Halftone on image transition to another image

Question

I want this:

but instead of the european flag, I want another flag. How can I achieve this halftone effect fading into the other flag in Illustrator?

I keep getting this result:

Answer 1

Edit: Didn't notice the OP tagged the question with Illustrator. I'll leave this answer here for referrence anyway. This is for Photoshop

Create a new greyscale image the same size as the flag, add a gradient from white to black, and apply the halftone filter, set the screen angle to 45º

Use that image as a layer mask for the rainbow flag, over a dark blue background.

Example:

Answer 2

Actually the idea is already shown by member Billy Kerr. It's for Photoshop. In Illustrator you can use opacity masks instead of layer masks.

Let's assume you have two flag images, the yellow one and the blue. Make a rectangle of the same size and insert a black-white gradient over the wanted transition zone:

(sorry for the vertical black line, it's my artboard edge)

Insert to the gradient effect Pixelate > Color Halftone. Let all RGB channels have the same angle, here it's 45 degrees. Use coarse enough max dot size, I used 30px (see NOTE1):

Place the result onto one of the flags and define it to be an Opacity mask. Option invert reverses the effect, it makes flag parts under black to stay visible:

Align both flags:

NOTE1: Color halftone is a raster effect, the resulted dot pattern is essentially a bitmap image, only wrapped in Illustrator's effect machinery to keep the effect editable. You must have defined high enough raster effect resolution in your document properties to prevent visible pixelation

Many Illustrator users hate to leave raster effects to their documents because they make the image non-freely scalable. Scaling easily causes pixelation or generates a gigantic file size. In high zoom you can see the pixelated edges without actual scaling:

It's essentially the same that you could get in Photoshop.

You can make it all-vector easily. At first rasterize the dot patter. Apply Object > Expand Appearance for it. The result has a large white frame, but you can get rid of it in vectorization.

Then trace it to vector (Object > Edit > Live Trace), Select option Ignore white:

Expand the result to fade the bitmap and the white periferia area finally. You get a vector which contains the black areas as a group:

The group of black dots works as an opacity mask, no white is a must. But you must invert the mask in the opacity mask options or change its color to white before using it. The result is in high zoom much cleaner than without the vectorization:

Another method to make the mask

Color halftone effect is not a must. As well you can make the dot chains with blending. It's vector in the beginning and the dot shape is arbitary. Here's the transition zone pattern made by blending.

On the top there's a blended stripe. In the middle there's a pair of them to get the interlacing.

The third one is the pair copied several times vertically. You need to insert a solid black rectangle to the right and make a group.