Saving PNG is extremely slow

[Micha]

Hi,

since a long time I have the problem that saving the output from the frame buffer is extremely slow. I thought it's a problem of PNG. But I observed that saving the same PNG, which needs approx. 49s to save from the frame buffer can be saved in approx. 7s from Photoshop (6000x3000). And if several channels needs to be saved - RGB, denoiser and result - all together needs 2..3min. Please try to bring down this times like PS do it. It's also important for rendering animations, where the difference between the time to save and the time to render an image is smaller and smaller. So, it can be that 50% the time for an animation is needed for saving.

[Gijs]

It looks like this has to do with image compression. Try to save out a png of that size with highest compression in photoshop, and you'll find similar times. So maybe they can add options for image compression to resolve this.

[Micha]

Can you test to save a PNG per PS too? Maybe I did something wrong. I chose "quick and large", but got quick and small.

[Gijs]

I just tried a file that saved a 7.49MB file from V-Ray. From Photoshop without compression this one becomes 7.98 MB, with medium compression 7.42 and slowest 7.41 MB. Both these are slower than the output from V-Ray. So to me it is pretty sure that V-Ray is using some compression and this makes the saves slower. Hope this gets addressed, I prefer speed over size.

[Micha]

OK, I did a new test - a large panorama image 16000x8000 16Bit 567MB.

PS fast&small 569MB 0:57 min:s
PS best&slow 568MB 4:35 min:s

V-Ray Curves+Exp used 255MB 4:40
V-Ray Exposure only 2:40
V-Ray nothing changed 2:40

I wonder that the V-Ray file sizes are so small and I found my loaded 16Bit PNG was saved in 8bit only. But this test showed, the VFB curve control slow down the save process and approx. doubled the time.

Now I converted the image to 8Bit and did the PS tests again:

PS fast&small 223MB 0:35
PS best&slow 223MB 2:29

OK, it looks like V-Ray compression isn't the best - it's nearly the same save time like at PS if no curve control is used. The big questions are - what would be if the V-Ray compression could be set to fast&big? How can the compression be changed to fast&big? Can the speed enhanced for using the curve correction?

[Micha]

Hi,

maybe the topic is overseen - is there a chance that the curve control doesn't slow down the save time? It's important for saving large renderings and for animations. Today I did an animation render test at low output res and the frames was so fast rendered ... it could be really great to get more speed for the save process.

[yavor.rubenov]

Do you get the slow down with a specific curve or having any curve causes the slow down ? If it is a specific curve - can you send it over ?

[Micha]

It's any curve. It's like the image processing is slowing down the save process.

Take a scene and render it at 6000x3000 (you don't need to render until it's finished, a noisy preview is fine) and save it with and without curve enabled. I get approx. 30s vs. 15s.

[yavor.rubenov]

It is indeed expected to slow down the process - we must apply the curve for every pixel of the final image. When saving to file we use a precise but slow method to calculate the curve for each pixel of the full resolution image (so 6000x3000 in your case). When doing image preview for the frame buffer we use a faster but less precise calculation and only apply it to the visible pixels so it is much faster.

We'll see if we can improve the file save case.

[Micha]

An improvement could be great. CPU and GPU are so powerful, but both don't show a strong usage. If I render large images and need to save all channels like Denoise, Effects and RGB than it can be that the save process needs a very long time (a few minutes, so that I must make a break.)

I think the problem will increase over the time, since render times goe's down. For example rendering an animation at full HD could be slowed down by the save process if the save time is 5s and the render time per frame 10s.

Good luck.