So you have generated a few Stable Diffusion AI images. They looked great but not quite what you want? You can use some customization. Here’s a primer for the basic generation parameters.
Stable Diffusion Software
We will focus on Stable Diffusion AI. While some parameters mentioned in this article are available in free online AI generators, all of them are available in this popular Stable Diffusion GUI (AUTOMATIC1111). See my quick start guide for setting up in Google’s cloud server.
CFG Scale
Classifier Free Guidance scale is a parameter to control how much the model should respect your prompt.
1 – Mostly ignore your prompt.
3 – Be more creative.
7 – A good balance between following the prompt and freedom.
15 – Adhere more to prompt.
30 – Strictly follow the prompt.
Below are a few examples of increasing the CFG scale with the same random seed. In general, you should stay away from the two extremes – 1 and 30.
Recommendation: Starts with 7. Increase if you want it to follow your prompt more.
Sampling steps
Quality improves as the sampling step increases. Typically 20 steps with Euler sampler is enough to reach a high quality, sharp image. Although the image will still change subtly when stepping through to higher values, it will become different but not necessarily higher quality.
Recommendation: 20 steps. Adjust to higher if you suspect quality is low.
Sampling methods
There’s a variety of sampling methods you can choose, depending on what GUI you are using. They are simply different methods for solving diffusion equations. They are supposed to give the same result but could be slightly different due numerical bias. But since there’s no right answer here – the only criteria is the image looks good, accuracy of the method should not be your concern.
Not all methods are created equal. Below are the processing time of various methods.
Below are images produced after 20 steps.
There are discussions in online community claiming that certain sampling methods tend to yield particular styles. This is without theoretical merit. My suggestion is to keep things simple: 20 steps with Euler.
Recommendation: Euler.
Seed
The random seed determines the initialize noise pattern and hence the final image.
Setting it to -1 means using a random one every time. It is useful when you want to generate new images. On the other hand, fixing it would result in the same images in each new generation.
How to find the seed used for an image if you use random seed? In the dialog box, you should see something like:
Steps: 20, Sampler: Euler a, CFG scale: 7, Seed: 4239744034, Size: 512×512, Model hash: 7460a6fa
Simply copy this seed value to seed input box. If you generate more than one image at a time, the seed value of the second image is this number incremented by 1, and so on. Alternatively, click the recycle button to reuse the seed from the last generation.
Recommendation: Set to -1 to explore. Fix to a value for fine-tuning.
Image size
The size of output image. Since Stable Diffusion is trained with 512×512 images, setting it to portrait or landscape sizes can create unexpected issues. Leave it as square whenever possible.
Recommendation: Set image size as 512×512.
Batch size
Batch size is the number of images generated each time. Since the final images are very dependent on the random seed, it is always a good idea to generate a few images at a time. This way, you can get a good sense of what the current prompt can do.
Recommendation: Set batch size to 4 or 8.
Restore faces
A dirty little secret of Stable Diffusion is that it often has issues with faces and eyes. Restore faces is a post-processing method applied to images using AI trained specifically to correct faces.
To turn it on, check the box next to Restore faces. Go to Settings tab, under Face restoration model, select CodeFormer.
Below are two examples. Without face restoration is on the left. With face restoration is on the right.
Recommendation: Turn restore faces on when you generate images with faces.
Summary
In this article, we have covered the basic parameters for Stable Diffusion AI. Check out this article for step-by-step guide in building high-quality prompt. Check out this article for more advanced prompting techniques.
