Edge color changes when overlapped but it can be solved using "Alpha add" mode

Report · Apr 23, 2017

Hello. I have a question.

I have 2 png images in After Effects. Those images are aliased images.Just like this image.

Those A and B shape is png images. Both images are ok when they are separated.

but when I move them and they are overlapped, strange thing happens. I painted problematic marginal area of shape B red.

Those red areas are the place where original colors are changed. This color change is really slight, like "R0 G0 B0" to "R1 G1 B1", but it definitely changes. Alpha value is not changed though.

I don't know why those color change happens. This happens when I set layer mode of shape B to "Normal".

When I change layer mode of shape B to "Alpha add", this color change won't happen. It must be related to alpha interpolation of those images.

I want to know why those color change happens and what is the right way to prevent this color change from happening. I am not sure "Alpha add" mode is the right answer.

Please help me . Thank you in advance.

Report · Apr 23, 2017

I painted problematic marginal area of shape B red.Those red areas are the place where original colors are changed.

it would be helpful to show us the problem at magnified resolution and not paint over it. show before and after.

as for alpha add, here's the resource file where you could read about what alpha add does: Use blending modes and layer styles in After Effects

Report · Apr 23, 2017

Alpha add changes the math used to blend transparent pixels. It is often used to fix the problems you are encountering. You can also use alpha add when you face opacity to create a dissolve. This will give you a more of a traditional film quality dissolve. The look will be completely different.

The Alpha Add blend mode only changes the aliased edge behavior in the alpha channel so there is no change in any of the other pixels in the image.

The original problem may be caused by the way you created the transparency in the first place. A complete workflow description with every step detailed may point us to offering a solution or at least a better explanation.

Report · Apr 23, 2017

Thank you for your answers. Then those images will be helpful, put those original images into After Effects and simply place them in a composition like I did in my first post. Check the line color of shape B, there will be 2 colors, R0 G0 B0 and R1 G1 B0.

I always edit aliased images, so this color change of fringe area is a problem to my workflow.

I made those image using Photoshop, and B shape line is completely black, color is R0 G0 B0. This only happens in After Effects. When I put those images in Photoshop,B shape line color does not change when overlapped.

Report · Apr 24, 2017

I'm not seeing any problems with the edges in AE other than pretty bad aliasing in the black stroke. Here is the image at 100% as a screen capture from a MacBook Pro R 15":