d;
else if (bar_drawing_a) bar_colr <= bar_colr_a;
else if (bar_drawing_b) bar_colr <= bar_colr_b;
else if (bar_drawing_c) bar_colr <= bar_colr_c;
else if (bar_drawing_d) bar_colr <= bar_colr_d;
else bar_colr <= 12'h000;
end
always_ff @(posedge clk) begin
if ((bar_drawing_a && bar_up_a) || (bar_drawing_b && bar_up_b) ||
(bar_drawing_c && bar_up_c) || (bar_drawing_d && bar_up_d))
bar_up <= 1;
else bar_up <= 0;
end
endmodule
This is a quick design for a demo, not an example of good hardware design practice! For one thing, I’m not proud of that huge if-else statement to determine bar drawing order!
What’s Next?
Check out my demos, FPGA graphics tutorials, and guide to FPGA sine lookup tables.
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