Pixaki

[rizer]

Hi, I'm the creator of a pixel art app called Pixaki. I'm working on a new version, and I have some questions about how things in the app should behave, so I thought "those Pixelation people seem nice, and clearly know a lot about pixel art, maybe I should ask them" 

A big new change I'm making is the option to have indexed colour mode documents. The app supports layers, and at the moment it's possible to adjust the opacity of a layer, and then merge it down, which obviously has the potential to produce a whole load more colours. So I thought that maybe in indexed colour mode there should be no semi-transparency in the layers or the selected colours, but in RGBA mode people can do what they like. Does that make sense? Would it be annoying to not be able to adjust a layer's opacity when working with indexed colours, or is that what you'd expect?

I'd love to hear what you think. Thanks for your time,

Luke

[eishiya]

When working in indexed/palette mode, I would expect it to be index painting, where the colours "snap" to the closest colour that is in the palette. This would apply not just to layer opacity, but "fuzzy" tools like gradients, soft brushes, etc.

[PixelPiledriver]

Yo, welcome to Pixelation. 

I was just looking at Pixaki a few days ago.
Nice to hear you are still supporting it. 
Will give it a try.

Would it be annoying to not be able to adjust a layer's opacity when working with indexed colours, or is that what you'd expect?

What works the best for me is to have RGB and A as separate elements with their own index tables.
That way alpha can be changed and controlled globally just like colors.

[Ai]

Would it be annoying to not be able to adjust a layer's opacity when working with indexed colours, or is that what you'd expect?

What works the best for me is to have RGB and A as separate elements with their own index tables.
That way alpha can be changed and controlled globally just like colors.

That looks to me like you answered a different question than he asked? You can have indexed color with opacity and layer modes (GIMP 2.9 does this), or without; and you can also quantize the result to existing palette, or add new colors. Your comment appears to address *document-wide* color / alpha rather than layer blending, or am I mistaken?

(Personally I'm a bit confused as how indexed alpha channel is supposed to work anyway, once you have a layer stack. Blend normally and quantize the result to the 'alpha palette'? Take the maximum of the lower and upper channel alpha? Something else?)

[Probo]

Would it be annoying to not be able to adjust a layer's opacity when working with indexed colours

I would absolutely, massively like the choice to change a layers opacity when working with indexed colours! I use Graphics Gale most of the time, and that doesn't let you control the alpha of a layer in indexed colour mode. Or rather it still lets you move the layer's opacity slider, but it dithers the layer by deleting pixels rather than changing opacity. Which would also be a nice option to still have I guess, but I don't use this dithering transparency often at all. Its not a particularly clever dithering algorithm either, maybe if it was id use it more.

[32]

I don't think I'd use opacity much as an artistic tool when I'm in indexed mode but it would definitely be nice to have available for the sake of being able to see through layers. Definitely get hamstrung by GraphicsGale sometimes because of the dithering system Probo mentioned. I think if you just had an option when merging layers to disregard opacity that would be ideal.

Behaviour like Eishiya mentioned would also probably come in handy now and then artistically.

[eishiya]

32's suggestion reminds me of onion-skinning, and that is a feature I'd like to see in more programs too. It would be very cool to be able to set a layer's true opacity (with the colour snapping like I described), as well as its apparent/onion skin opacity, in which case it's purely a UI thing like selections (marching ants are displayed on the image but aren't actually a part of it), except for opacity. When flattening, picking colours, etc, the onion skin opacity would be ignored, while the true opacity would be taken into account. This is something that would be useful both for RGBA and indexed modes.

[PixelPiledriver]

That looks to me like you answered a different question than he asked?

Yep, my bad.
Def agree with everyone else's suggestions.

Personally I'm a bit confused as how indexed alpha channel is supposed to work anyway

It's a palette of black to white blocks.
Just like colors except they represent alpha steps instead.
So you have a restricted set of alpha values available.

[Ai]

Personally I'm a bit confused as how indexed alpha channel is supposed to work anyway

It's a palette of black to white blocks.
Just like colors except they represent alpha steps instead.
So you have a restricted set of alpha values available.

Yes, I understand the concept, but not how it's supposed to work in context of a multi layer document. That was what this bunch of questions: "Blend normally and quantize the result to the 'alpha palette'? Take the maximum of the lower and upper channel alpha? Something else?" was addressed at). So basically I'm having difficulty seeing how you're supposed to combine [indexed alpha] + [layering] in an intuitive/consistent way -- how compositing should interact with the alpha palette.

@rizer:
Getting back to answering OP's question, personally I have a use for a) onionskinning, b) layer opacity with quantization to palette, c) layer opacity without quantization to palette.
In that order of priority (but I think b might depend on c to work sensibly? GIMP does c and then applies the quantization only when you merge down the layer, so what you get when you merge isn't always exactly what you expected.)

I can't comment on experiences with your app cause I don't have a iPad; keep that in mind if I say something ignorant.

If you don't have something like GraFX2's QShade mode, then some limited selection of layer modes (eg. add, sub, grain merge, grain extract) would be appreciated. If you do have a QShade-alike, then it's cool (qshade covers most of the same use cases)

To address what Probo said about alpha dither: I don't think it's particularly important, but would be neat to have -as an option- (like a variant of 'Dissolve' blend mode that's not randomized, instead based on a standard 4x4 Bayer matrix.)

[rizer]

Wow, amazing, thanks so much everyone! This is a real treasure trove 

So putting everything together, here's what I'm thinking I'll do:

Keep layer opacity as an option, just like in RGBA mode. If you choose to export the document and you have layers with semi-transparency, then you'll just get an image with more colours than you have in your palette — I don't think I need to tell the user that they've done this, I think it should be pretty obvious.

If you select to merge a layer at 100% or 0% opacity, it will just merge down (the latter will just delete the layer).

If you select to merge a layer with any other opacity, you'll be presented with some sort of dialogue asking if you'd like to add the colours to the palette or quantise to existing colours.

Onion skinning will be available if you have more than one frame of animation.

Which leads me on to a few more questions:

• Does anyone have any thoughts on how the colour quantisation algorithm should work? My first thought was maybe to calculate the difference in hue, saturation, and brightness for each colour in the palette and the target colour, then take the palette colour with the smallest ΔH + ΔS + ΔB?
  
  
• I'm planning to allow the user to be able to import a palette from a list of palettes. If the current document palette has more colours than the imported palette, I thought it would be best to append the extra colours. So if you were working with a palette that was [#9CBD0F, #8CAD0F, #306230] and you import [#CC0000, #00CC00], you'd end up with [#CC0000, #00CC00, #306230]. I guess I could quantise, but it seems like it would be more annoying to lose colour distinctions in the document without being very explicit about doing so. I'm planning some sort of interactive colour merge feature too, so you could just merge the new colours if you didn't want them. What do you think?
  
  
• With regards to onion skinning, what options would you want to see there? My current plan is that I'll show the frame in front and the frame behind (which will loop if you're on the first or last frame), and have a single opacity option that will change the opacity of both frame previews. I know there's lots more I could do, but would this cover what most people would be happy with?
  
  

Thanks again!