translucent png images for markers has a dark tint

[zainnadeem]

TO REPRODUCE THE ISSUE, FOLLOW THESE STEPS:

I am trying to add a png image that has an opacity of %20 percent.

RESULT:

The png is coming out dark. It seems anytime you add an image to a marker with an alpha other than 1, it darkens the image.

EXPECTED RESULT:

I expect the image to have a fully transparent background and should not tint my png. It should look like this:

ENVIRONMENT:

• Xcode 9.3 swift

[matteblair]

Hi @zainnadeem, thanks for filing an issue! I'm trying to understand the situation here. From your screenshots it looks like there is one image with a grid of dots and one image that looks like a round pink button - is that right? Are you able to share these images for us to investigate the issue?

[zainnadeem]

**please ignore the pink dot located in the center. I've replaced that image. The issue is with the larger circle not rendering pink but instead having a dark tint.

I've attached the image below. It's a pink circle with dots in the center. When I add it as a marker to the map, it renders dark. I am using the sdk-point-overlay style in the yaml file. I've attached the png for you to test.

Desired outcome:

Outcome:

[zainnadeem]

Thanks @matteblair let me know if I can provide anything else that might be useful.

[matteblair]

With some help, I think I've found the source of this problem (thanks @karimnaaji!). When we set an icon for a map marker, we go through a series of steps to get the raw pixel data from the UIImage so that it can be passed into the cross-platform C++ core. In that process we create a CGBitmapContext and render the UIImage into it with a set of options, one of which is the alpha info: https://github.com/tangrams/tangram-es/blob/master/platforms/ios/src/TangramMap/TGMarker.mm#L158

The pixel data that we want to get out is 32bpp RGBA, not alpha-premultiplied. This is represented by the kImageAlphaLast entry in the CGImageAlphaInfo enum but according to the Quartz2D programming guide this option is not supported on iOS (I tried it anyway and sure enough, it generates an error and fails on iOS).

Since iOS only supports alpha-premultiplied CGBitmapContext configurations, we will have to either:

• un-multiply the alpha from the resulting pixels by looping over the entire image manually, or
• find a way to extract pixel data from a UIImage that doesn't use CGBitmapContext.

I'll update when I've got a working fix!

[tallytalwar]

This "iOS only supports alpha-premultiplied CGBitmapContext configurations" is kinda a bummer!!