Pixel is short for picture element and although due to its popularity
it has many meanings depending on context, when talking about pixels in the context of druid
a pixel means always only one thing. It is the smallest configurable area of color
that the underlying platform allows
druid-shell to manipulate.
The actual physical display might have a different resolution from what the platform knows or uses. Even if the display pixel resolution matches the platform resolution, the display itself can control even smaller elements than pixels - the sub-pixels.
The shape of the physical pixel could be complex and definitely varies from display model to model. However for simplicity you can think of a pixel as a square which you can choose a color for.
As technology advances the physical size of pixels is getting smaller and smaller. This allows display manufacturers to put more and more pixels into the same sized screen. The pixel densities of displays are increasing.
There is also an increasing variety in the pixel density of the displays used by people. Some might have a brand new 30" 8K UHD (7680px * 4320px) display, while others might still be rocking their 30" HD ready (1366px * 768px) display. It might even be the same person on the same computer with a multi-display setup.
For a very long time UIs have been designed without thinking about pixel density at all. People tended to have displays with roughly similar pixel densities, so it all kind of worked most of the time. However it breaks down horribly in a modern world. The 200px * 200px UI that looks decent on that HD ready display is barely visible on the 8K UHD display. If you redesign it according to the 8K UHD display then it won't even fit on the HD ready screen.
Some platforms have mitigations in place where that small 200px * 200px UI will get scaled up by essentially taking a screenshot of it and enlarging the image. This will result in a blurry UI with diagonal and curved lines suffering the most. There is more hope with fonts where the vector information is still available to the platform, and instead of scaling up the image the text can be immediately drawn at the larger size.
The application should draw everything it can with vector graphics, and have very large resolution image assets available where vectors aren't viable. Then at runtime the application should identify the display pixel density and resize everything accordingly. The vector graphics are easy to resize and the large image assets would be scaled down to the size that makes sense for the specific display.
Druid aims to make all of this as easy and automatic as possible.
Druid has expressive vector drawing capabilities that you should use whenever possible.
Vector drawing is also used by the widgets that come included with druid.
Handling different pixel densities is done at the
druid-shell level already.
In fact pixels mostly don't even enter the conversation at the
druid coordinate system is instead measured in display points (dp),
e.g. you might say a widget has a width of 100dp.
Display points are conceptually similar to Microsoft's device-independent pixels,
Google's density-independent pixels, Apple's points, and CSS's pixel units.
You describe the UI using display points and then druid will automatically translate that into pixels based on the pixel density of the platform. Remember there might be multiple displays connected with different pixel densities, and your application might have multiple windows - with each window on a different display. It will all just work, because druid will adjust the actual pixel dimensions based on the display that the window is currently located on.
TODO: Write this section after it's more clear how this works and if its even solved.