Photoshop does the same thing.
simply, double tap to fully paint it
yeah but sometimes it just covers the lines when you do that
Double tapping does nothing extra for me in FlipaClip. It does in Photoshop. The issue is because everything we use to draw in FlipaClip has smoothing on the outer edges so you get pixels that blend with the background color. If we had the option to draw with a line pencil or something similar that had absolute hard edges with no smoothing (meaning every pixel it draws is 100% that color) then you would get no white pixels. But it would also look very rough and pixely. Its not really a FlipaClip problem. It is just how it is when working with rasterized images as opposed to vector. Most artists just take the time and care to paint their art so it looks good. You might consider drawing the color on a cleanup layer behind the problematic layer to cover the white pixels up so they dont show white.
In smoothing, those edge pixels might have alpha transparency levels. Its like paintbucket sees there is something in a pixel already so it doesnt mess with it. Even if the pixel only has a color with low 10% alpha. If the paint tool could analize those partially transparent pixels and fill the transparent portion with its own color to make the pixel alpha 100% that would maybe fix it. Rather than completely ignoring. But unless the lines you paint between are black, you might then get odd color mixtures...well..like real life color mixing/color bleed i guess.
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