Adds Negative Prompts (#61)

* Synced to main branch and minimizes line changes

* Adds negative prompt argument to CLI

Co-authored-by: Wanaldino Antimonio <[email protected]>

Author: JustinMeans

swift/StableDiffusion/pipeline/StableDiffusionPipeline.swift

@@ -108,6 +108,7 @@ public struct StableDiffusionPipeline: ResourceManaging {
     ///
     /// - Parameters:
     ///   - prompt: Text prompt to guide sampling
+    ///   - negativePrompt: Negative text prompt to guide sampling
     ///   - stepCount: Number of inference steps to perform
     ///   - imageCount: Number of samples/images to generate for the input prompt
     ///   - seed: Random seed which
@@ -117,6 +118,7 @@ public struct StableDiffusionPipeline: ResourceManaging {
     ///            The images will be nil if safety checks were performed and found the result to be un-safe
     public func generateImages(
         prompt: String,
+        negativePrompt: String = "",
         imageCount: Int = 1,
         stepCount: Int = 50,
         seed: UInt32 = 0,
@@ -125,17 +127,18 @@ public struct StableDiffusionPipeline: ResourceManaging {
         progressHandler: (Progress) -> Bool = { _ in true }
     ) throws -> [CGImage?] {
 
-        // Encode the input prompt as well as a blank unconditioned input
+        // Encode the input prompt and negative prompt
         let promptEmbedding = try textEncoder.encode(prompt)
-        let blankEmbedding = try textEncoder.encode("")
+        let negativePromptEmbedding = try textEncoder.encode(negativePrompt)
 
         if reduceMemory {
             textEncoder.unloadResources()
         }
 
         // Convert to Unet hidden state representation
+        // Concatenate the prompt and negative prompt embeddings
         let concatEmbedding = MLShapedArray<Float32>(
-            concatenating: [blankEmbedding, promptEmbedding],
+            concatenating: [negativePromptEmbedding, promptEmbedding],
             alongAxis: 0
         )
 

swift/StableDiffusionCLI/main.swift

@@ -19,6 +19,9 @@ struct StableDiffusionSample: ParsableCommand {
     @Argument(help: "Input string prompt")
     var prompt: String
 
+    @Option(help: "Input string negative prompt")
+    var negativePrompt: String
+
     @Option(
         help: ArgumentHelp(
             "Path to stable diffusion resources.",
@@ -85,6 +88,7 @@ struct StableDiffusionSample: ParsableCommand {
 
         let images = try pipeline.generateImages(
             prompt: prompt,
+            negativePrompt: negativePrompt,
             imageCount: imageCount,
             stepCount: stepCount,
             seed: seed,