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electrotrains
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Joined: 29/07/2020
Location: United Kingdom
Posts: 13

Posted: 12:07pm 29 Jul 2020

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Hi all,

I'm attempting to make a game with my son on the CMM2 to have some fun and so he can learn some programming.

However, I'm not very clear on how transparency works with BLITs in 16-bit colour mode (MODE 3,16).

I've created a sprite sheet as a PNG and loaded it offscreen (PAGE WRITE 1). The PNG has transparent pixels where there is no sprite.

We then blit individual frames of animation onto the screen using PAGE WRITE 0, then BLIT from page 1. This is working fine so far.

What I'm not very clear about is how the transparent mode for BLIT will work in 16-bit colour mode - since presumably the tranparency information was lost when I loaded the PNG.

The graphics tutorial posted on this forum (here) has some code that says:

' move the text but only non-transparent (non-zero) pixels
blit 100,100,200,200,32,50,,4

which implies anything zero (i.e. black?) counts as transparent. Is this simply the case for BLITs in 16-bit mode? in which case my sprite sheets should really use black as the transparent colour, rather than the PNG alpha channel.

Any pointers appreciated!

Robin

https://github.com/robinhedwards

matherp
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Joined: 11/12/2012
Location: United Kingdom
Posts: 10580

Posted: 12:19pm 29 Jul 2020

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In 8 and 16 bit modes black is considered transparent

electrotrains
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Joined: 29/07/2020
Location: United Kingdom
Posts: 13

Posted: 12:27pm 29 Jul 2020

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Thanks - good to have that confirmed.

It does means that my (blitted) sprites and tiles can't contain true black, though I suppose I can use the darkest grey instead if I needed to.

It would be good to have a command to set the colour to be treated as transparent.

Unless there's a better way to go about this? Maybe 12-bit mode helps here?

https://github.com/robinhedwards

matherp
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Joined: 11/12/2012
Location: United Kingdom
Posts: 10580

Posted: 12:33pm 29 Jul 2020

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12-bit mode is definitely the way to go if you want to really use transsparency. For other modes it is just a conditional copy pixel by pixel which is inherently inefficient. I did start with having the concept of a "selectable" transparent colour but that just added another layer of complexity so using a dark colour for black is the workround

electrotrains
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Joined: 29/07/2020
Location: United Kingdom
Posts: 13

Posted: 12:43pm 29 Jul 2020

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Peter,

Great to get such prompt responses from you! Much appreciated.

I'm hoping this machine will be as much fun to make games with as STOS/AMOS on the Atari ST/Amiga were in my youth. So far its looking promising, so thanks for all you've done.

I'll persist with 16-bit mode for a bit at least, and try to get my head round 12-bit mode later.

Robin

https://github.com/robinhedwards

capsikin
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Joined: 30/06/2020
Location: Australia
Posts: 342

Posted: 11:56pm 29 Jul 2020

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matherp said In 8 and 16 bit modes black is considered transparent

For 8 bit mode can I check something?
Only colour 0 is considered transparent, even if you set colour 1 to also be black using MAP(1)=RGB(0,0,0), right?

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