Clarifications about android drawable folders

Question

Until now, I did not pay too much attention to how to store drawable resources.

I usually generate multiple versions of an icon and store them under the drawable-mdpi, drawable-hdpi, drawable-xhdpi, ... folders. For other images for which I don't have multiple versions, I inconsistently store them under drawable-nodpi or drawable.

However, I recently encountered an issue related that bring all my attention to that. I stored a 100KB image under the drawable folder. However, my app was regularly crashing, stating it could not allocate 18MB!

After some searches, the reason was that the image was scaled to fit the screen resolution and it resulted in a way heavier image. The fix was to move it under the drawable-nodpi folder which prevents that scaling.

So now, I am trying to better understand where I should locate my images and how this scaling effect works to optimize my app on that part.

I have done plenty of searches, but resources are limited or unclear on that subject and the official documentation kinda really sucks. I am aware of the official explanation for the drawable or drawable-nodpi, but it does not clarify everything.

Typically:

• How does the scaling work? Let's say I have res/drawable-mdpi/image.png. Does that mean the image is scaled if I have a screen different from mdpi, or will it also be scaled on mdpi screens resulting in possibly heavier image size on every device?
• Following previous question, if the image is not scaled for mdpi screens but scaled for any others, then does that mean that if I provide a version of this image for every screen density, the scale will never happen? Or at the opposite, if the image is scaled also on mdpi devices, then having different version of the same image for each screen density will still scale the image, but using the version of the image matching the screen density of the device?
• How to deal with icons for which I do not have multiple versions? I am afraid that if I put this single version in mdpi, it just scales it and uses so much more memory than necessary. In such case, should I put any icon for which I do not have different versions under nodpi?
• On the other hand, if whenever you put an image in mdpi, hdpi, ... it scales it even on devices matching the density, then should I just move everything under nodpi or some high resolution like xxxhdpi where it can only be downscaled?

Thanks

Answer 1

Let's say I have res/drawable-mdpi/image.png. Does that mean the image is scaled if I have a screen different from mdpi

Yes.

or will it also be scaled on mdpi screens resulting in possibly heavier image size on every device?

No.

if I provide a version of this image for every screen density, the scale will never happen?

Yes. Your APK will be larger, due to the 7 copies of the drawable.

How to deal with icons for which I do not have multiple versions?

Option #1: Decide what density that particular version belongs in, and put it there. Android will upsample or downsample the image for devices operating at other densities.

Option #2: Put it in -anydpi or -nodpi, in which case Android will not upsample or downsample it. However, in this case, you need to be controlling the size of the image yourself (e.g., in the ImageView).

Option #3: Replace the icon with one that either you have all relevant densities or one that is an SVG that works as a vector drawable when imported using the Vector Asset Wizard in Android Studio.

The decision-making needs to be based both on memory consumption and what the result looks like. A low-memory solution that looks awful is unlikely to be a good choice.

should I just move everything under nodpi

Probably not.

or some high resolution like xxxhdpi where it can only be downscaled?

Probably not. It is unlikely that a massively downscaled version of your icon will look very good.