9 posts / 0 new
Last post
Why did the colors bleed on the C-64?

Why did the colors bleed on the C-64?
 
When doing hi-resolution bitmapped drawings in color, if I painted too many colors in one area, the colors would bleed.  I think if I put three or four colors too close together, they would bleed together.
 
Companies like Electronic Arts were always better at color drawings in their games.  I don't know why but their pictures didn't bleed.
 
Is there a clear answer on this?
 

It was to save RAM and money.

It was to save RAM and money. The C64 AND C128 both use the VIC-II chip, although the exact versions of these chips are different. In the hires mode of 320 x 200, you could only have one foreground colour in each 8x8 character mapped cell. The VIC-II graphic screens each take up 8K. Having the facility to use more than one foreground colour per 8x8 character cell would have taken 16K for 4 colours, and possibly 32K for 16 colours. The lores mode of 160 x 200 could have 3 foreground colours per 8x8 character mapped cell. I think that allowing any of the 16 colours in that mode to be in the same 8x8 character cell would have taken up 16K RAM. The alternative way of doing things is having 16, 8, 4, or 2 colours anywhere on the screen, but taking more than 8K. This was how Atari and various other companies did it. 
The reason why Electronic Arts graphics screens didn't bleed was because they planned them very carefully, using either the lores mode, or possibly hires mode with careful placing of each colour, or even large areas of the screen being the same colour. If you can load some of their screens into a graphics program, then check the coordinates and you'll see what I mean. If not, then they developed their own alternative software driven display modes, as done by some demo creators in recent years. 
 

--

Paul Alba

Hi all,

Hi all,
Thanks for that answer Paul, I dont think I would have known what kind of bleed was being talked about.
On the two wire interface that used to go in the front of the 1702 monitor the chroma signal was timed differently than the luminence and the Commodore monitors actually unskewed the signals specifically (due to signal mixing in the Mitsumi modultaor on the main board).  However a regular TV did not unskew the signals  so the color bled to the right usually,
And yes the images of the day were carefully crafted.
 
Bil

This effect was well known.

This effect was well known. It was also called attributes, or dot creep. The Sinclair Spectrum (not popular in North America) suffered from this more than any other computer I can name, because it only had one graphics display mode. The screen was divided into 32 x 24 or 25 character cells. Here's an example from the game Barbarian by Palace Software. You can tell the colours have been carefully positioned to avoid this effect.
 
http://prostoklevo.ru/uploads/posts/2013-03/1364448691_o6ionyk0ldurw7d.jpg
In spite of this, as time went by, programmers and artists on the C64 AND the SInclair Spectrum developed new display modes. Examples of various artwork from games, demos, parties, etc can be seen on www.c64pixels.com where a lot of the graphics are theoretically impossible on the C64 or VIC-II chip. One of the tricks is interlace, like on the Amiga, but I don't think the horizontal resolution can be increased above 320. If you look at them in chronological order, then you can see how the quality improves through the years.

--

Paul Alba

We probably didnt call this

We probably didnt call this bleed since we could "see" the attriubute boundries when we looked at the screens having done so much debugging.  There was a kind of smear we called bleed or artifact which was where the chroma changed faster than the chroma color refernce of 3.58mhz.  Depending on where the edge was you would get a red or a green smear.  I am told that the first Apples used this to make their red and green colors.
Later the Amiga would bypas this limitation with the HAM Hold and Modify mode where they would keep a color the same for one entire cell, but modify the brightness  for the second half of the cell, in effect a different shade.  This made use of the fact that a TV at the time had a higher bandwidth for luma, around 4.5Mhz instead of 3.58 ffor color.

I'm a bit confused by the

I'm a bit confused by the last comment about the Amiga HAM mode. I have recently been doing some more artwork on the Amiga, although the A1200 also has an improved HAM mode, as well as the original HAM mode. Of course, my Amigas are PAL and I've used them with TVs as well as a Commodore 1084S monitor. The original HAM mode certainly worked for me years ago on a 1084S monitor and PAL Amiga. I assume the bandwidths are different to NTSC. The limitations of the original HAM mode were that certain colours couldn't be placed next to each other, or only if these colours were placed into the first 16 colours of your palette. There was often a transition or graduation effect over 2-3 pixels before black could change to white, as well as for some other colours. Apart from this, the early Apple ][ computers couldn't display colour on PAL TV sets unless a graphics card was fitted, because the trick you described would only work on NTSC TV sets.

--

Paul Alba

Too funny!  I never thought

Too funny!  I never thought about PAL Apple, where Phase Alternating Line (PAL) was created to be immune to color  artifacts!  I always thought it was “chintzy” to rely on artifacts to begin with.
 
Take what you said about Amiga and remember that there is room for only about 320 color changes but close to 640 luminance changes… within reason (probably a gain power bandwidth issue, I.E. you can change a certain size at a certain speed, but not faster or more than a certain amount)
 
One day no-one will remember that the original home computers worked on an NTSC TV or that the Internet was through a 28.8 Modem.
 
Bil

Re: Too funny! I never thought

Bil wrote:

> One day no-one will remember that the original home computers worked on an NTSC TV or that the Internet was through a 28.8 Modem.

Heh, when not connected to a 13-inch Magnavox Color-80 monitor (composite and RGB digital), my C128DCR is connected to a 20-inch Sears stereo t.v./monitor (RF, composite, RGB digital). My DISH t.v. box works very well through it. :) Also the C128DCR has a Zoom 56K modem attached to it via a Turbo-232 cartridge. When I tell people I use dial-up at home, they stare at me in disbelief (or they don't know about dial-up, because they only know about high-speed connections). For CommVEx 2011 and CommVEx 2012, I demonstrated how to use the Commodore connecting to an ISP with dial-up. It was like demonstrating a lost art!

Truly,
Robert Bernardo
Fresno Commodore User Group
http://videocam.net.au/fcug

Yeah give me a big piece of

Yeah give me a big piece of flint I can carve a stone age keyboard. >:)

Log in or register to post comments