If David Hockney had a diary, he points out, every day's entry would read the same: painting and drawing. At 76 he continues to work at an unflagging pace, as I observed during a recent stay at his home in Los Angeles.
Every few days he finishes another in the epic cycle of portraits on which he has recently embarked. "I am working on a roll now," he says, "and can't really stop."
The portraits come on the heels of a decade-long project which saw Hockney living in Bridlington, Yorkshire, while depicting with enormous energy the quiet scenery of the Wolds. He worked in an astonishing range of media, including oils, watercolour, iPhone, iPad, and high-definition video.
But all those, he says, depended on drawing, so it is fitting that the final works he made in those years were a sequence of large-format landscapes in one of the most time-honoured of graphic techniques: charcoal. Titled The Arrival of Spring 2013, the drawings have gone on show in London's Annely Juda Fine Art Gallery (to July 12), alongside pictures from his previous series, The Arrival of Spring in Woldgate, East Yorkshire, in 2011, drawn on his iPad and first shown at the Royal Academy.
When I visited LA, the walls of Hockney's living rooms were hung with full-scale photographic reproductions of the charcoal drawings. "This," he said one day, gesturing towards the images, "is one of my great works." He's not wrong. The Arrival of Spring 2013 is a triumph of landscape drawing, and a peak in Hockney's art. The monochrome drawings seem to hold all the richness of light and the texture of foliage that Hockney's paintings do - perhaps more.
The achievement is all the more extraordinary considering that The Arrival of Spring 2013 was completed under the most difficult of circumstances. In the autumn of 2012 Hockney had a minor stroke which left him almost unable to speak. His speech quickly returned, although he continues to feel less talkative. Shortly afterwards one of Hockney's favourite subjects, a high tree stump in Woldgate Woods, which he called the Totem, was felled by vandals with a chainsaw. The stump, standing like a silent watcher in the landscape, at the height of a person, had a distinctly human air.
It is not hard to guess that the Totem had been a surrogate for the artist himself. Now that it was lying on the ground that seemed even more the case. When Hockney was told the news he took to his bed in a black depression. He has described his state of mind at that time as being "very dark and I felt about as bad as I had in many years".
After two days, Hockney got up and began drawing the vandalised Totem in charcoal. These drawings of the felled trunk in a bare, midwinter landscape have a sombre, elegiac feeling to which soft strokes of black and grey are perfectly matched.
The drawings were a prelude to The Arrival of Spring on which Hockney began work the following January. He began to chart the seasonal changes from five familiar vantage points. Two months into the series, however, Hockney was hit by another appalling blow: the sudden death of Dominic Elliot, one of his assistants. Subsequently, Hockney almost gave up. Keeping going, he told me later, had been "hard work".
But persevere he did, making images that seem to buzz with returning life. The whole sequence of charcoal drawings feels like both a climax and a farewell: they are filled with the knowledge Hockney gained in all those years of looking harder and harder at these places, but also with a sense of loss.
Hockney has been reflecting on mortality of late, and coming up with some typically unconventional thoughts on the subject. "Why is everything now geared to longevity?" he asks. "If everything's directed at longevity, you're denying life." He approves of a remark that the photographer Henri Cartier-Bresson made at the age of 93: "It's not longevity, it's intensity that counts."
Hockney reflected: "It's a different attitude to time. That's what I have. We all get a lifetime. They're different, but we all get one."
Shortly after completing The Arrival of Spring, he departed for California. In the autumn, during a break in the installation of his exhibition in San Francisco, he returned to Los Angeles. "There was nothing for me to do for a week or two, so I came back and painted an old friend." Then he did another portrait, and another. Thus his latest project began.
This is, in a way, the reverse of The Arrival of Spring: indoor not outdoor, and painting in acrylic, not drawing. Once more Hockney has switched mediums to explore the possibilities of something new. For the portraits he has returned to acrylic, in which he has been depicting a diverse range of sitters, all in the same format, seated in the same chair on a low stage.
When I was visiting he had just decided to dub the sequence La Comedie humaine. Hockney was reading a biography of Balzac and, musing on time and intensity, took the French novelist as an example. "Look at Balzac, dead at 51, but he produced 100 years of writing."
The portraits, which the artist feels are not yet ready to exhibit, already number several dozen. "I'll just go on until I'm bored, and I think it will be a long time," he says. "I might do 100. I work in series and sequences. I always need a project to get me going as this has got me going now. It's something I can see is endlessly fascinating."
In an email the other day, Hockney reflected with satisfaction: "The studio is looking very good, indeed better than it's ever looked before. I like to think the late work is the best, but I really think so at the moment."