[Oberon] FPGA - Display.Mod 4-bit colour

[Tomas Kral]

On Tue, 2 Oct 2018 11:40:22 +0200
Tomas Kral <thomas.kral at email.cz> wrote:

> IF SW[6] THEN dpt := 4 (*4 bit*) 
>   ELSE dpt := 1 (*1 bit*) END 

Hi,

I am experimenting with 1-bit/4-bit colour map switch. It does things,
display is alright. But colours are mapped as thus, the names are not
exact but they appear to me like that.

 map entry colour
 00000000  black
 00000080  red
 00008000  green
 00008080  yellow
 00800000  cyan
 00800080  violet
 00808000  light cyan
 00808080  light violet
 00C0C0C0  light light violet
 000000FF  light red
 0000FF00  light yellow
 0000FFFF  orange
 00FF0000  dark red
 00FF00FF  yet another shade of violet
 00FFFF00  turquoise
 00FFFFFF  white

The good point is, I can now set mono display with two colours other
than black/white, e.g. green/black or yellow/black, etc.

While I would expect the colours be more or similar like this.

 map entry colour
 00000000  black
 00000080  blue
 00008000  green
 00008080  cyan
 00800000  red
 00800080  violet
 00808000  yellow
 00808080  grey
 00C0C0C0  light grey
 000000FF  light blue
 0000FF00  light green
 0000FFFF  light cyan
 00FF0000  light red
 00FF00FF  light violet
 00FFFF00  light yellow
 00FFFFFF  white

The colours are added by these lines in verilog for both 1-bit/4-bit.

assign vidm = (pixbuf[0] ^ inv); // mono
assign vidc = pixbuf[3:0] ^ {inv, inv, inv, inv}; // 4-bit
...
// RGB[7:0] = RRR GGG BB
assign RGB = {color[23:21], color[15:13], color[7:6]};
...
if (cmode) vid <= vidc else vid <= {3'b000, vidm};
...

and looked up in the map
...
if (~hblank & ~vblank)
  case (vid)
    4'h0: color <= col0;
    ...
    4'hf: color <= col15;
  endcase
else
  color <= 24'h000000;
...

Noticeably, Display does not seem to have the same intensity down
across, at the the 1/3 upper top appears slightly darker.
 
I am becoming quite confused, I would be grateful for any hints.

Many thanks in advance.

-- 
Tomas Kral <thomas.kral at email.cz>