UPDATED: 15/06/2023 following experiments with different resolutions on the Agon Light

I’ve been given this a lot of thought (and background reading!) as I can only get MODE 3 working properly on my Agon and VGA monitor.

I submit the following thoughts for consideration (sorry, long post!):

BBC Modes

While the BBC modes with 256 vertical pixels have a nostalgic vibe, they don’t appear to be that compatible with most VGA monitors. Since the Agon isn’t trying to be a direct clone of the BBC, it would probably be best to ignore these and build on the VGA standard.

VGA Modes

Of the modes that are included in the VGA standard, I’d suggest the following as being the most useful resolutions:

640x480 at 60Hz (current MODE 3) that maps to VGA Mode 12h
320x240 at 60Hz (proposed by dsnopek) that maps to QVGA/VGA Mode X

This would give a “high resolution” mode for displaying 80x25 text and graphics and a decent lower resolution with square pixels suitable for games. I think it would be reasonable to expect these modes to be compatible with pretty much all monitors.

It may also be useful to add:

640x240 at 60Hz. This is supported in FabGL and gives a 1:2 pixel resolution with tall, thin text that is similar to classic computers such as the BBC Micro, Amstrad CPC, Atari ST and early PCs.
320x200 at 70Hz that maps to VGA Mode 13h (albeit with reduced colours on the Agon) and is a resolution used by a lot of retro systems and games. As part of the VGA standard, this should also be supported by all monitors.

Logical Co-ordinates

The new resolutions could mean a change to the logical co-ordinates from 1280x1024 used by the BBC, but it appears that Acorn tackled this problem in RISC OS using its “OS Units” concept (https://www.riscosopen.org/wiki/documentation/show/Mode%20Variable%20Numbers) that allowed different screen resolutions to use different logical coordinates depending on the resolution aspect ratio.

For example:

RISC OS Mode 27: 640x480 = 1280x960 OS units
RISC OS Mode 46: 640x200 = 1280x800 OS units

Colours

The limited amount of memory in the ESP32 means that low colour modes are still useful. Suggested colour depths are 2, 4, 16 and 64 colours.

Proposed Modes

The following modes are suggested and should be compatible with all VGA monitors. In the testing I've done, if the monitor is running at 60Hz, then these modes appear to be scaled to 640x480. As the 640x240 and 320x240 resolutions can do this using pixel doubling, the picture is very crisp and clear.

For modes with 200 lines, the monitors I've tested display using a 720x400 resolution which introduces some display artefacts and is less crisp.

Mode	Details	FabGL Mode
0	640x480 in 16 colours at 60Hz	VGA_640x480_60Hz
1	640x480 in 4 colours at 60Hz	VGA_640x480_60Hz
2	640x480 in 2 colours at 60Hz	VGA_640x480_60Hz
3	640x240 in 64 colours at 60Hz	VGA_640x240_60Hz
4	640x240 in 16 colours at 60Hz	VGA_640x240_60Hz
5	640x240 in 4 colours at 60Hz	VGA_640x240_60Hz
6	640x240 in 2 colours at 60Hz	VGA_640x240_60Hz
7	Reserved for Teletext mode currently under development
8	320x240 in 64 colours at 60Hz	QVGA_320x240_60Hz
9	320x240 in 16 colours at 60Hz	QVGA_320x240_60Hz
10	320x240 in 4 colours at 60Hz	QVGA_320x240_60Hz
11	320x240 in 2 colours at 60Hz	QVGA_320x240_60Hz
12	320x200 in 64 colours at 70Hz	VGA_320x200_70Hz
13	320x200 in 16 colours at 70Hz	VGA_320x200_70Hz
14	320x200 in 4 colours at 70Hz	VGA_320x200_70Hz
15	320x200 in 2 colours at 70Hz	VGA_320x200_70Hz

In this configuration, modes 0-2 could be described as "Agon High Resolution", modes 3-6 as "Agon Medium Resolution" and modes 8-11 as "Agon Low Resolution". Modes 12-15 could be "Agon Compatibility Modes" as they are likely to be most useful for porting from other platforms.

I did not include a 640x480 in 64 colours at 60Hz mode as this will require 300K for the frame buffer and I assume there isn’t sufficient memory in the ESP32 for this.

Additionally, beyond VGA, I have had success with the following:

Mode	Details	FabGL Mode
16	800x600 in 4 colours at 60Hz	SVGA_800x600_60Hz
17	800x600 in 2 colours at 60Hz	SVGA_800x600_60Hz
18	1024x768 in 2 colours at 60Hz	SVGA_1024x768_60Hz

Hope this is helpful!

Uh oh!

Increase quantity and quality of default modes in Agon #16

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