Bending is caused by 8-bit color. Try changing your project settings to 16 or 32 bit. If that doesn't do it then you will have to add some noise to hide the banding.
HAHA,here you again,thank you for your answer first.
I have already changed channel to 16 or 32,but not used.
Add noise is a good help,there is no stripe.
Although the noise make the image quality a little bit worse,but it solves problems.
So is it a technical problem?
Thank you again,I will asking for your help next time
Problem with a 32 bit project is your monitor is probably 8 bit. That means the maximum difference between tones is 256 steps. You have a dark gray and a darker gray in your sample which means the values may be going from say 15 to 5 across 1080 pixels. That means you will have 10 distinct bands displayed. Increase the bit depth and the footage may render with no bands but if you don't have a 10 bit or better display, you're still stuck with the 10 possible values for the color. It's as simple as that.
I use macbook pro with retina display, so I don't know the bits of it. I know it may be a problem of 256.
But the question is if I don't have a professional display ,and my client maybe don't have,too.
Is any other ways to fix it like add noise?Is it a technical problem that can't solved unless change better display?
Or I there are some values that we can chose or not chose to avoid this kind of situation.
Like that we don't use some kind of gray,or we don't use over 3-5 colors,and something else.
I mean most situations,the video can not have problem like this.
But there is still some situations, we need color gradient.
We can see many videos like solid color BG,a scene in a studio and others.
So I need some ways to fix it.
Is it just one way (add noise)?
Is this a problem with graphics card or encoder?
If you are going to have very slight color differences in a gradient then your best option is to tint the dark toward something like blue and the brighter toward orange just slightly so that the bands break up between the r g and b channels. Then you add some noise. It doesn't matter what kind of display you have if you compress your video for delivery using an mpeg format like h.264 because that is an 8 bit codec so color artifacts will show up. Noise is your friend and unequal rgb values for each channel are also your friend so you can break up the differences between the channels in more steps. The biggest problem with digital displays is that all color values are quantified so you have to learn to work with your design to minimize problems.
I hope you understand what I am saying. A gradient with RGB values of 2, 4, and 9 at one end blending to 22, 35 and 19 at the other end are going to have a lot more bands in the image than a gradient that moves all values up 10 or 15 points equally because the banding in red is going to be in a different position than the banding in blue and so on. This helps hide the banding and also improves the chances that compressing to mp4 for delivery will build up fewer blocky color artifacts.
Another trick is to use a feathered mask on a solid color as an overlay on an image that has a texture. Something simple like rice paper or even a fractal with a blended (multiply blend mode) dark overlay with a feathered mask will work much better than a simple gradient background.
OK,I understand what you said.And I try them, noise is the best.