Water, mixing, tilt

Four things that felt wrong on the phone, found in the simulator, what caused each one, and what changed.

Every picture is the app painting a scene by itself with no hand on the glass, screenshotted a set number of seconds after launch. Before and after are the same strokes at the same moments. Before is the engine as it was; after is the engine with these fixes.

1Water left a grey mark that never went away

What you saw

Painting water left a grey, damp-looking band that stayed after everything else had dried.

Why

The sheet stores its water in 256 steps per pixel. Running at 120 frames a second, the loop took about a two-thousandth of a step off each pixel per frame, and that rounds straight back to nothing. Water never evaporated. The grey band was paper that stayed damp for good.

What changed

The simulation now steps 30 times a second whatever the screen does, and every value is nudged by a random fraction of a step before it is stored. A change of a tenth of a step now happens one step in ten, instead of never. Paper that has dried also fixes its paint in one go, which keeps most dry marks smooth.

In the pictures

Water over a blue mark and on clean paper. The water shows grey while it is wet, and is gone by 45 seconds.

Before

After

2Colours sat on top of each other instead of mixing

What you saw

Now and then two colours mixed, but mostly one just darkened the other where they overlapped.

Why

The sheet only knew two pigments at a time: the two of the set in hand. Picking a colour from anywhere else froze everything already painted into a flat picture, and the new colour could only glaze over it. Mixing happened only when both colours came from the same set, which is why it seemed to work sometimes.

What changed

Every point of the sheet now holds how much paint is there and what colour that paint is. When wet paint moves, its colour moves with it and averages with whatever it meets, using the same pigment model as before, so yellow and blue give green. Any colours that meet while wet mix, whichever card they came from.

In the pictures

Top: two colours of the same set. Middle and bottom: colours from different sets, which used to meet as a dark overlap.

Before

After

3Paint dried too fast

What you saw

Marks stopped moving within a few seconds, so there was little time to paint into them.

Why

The same rounding. Slow movement at the edge of a mark rounded to zero, so a mark stopped spreading while it was still wet, and looked finished before it was.

What changed

The same fix as the water. Nothing about drying was retuned: the numbers that were always there now actually take effect.

In the pictures

A blue stroke, then red strokes across it after 3 and 8 seconds. Before, they stay thin and overlap as dark squares. After, the blue is still wet at 8 seconds and the colours run into each other.

Before

After

4Tilting took a big lean

What you saw

The phone had to be tipped a long way before any colour moved.

Why

The pull of gravity was multiplied by the length of a frame. It was tuned when the app ran at 7 frames a second. At 120, each push was a seventeenth as strong.

What changed

Gravity is now a fixed push, the same on any phone at any frame rate.

In the pictures

A lean of about 6 degrees, held. The simulator has no motion sensor, so this scene sets the lean directly.

Before

After