Friday, October 27, 2017

Pitting Profilers against each other.

In the past, I have been using Remotery, an in-app profiler. I recently became aware of Minitrace, a similar app. So I decided to compare results.

The good news is that when my ray tracer is working in single-threaded mode, the results are in agreement. 6ms or so is spent on uploading the image as texture to OpenGL. The rest of the time is spent rendering scanlines.


I can also run my app in multi-threaded mode. The scanlines are then rendered in 100 work batches. The batches are processed by four worker threads, that are alive during the lifetime of the app.


The Minitrace run shows that the worker threads were fully busy during the generation of the image. Sometimes, I see a chunk that take a lot more time (> x10) than normal, which made me doubt the measurement. This was the reason I decided to compare to Remotery. However, now I no longer think this is a bad measurement. One of the worker-threads probably got pre-empted by the OS or something.

The Remotery run, on the other hand, seems to be missing data? Could it be a race-condition between worker threads trying to record events? I'll be creating a github issue, but wrote this blog post first, so that the images are hosted.

OS: 64bit Ubuntu Linux.
CPU: Intel(R) Core(TM) i5-4570 CPU @ 3.20GHz
Both Minitrace and Remotery latest version from github as of oct27, 2017.

Thursday, October 26, 2017

sRGB Colour Space - Part Deux.

In part one, I described what you need to do after simulating light before you can show it on a monitor. You need to brighten it by going from linear colour space to sRGB colour space.

Here I talk about how to avoid polluting your calculations with non-linear data. Light intensities can be added or multiplied. But sRGB values cannot. As Tom Forsyth puts it: just as you would not add two zip files together, you would also not add sRGB values.

If you were to take a photograph of a certain material, the image from the camera will typically be in the sRGB colour space. If you want to render a 3D object that has this texture applied to it, then for the lighting calculations you need to get a texel in the linear colour space.

Fortunately, OpenGL can help you out here. If you sample from an sRGB encoded texture, there will be an automatic sRGB->linear conversion applied, so that after the texel fetch, you can actually do calculations with it. To trigger this automatic conversion you need to pass the correct parameters when creating the texture using the glTexImage2D() function. Instead of using GL_RGB8 or GL_RGBA8, you specify the internal format as GL_SRGB8 or GL_SRGB8_ALPHA8. There are also compressed variants: GL_COMPRESSED_SRGB and others.

Be careful that you do not use sRGB textures for content that is not in sRGB. If the content is linear, like maybe a noise map, or a normal map, then you don't want OpenGL meddling with that content by doing a sRGB to Linear conversion step. This kind of data needs to be in a texture with linear colour encoding.

Lastly, when creating mip maps for sRGB, you need to be careful.

sRGB versus Linear Colour Space

I keep screwing up my colour spaces, so I forced myself to write down the rationale behind it. A lot of it comes from Tom Forsyth.

CRT monitors respond non-linearly to the signal you drive it with. If you send it value 0.5 you get less than half the brightness (photons) of a pixel with a 1.0 value. This means, you cannot do light calculations in sRGB space. You need to do them in a linear space.

Once you have calculated the light for your rendered image, you need to send it to the monitor. LCD monitors are made to respond the same way as the old CRT monitors. So the values you send to your framebuffer will end up producing too few photons (too dark.)

To account for this, you need to convert your rendered image from linear colour space to sRGB colour space. This means that all dark pixels need to be brightened up. One way to do this, which avoids manual conversion, is to have OpenGL do this for you. You create a framebuffer that is sRGB capable. With SDL2 you do this with the SDL_GL_FRAMEBUFFER_SRGB_CAPABLE flag to SDL_GL_SetAttribute() function. In iOS you can use kEAGLColorFormatSRGBA8 drawable property of the CAEAGLLayer.

Once you have this special framebuffer, you tell OpenGL Core Profile that you want the conversion to happen. To do this, you use glEnable( GL_FRAMEBUFFER_SRGB );

Note that OpenGL ES3 does not have this glEnable flag. If the ES3 framebuffer is sRGB capable, the conversion is always enabled.

When my renderer does the lighting calculations, it will work in a linear colour space. After rendering, it would produce this linear image:

For proper display on a monitor, we need to account for the monitor's response curve, so we change it into the sRGB colour space. After conversion, the dark colours are brighter:

Yes, much brighter! But hey! What's up with those ugly colour bands? Unfortunately, by converting the values into sRGB, we lose a lot of precision, which means that 8-bit colour channels are no longer adequate. In 8-bits, the three darkest linear values are 0x00, 0x01 and 0x02. After converting these values to sRGB, they are mapped to 0x00, 0x0c and 0x15. Let that sink in... there is a gap of "1" between linear colours 0x00 and 0x01, but a gap of "12" between corresponding sRGB neighbours.

So when we convert from linear to sRGB, we should never convert from 8bit linear to 8bit sRGB. Instead, we convert using floating point linear values. If OpenGL is rendering to a sRGB capable framebuffer, it just needs to read from floating point textures. In my game, the ray tracer now renders to a floating point texture. This texture is then put on a full screen quad onto the sRGB framebuffer, resulting in a correct image:

And that's it for this installment, people. In the future I could perhaps gain more precision by rendering to a framebuffer with 10 bit channels. But for now this is good enough.

Please see part two of this blog post series where I explain what you need to do if you sample textures in your shader and want to do light calculations on that.