I find these passages quite interesting from Venerable Kumārajīva’s "Chán fǎ yào jiě (Essential Explanation of The Method of Dhyāna), regarding how the body begins to change to the subtle form of the rūpa-loka during Jhāna
Question: How can one recognize the signs of single-mindedness?
Answer: When the mind dwells on an image, the body would be soft, gentle, and blissful. All anger, anxiety, grief, and other afflictive mental dharmas are ceased.174 The mind acquires swift blissfulness never before experienced, which surpasses the five desires. Because the mind is pure without any defilement, the body will shine brightly. It is like a pure and clean mirror [shining] the light externally, or like the shining light of bright pearl that appears, illumines, and manifests in the pure water. After having seen these signs, the cultivator‘s mind is calm, tranquil, joyful, and delightful. It is like a thirsty person, who digs the earth searching for the water. If he sees the moist mud, then he will get the water soon. The cultivator practices in a similar way as such; at the beginning of the practice, it is like digging a dry earth for a longtime without stopping; as he sees the signs of moisture, he knows himself that he will get the water soon. Having known by oneself that one will attain the meditative samādhi soon, one must diligently concentrate, joyfully believe, gather the mind, and move it to enter the deep samādhi. Give rise to the thought that ―I have already cursed the five desires.‖ See those who seek after their desires as extremely detestable, as one sees a dog, who, unable to get good food, chews on stinking manure. According to these various conditions, you should curse desire as a fault. One‘s mind gives rise to sympathize with those who experience the five desires. Their own minds have blissfulness already, but they do not know how to seek for it. Instead, they seek for the external impurity and faulty joyfulness. Throughout day and night, the cultivator should always practice diligently various wholesome dharmas, which support the achievement of meditative samādhi.
Question: What are the marks of attaining the first dhyāna?
Answer: At first, one uses proper mindfulness to admonish and halt five desires. Although one has not attained the ground [of the first dhyāna], the mind is joyful, delightful, soft, harmonious, and gentle; the body has bright light. **When one attains the first dhyāna, its mark is that it continuously changes, increases, and excels [than before]. cause the four elements of the Desire Realm spread fully all over the body, which is soft, harmonious, gentle, and joyful signs, and the mind leaves bad desire and unwholesome deed, then the samādhi of single-minded thought can cause one having joy and happiness.183 Forms created in the Form Realm have the feature of bright light. Hence, the cultivator sees the wonderful and bright light emitting from the body internally and externally. The mind of the cultivator changes differently. Within the angry situation, one does not get angry. Within the joyful situation, one does not have [much] joy. The eight kinds of worldly dharmas cannot move the cultivator.184 Faith, respect, shame, and conscience largely change and multiply. As for the clothes, food, and drink, one does not crave and attach to them. One only considers various wholesome deeds and meritorious morality as valuable, and others are worthless. One does not attach to even the five celestial desires, how much more the five impure desires of the secular world. For those who have attained the first dhyāna, these are the features.
Again, when one attains the first dhyāna, the mind is greatly surprised and joyful. As a poor man at last acquires the treasury storage, he is greatly surprised and joyful. He thought that: ―During the beginning, middle, and last watches of the morning, I have cultivated diligently and ascetically the first dhyāna. Now I have attained the good retribution, which is true without falsity. These wonderful and joyful experiences are as such, but other sentient beings are insane, confused, stubborn, and foolish. They are sunk into the impurity and non-blissfulness of five desires. How pitiful they are.‖ The blissfulness of the first dhyāna is spread all over the body internally and externally. As the water soaks into dry earth, it is wet and moist inside and outside. The experience of blissfulness of the Desire Realm cannot spread through the body and mind. The fire of sexual desire and anger in the Desire Realm burns the body. Entering the cooling and blissful pool of first dhyāna is the foremost way in extinguishing the fire of mental afflictions. As when it is too hot, one jumps into the cooling and pure pool. After one has attained the first dhyāna already, one thinks about the original practice of spiritual path or other conditions, namely the samādhi of Buddha name‘s recitation, or the mindfulness of the body‘s impurity, or the contemplation of the loving-kindness mind, or others. Why is that? Utilizing the power of contemplation helps the cultivator to attain the meditative samādhi and again enter deeply. Then, the original contemplations will become many times more pure and clear.
There is a form of nimitta here, but there is also a bodily experience. The body and the external elements start to being subtle, translucent-like but a physical side to the experience still remains. Rather than a mental image, the body and external forms are becoming transformed to the meditator. We also see something like this talked about in the Visuddhimagga, but its under the section on “corruptions of insight”, where the body is blissful and starts to emit light.