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page"},{"oldId":"b-454fef6ab484818ed13d9957","targetId":"b-454fef6ab484818ed13d9957","page":"h-i-pca-task-information","operation":"move","beforeSHA256":"23fb328ddea3c580d00a3d6063033a3a285a22d74ef6bf1a9caf5a2b575485d0","afterSHA256":"23fb328ddea3c580d00a3d6063033a3a285a22d74ef6bf1a9caf5a2b575485d0","reason":"Same source body in authored semantic page"},{"oldId":"b-dc2cf8a73d110b9f1f8af1c7","targetId":"b-dc2cf8a73d110b9f1f8af1c7","page":"h-i-pca-task-information","operation":"move","beforeSHA256":"3ee6b967c2180d7471f95f260440fb5df461b94fb66bc1cad9c30e0cf8519c32","afterSHA256":"3ee6b967c2180d7471f95f260440fb5df461b94fb66bc1cad9c30e0cf8519c32","reason":"Same source body in authored semantic 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[复制输入为何不是好表示｜核心改动怎样工作](/supplement.html?return=l3-r2-p19/0#/l3-autoencoder-denoise-mechanism/0) 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[复制输入为何不是好表示｜一起算一个完整例子](/supplement.html?return=l3-r2-p19/0#/l3-autoencoder-denoise-example/0) 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重建误差最小，未必保留任务所需差异","sha256":"9087c28fd76e5e2fed0eb5a5ed4539e624058d2e3664168f94241f25c33f8191","chapter":{"id":"I","title":"学习信号与表示"},"unit":{"id":"i-masked-prediction","title":"限制可见信息，怎样构成可验证的重建任务？"},"page":{"id":"h-i-pca-task-information","title":"重建误差最小，未必保留任务所需差异","number":2},"sectionTitle":"重建误差最小，未必保留任务所需差异","currentQuestion":"限制可见信息，怎样构成可验证的重建任务？","sourceSlides":["l3-r2-p19"],"knowledgeBlock":null,"formulas":[],"previousBlock":null,"nextBlock":"b-8aae2bc79f501ae2b0dded51","prerequisites":[{"id":"b-8aae2bc79f501ae2b0dded51","url":"#b-8aae2bc79f501ae2b0dded51","role":"problem-context"}],"relatedBlocks":[]},{"id":"b-8aae2bc79f501ae2b0dded51","type":"p","scope":"h-i-pca-task-information","url":"https://codingai-lec03.pages.dev/course/pages/l3-r2-p19.html#b-8aae2bc79f501ae2b0dded51","content":"用四个确定点，把重建目标与未来分类目标分开。","sha256":"1890f0f99ff32e3aad93ad0af20d3407ad1320a23233c0e0a964e68ecb92b9dd","chapter":{"id":"I","title":"学习信号与表示"},"unit":{"id":"i-masked-prediction","title":"限制可见信息，怎样构成可验证的重建任务？"},"page":{"id":"h-i-pca-task-information","title":"重建误差最小，未必保留任务所需差异","number":2},"sectionTitle":"重建误差最小，未必保留任务所需差异","currentQuestion":"限制可见信息，怎样构成可验证的重建任务？","sourceSlides":["l3-r2-p19"],"knowledgeBlock":null,"formulas":[],"previousBlock":"b-18d17895eb2ba4c95671efea","nextBlock":"b-e37728f5558f62dff8a9098f","prerequisites":[{"id":"b-18d17895eb2ba4c95671efea","url":"#b-18d17895eb2ba4c95671efea","role":"section-title"}],"relatedBlocks":[]},{"id":"b-e37728f5558f62dff8a9098f","type":"h3","scope":"pca-reconstruction-worked-example","url":"https://codingai-lec03.pages.dev/course/pages/l3-r2-p19.html#b-e37728f5558f62dff8a9098f","content":"### 保留最大方差，不等于保留任务答案","sha256":"664d300085b6fa401332b40e632f3859ec92475c6506f200390ca1c98a727720","chapter":{"id":"I","title":"学习信号与表示"},"unit":{"id":"i-masked-prediction","title":"限制可见信息，怎样构成可验证的重建任务？"},"page":{"id":"h-i-pca-task-information","title":"重建误差最小，未必保留任务所需差异","number":2},"sectionTitle":"保留最大方差，不等于保留任务答案","currentQuestion":"限制可见信息，怎样构成可验证的重建任务？","sourceSlides":["l3-r2-p19"],"knowledgeBlock":"pca-reconstruction-worked-example","formulas":[],"previousBlock":"b-8aae2bc79f501ae2b0dded51","nextBlock":"b-40f305e6ba975ab93dcf0b96","prerequisites":[{"id":"b-18d17895eb2ba4c95671efea","url":"#b-18d17895eb2ba4c95671efea","role":"section-title"},{"id":"b-8aae2bc79f501ae2b0dded51","url":"#b-8aae2bc79f501ae2b0dded51","role":"problem-context"}],"relatedBlocks":[{"id":"b-40f305e6ba975ab93dcf0b96","url":"#b-40f305e6ba975ab93dcf0b96","relation":"same-reasoning"},{"id":"b-454fef6ab484818ed13d9957","url":"#b-454fef6ab484818ed13d9957","relation":"same-reasoning"},{"id":"b-dc2cf8a73d110b9f1f8af1c7","url":"#b-dc2cf8a73d110b9f1f8af1c7","relation":"same-reasoning"}]},{"id":"b-40f305e6ba975ab93dcf0b96","type":"p","scope":"pca-reconstruction-worked-example","url":"https://codingai-lec03.pages.dev/course/pages/l3-r2-p19.html#b-40f305e6ba975ab93dcf0b96","content":"取四点 $(2,0),(-2,0),(0,1),(0,-1)$。均值为零，按四个样本取平均的协方差为diag(2,0.5)。PCA只保留一维时选择横轴：前两点不变，后两点投影为原点。平均平方重建误差为 $(0+0+1+1)/4=0.5$；若选纵轴，则为2。","sha256":"afb4b88eb02b7c896b6906f158c3b8e8f4b422a9c200eba1c04397589e0b2461","chapter":{"id":"I","title":"学习信号与表示"},"unit":{"id":"i-masked-prediction","title":"限制可见信息，怎样构成可验证的重建任务？"},"page":{"id":"h-i-pca-task-information","title":"重建误差最小，未必保留任务所需差异","number":2},"sectionTitle":"保留最大方差，不等于保留任务答案","currentQuestion":"限制可见信息，怎样构成可验证的重建任务？","sourceSlides":["l3-r2-p19"],"knowledgeBlock":"pca-reconstruction-worked-example","formulas":[{"latex":"(2,0),(-2,0),(0,1),(0,-1)","definitionContext":"取四点 $(2,0),(-2,0),(0,1),(0,-1)$。均值为零，按四个样本取平均的协方差为diag(2,0.5)。PCA只保留一维时选择横轴：前两点不变，后两点投影为原点。平均平方重建误差为 $(0+0+1+1)/4=0.5$；若选纵轴，则为2。\n在“尽量重建输入”这个题目下，横轴合理。但若任务要区分纵坐标正负，两点落在原点后就无法再区分。问题不在PCA算错，而在重建目标和未来用途不完全一致。\n这个反例同样提醒我们理解深层重建：低像素误差是一个有意义的成绩，却不是所有语义任务的替代指标。需要另外检验表示能否支持类别、颜色或空间读出。"},{"latex":"(0+0+1+1)/4=0.5","definitionContext":"取四点 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架构 | 监督训练 | FCMAE预训练后微调 |\n| --- | --- | --- |\n| V1-Base | 83.8 | 83.7 |\n| V2-Base | 84.3 | 84.6 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沿这个问题继续","sha256":"cf44fcc62782fb4f6bdc42c16c5187d57556a03cb7ce6065e7bd9c36d8e7d733","chapter":{"id":"I","title":"学习信号与表示"},"unit":{"id":"i-extensions","title":"表示学习的选读问题"},"page":{"id":"h-i-architecture-and-target","title":"选读：结构和重建目标需要共同检验","number":4},"sectionTitle":"选读：结构和重建目标需要共同检验","currentQuestion":"哪些细节需要进一步对照？","sourceSlides":["l3-r2-p19","l3-r2-p22"],"knowledgeBlock":null,"formulas":[],"previousBlock":"b-a986ded404b9643d3c19dc3c","nextBlock":"b-ff51fa502d11016b37694b08","prerequisites":[{"id":"b-9f29332cd029e6f534a361a9","url":"#b-9f29332cd029e6f534a361a9","role":"section-title"},{"id":"b-2e612e34c0c5842b28cab417","url":"#b-2e612e34c0c5842b28cab417","role":"problem-context"}],"relatedBlocks":[]},{"id":"b-ff51fa502d11016b37694b08","type":"p","scope":"practice","url":"https://codingai-lec03.pages.dev/course/pages/l3-r2-p19.html#b-ff51fa502d11016b37694b08","content":"CLS 05提供连续推导；想做最透明优化选LAB 05，想做图像模型选LAB 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按问题回查的讲解与短练习（1）","sha256":"0480869f0a66009b015a757d67aed6061011e378987f98a024f4e8bfc5386152","chapter":{"id":"I","title":"学习信号与表示"},"unit":{"id":"i-extensions","title":"表示学习的选读问题"},"page":{"id":"h-i-architecture-and-target","title":"选读：结构和重建目标需要共同检验","number":4},"sectionTitle":"选读：结构和重建目标需要共同检验","currentQuestion":"哪些细节需要进一步对照？","sourceSlides":["l3-r2-p19","l3-r2-p22"],"knowledgeBlock":null,"formulas":[],"previousBlock":"b-ff51fa502d11016b37694b08","nextBlock":null,"prerequisites":[{"id":"b-4b89afe2366cd5948483bb3f","url":"#b-4b89afe2366cd5948483bb3f","role":"section-title"},{"id":"b-2e612e34c0c5842b28cab417","url":"#b-2e612e34c0c5842b28cab417","role":"problem-context"}],"relatedBlocks":[]}]}