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feat: LLM-powered image prompt rewriting for FLUX.1

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Replaces the rule-based prompt builder with a cheap LLM call
(analysis model via OpenRouter) that distills article content into
concise visual scene descriptions. The old rule-based method becomes
the fallback if the LLM call fails.

Key design decisions:
- System prompt teaches the LLM to describe objects/setting/lighting
  instead of abstract concepts, and to avoid faces (FLUX.1 weakness)
- Uses the analysis model (gemini-2.5-flash-lite) at ~$0.001/call
- Cost is logged to the analytics dashboard via log_api_call()
- Style suffix is still appended separately so the rewriter only
  handles scene composition

Cost impact: +~$0.001 per image on top of ~$0.0008 generation cost.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
This commit is contained in:
peptiderepo
2026-04-18 02:06:22 +07:00
parent 0e44c52ad8
commit dcc8b7b183
1 changed files with 155 additions and 49 deletions
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204
includes/core/class-image-prompt-builder.php
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@@ -4,23 +4,60 @@ declare(strict_types=1);
/** /**
* Builds image prompts from article content or source data. * Builds image prompts from article content or source data.
* *
* Generates concise visual concepts suitable for FLUX.1 image generation, * Uses a cheap LLM (via OpenRouter) to distill article content into concise
* extracting key themes from article titles/content and Reddit source data. * visual scene descriptions optimised for FLUX.1. Falls back to rule-based
* synthesis if the LLM call fails, so image generation never blocks on a
* prompt-rewriting outage.
* *
* Triggered by: PRAutoBlogger_Image_Pipeline during content generation run. * Triggered by: PRAutoBlogger_Image_Pipeline during content generation run.
* Dependencies: None — pure data transformation. * Dependencies: PRAutoBlogger_OpenRouter_Provider (LLM call),
* PRAutoBlogger_Cost_Tracker (logs prompt-rewrite cost),
* PRAutoBlogger_Logger (diagnostics).
* *
* @see core/class-image-pipeline.php — Consumes build_article_prompt() and build_source_prompt(). * @see core/class-image-pipeline.php — Consumes build_article_prompt() and build_source_prompt().
* @see providers/class-open-router-provider.php — LLM provider used for rewriting.
* @see ARCHITECTURE.md — Image generation data flow. * @see ARCHITECTURE.md — Image generation data flow.
*/ */
class PRAutoBlogger_Image_Prompt_Builder { class PRAutoBlogger_Image_Prompt_Builder {
/**
* System prompt that teaches the LLM how to write image-gen prompts.
*
* Why this lives here instead of in wp_options: it's engineering-level
* instruction, not a user-facing setting. Changing it should require a
* code review, not an admin-panel click.
*/
private const REWRITER_SYSTEM_PROMPT = <<<'PROMPT'
You are an image-prompt specialist for FLUX.1, a text-to-image model.
Given an article title and summary, output ONE short paragraph (2-4 sentences, under 60 words) describing a concrete visual scene that would make a compelling featured image for the article.
Rules:
- Describe OBJECTS, SETTING, LIGHTING, COMPOSITION, and MOOD — not abstract concepts.
- Never include text, words, labels, logos, or watermarks in the scene.
- Never describe people's faces in detail (FLUX.1 struggles with faces). Use silhouettes, hands, or over-the-shoulder framings instead.
- Focus on the article's core SUBJECT as a physical thing: lab equipment, supplements, syringes, molecules, gym settings, packaging, etc.
- Do NOT include any style direction — that is appended separately.
- Output ONLY the scene description. No preamble, no explanation, no quotes.
PROMPT;
/**
* Max tokens for the rewriter response. Scene descriptions are short.
*/
private const REWRITER_MAX_TOKENS = 120;
/**
* OpenRouter provider for LLM calls. Null until first use.
*
* @var PRAutoBlogger_OpenRouter_Provider|null
*/
private ?PRAutoBlogger_OpenRouter_Provider $llm = null;
/** /**
* Build a visual prompt from finished article content. * Build a visual prompt from finished article content.
* *
* Extracts the title and first paragraph to synthesize a product-marketing * Tries LLM rewriting first; falls back to rule-based synthesis on failure.
* angle visual concept suitable for a featured image. Appends the style suffix * Appends the style suffix from plugin options.
* from plugin options.
* *
* @param array{ * @param array{
* post_title?: string, * post_title?: string,
@@ -28,19 +65,15 @@ class PRAutoBlogger_Image_Prompt_Builder {
* suggested_title?: string, * suggested_title?: string,
* } $article_data Article data with title and HTML content. * } $article_data Article data with title and HTML content.
* *
* @return string Concise prompt (under 200 words) with style suffix appended. * @return string Concise prompt with style suffix appended.
*/ */
public function build_article_prompt( array $article_data ): string { public function build_article_prompt( array $article_data ): string {
$title = $article_data['post_title'] ?? $article_data['suggested_title'] ?? 'Product'; $title = $article_data['post_title'] ?? $article_data['suggested_title'] ?? 'Product';
$content = $article_data['post_content'] ?? ''; $content = $article_data['post_content'] ?? '';
// Extract first paragraph from HTML content.
$first_para = $this->extract_first_paragraph( $content ); $first_para = $this->extract_first_paragraph( $content );
// Build visual concepts from title + first paragraph. $concepts = $this->rewrite_via_llm( $title, $first_para );
$concepts = $this->synthesize_visual_concepts( $title, $first_para );
// Append style suffix.
$style_suffix = get_option( 'prautoblogger_image_style_suffix', PRAUTOBLOGGER_DEFAULT_IMAGE_STYLE_SUFFIX ); $style_suffix = get_option( 'prautoblogger_image_style_suffix', PRAUTOBLOGGER_DEFAULT_IMAGE_STYLE_SUFFIX );
return trim( $concepts . ' ' . $style_suffix ); return trim( $concepts . ' ' . $style_suffix );
@@ -49,9 +82,8 @@ class PRAutoBlogger_Image_Prompt_Builder {
/** /**
* Build a visual prompt from source Reddit thread data. * Build a visual prompt from source Reddit thread data.
* *
* Extracts the Reddit thread title and top comments to synthesize an * Tries LLM rewriting first; falls back to rule-based synthesis on failure.
* editorial-cartoonist angle visual concept. Appends the style suffix * Appends the style suffix from plugin options.
* from plugin options.
* *
* @param array{ * @param array{
* title?: string, * title?: string,
@@ -59,24 +91,99 @@ class PRAutoBlogger_Image_Prompt_Builder {
* comments?: string[], * comments?: string[],
* } $source_data Reddit source data with title and comments. * } $source_data Reddit source data with title and comments.
* *
* @return string Concise prompt (under 200 words) with style suffix appended. * @return string Concise prompt with style suffix appended.
*/ */
public function build_source_prompt( array $source_data ): string { public function build_source_prompt( array $source_data ): string {
$title = $source_data['title'] ?? 'Reddit Discussion'; $title = $source_data['title'] ?? 'Reddit Discussion';
$comments = $source_data['comments'] ?? []; $comments = $source_data['comments'] ?? [];
$context = is_array( $comments ) && ! empty( $comments ) ? $comments[0] : '';
// Build visual concepts from title + top comments. $concepts = $this->rewrite_via_llm( $title, $context );
$concepts = $this->synthesize_visual_concepts(
$title,
is_array( $comments ) && ! empty( $comments ) ? $comments[0] : ''
);
// Append style suffix.
$style_suffix = get_option( 'prautoblogger_image_style_suffix', PRAUTOBLOGGER_DEFAULT_IMAGE_STYLE_SUFFIX ); $style_suffix = get_option( 'prautoblogger_image_style_suffix', PRAUTOBLOGGER_DEFAULT_IMAGE_STYLE_SUFFIX );
return trim( $concepts . ' ' . $style_suffix ); return trim( $concepts . ' ' . $style_suffix );
} }
/**
* Use a cheap LLM to distill title + context into a visual scene.
*
* Falls back to rule-based synthesis if the LLM call fails for any
* reason (network, auth, timeout, unexpected response shape).
*
* Side effects: one OpenRouter API call; one cost-tracker log entry.
*
* @param string $title Article or thread title.
* @param string $context First paragraph or top comment.
* @return string Visual scene description (without style suffix).
*/
private function rewrite_via_llm( string $title, string $context ): string {
$title = trim( sanitize_text_field( $title ) );
$context = trim( sanitize_text_field( $context ) );
// Truncate context to keep prompt tokens low.
if ( strlen( $context ) > 300 ) {
$context = substr( $context, 0, 300 ) . '...';
}
$user_message = "Article title: {$title}";
if ( '' !== $context ) {
$user_message .= "\n\nSummary: {$context}";
}
try {
$llm = $this->get_llm_provider();
$model = get_option( 'prautoblogger_analysis_model', PRAUTOBLOGGER_DEFAULT_ANALYSIS_MODEL );
$result = $llm->send_chat_completion(
[
[ 'role' => 'system', 'content' => self::REWRITER_SYSTEM_PROMPT ],
[ 'role' => 'user', 'content' => $user_message ],
],
$model,
[
'temperature' => 0.7,
'max_tokens' => self::REWRITER_MAX_TOKENS,
]
);
$scene = trim( $result['content'] ?? '' );
// Log the rewrite cost so it shows in the analytics dashboard.
( new PRAutoBlogger_Cost_Tracker() )->log_api_call(
null,
'image_prompt_rewrite',
'openrouter',
$model,
$result['prompt_tokens'] ?? 0,
$result['completion_tokens'] ?? 0
);
$cost = $llm->estimate_cost(
$model,
$result['prompt_tokens'] ?? 0,
$result['completion_tokens'] ?? 0
);
PRAutoBlogger_Logger::instance()->debug(
sprintf( 'Image prompt rewritten (%d→%d chars, $%.6f): %s', strlen( $user_message ), strlen( $scene ), $cost, substr( $scene, 0, 120 ) ),
'image_prompt_builder'
);
if ( '' !== $scene ) {
return $scene;
}
} catch ( \Exception $e ) {
// LLM failure is not fatal — fall back to rule-based synthesis.
PRAutoBlogger_Logger::instance()->warning(
'Image prompt LLM rewrite failed, using fallback: ' . $e->getMessage(),
'image_prompt_builder'
);
}
return $this->synthesize_visual_concepts_fallback( $title, $context );
}
/** /**
* Extract the first paragraph from HTML content. * Extract the first paragraph from HTML content.
* *
@@ -84,18 +191,13 @@ class PRAutoBlogger_Image_Prompt_Builder {
* or 200 characters, whichever comes first. * or 200 characters, whichever comes first.
* *
* @param string $html HTML content. * @param string $html HTML content.
*
* @return string Plain text of the first paragraph. * @return string Plain text of the first paragraph.
*/ */
private function extract_first_paragraph( string $html ): string { private function extract_first_paragraph( string $html ): string {
// Remove HTML tags. $text = wp_strip_all_tags( $html );
$text = wp_strip_all_tags( $html ); $paras = preg_split( '/\n\n+/', $text );
$para = isset( $paras[0] ) ? trim( $paras[0] ) : '';
// Split on double-newline and take the first paragraph.
$paras = preg_split( '/\n\n+/', $text );
$para = isset( $paras[0] ) ? trim( $paras[0] ) : '';
// Limit to first 200 chars to keep prompt concise.
if ( strlen( $para ) > 200 ) { if ( strlen( $para ) > 200 ) {
$para = substr( $para, 0, 200 ) . '...'; $para = substr( $para, 0, 200 ) . '...';
} }
@@ -104,33 +206,37 @@ class PRAutoBlogger_Image_Prompt_Builder {
} }
/** /**
* Synthesize visual concepts from a title and supporting text. * Rule-based fallback when LLM rewriting is unavailable.
* *
* Combines key words and themes into a short visual narrative that FLUX.1 * Kept deliberately simple — this only runs if OpenRouter is down.
* can render. This is intentionally rule-based rather than LLM-driven to
* avoid another API call per image generation.
*
* @param string $title Main heading / topic.
* @param string $supporting_text Additional context (first para or comment).
* *
* @param string $title Main heading / topic.
* @param string $supporting_text Additional context.
* @return string Synthesized visual prompt concept. * @return string Synthesized visual prompt concept.
*/ */
private function synthesize_visual_concepts( string $title, string $supporting_text ): string { private function synthesize_visual_concepts_fallback( string $title, string $supporting_text ): string {
$title = trim( sanitize_text_field( $title ) );
$text = trim( sanitize_text_field( $supporting_text ) );
// Build a simple declarative concept from title and text snippets.
$prompt = "A visual representation of: {$title}"; $prompt = "A visual representation of: {$title}";
if ( ! empty( $text ) ) { if ( '' !== $supporting_text ) {
// Take first 150 chars to avoid bloating the prompt. $snippet = strlen( $supporting_text ) > 150
$snippet = strlen( $text ) > 150 ? substr( $supporting_text, 0, 150 ) . '...'
? substr( $text, 0, 150 ) . '...' : $supporting_text;
: $text;
$prompt .= ". Context: {$snippet}"; $prompt .= ". Context: {$snippet}";
} }
return trim( $prompt ); return trim( $prompt );
} }
/**
* Lazy-load the OpenRouter provider.
*
* @return PRAutoBlogger_OpenRouter_Provider
*/
private function get_llm_provider(): PRAutoBlogger_OpenRouter_Provider {
if ( null === $this->llm ) {
$this->llm = new PRAutoBlogger_OpenRouter_Provider();
}
return $this->llm;
}
} }