Common mistakes and misunderstandings, problem solving¶
The workspace must be the same as the file to be delivered.¶
This is not true: when working and calculating colors, it is necessary to work in a space “larger” than the delivery; indeed, the precision during calculations must be greater than that of the data to obtain. To be able to work in a larger space, as well in gamut* as in depth* guarantees a better respect of the colors at the time of export.
If the workspace is not larger than the output workspace, flaws may appear, such as banding, the appearance of visible bands in fine gradients.
If the workspace is linear, its depth must also be greater than that of the output workspace, and this is in any case recommended even for a non-linear workspace. Thus, for an 8 bpc output, it is preferable to work at least in 16 bpc, or in 32 bpc for an 8 bpc output.
It is equally false to believe that working in a larger space will make us select and use colors “outside” the output format. Cf. next question.
Working in a color space larger than the output color space is dangerous because we might choose colors that cannot be reproduced.¶
This is not true: even if the workspace is very large (such as ACEScg for example), the colors displayed by the screen are the result of a “live” conversion to the screen space, which is necessarily smaller (sRGB in most cases). It’s impossible to have variation at the time of the export, which will undergo exactly the same transformation.
It should be noted that the majority of the screens (except HDR and other screens in P3) cannot display the colors used in HDR and cinema; the problem is thus rather opposite: how to work on colors apart from the display space of the screen…
However, what is true is that one must be careful in the selection of colors in the rendering engines (3D): the colors chosen on the screen are colors displayed in sRGB, and it’s easy to select colors that are too intense or too saturated without realizing it, because they are actually outside the sRGB space in values. One way to overcome this problem is to make sure that the color pickers of the application are limited to sRGB for example (the application taking care of the conversion from sRGB to the workspace); in this case, the selection of an intense green for example will still be far from the extremes of the larger workspace, and the color will not risk to brighten and saturate the scene too much.
You must choose a Rec.709 display space because the video output will be Rec.709.¶
The display color space must be that of the connected screen (sRGB in most cases in computing).
This display area is used for conversion from the workspace to the screen display.
When the video file is output, a conversion is made from the workspace to the video space, Rec.709 in this example. And it is when the video is played that a new conversion takes place again from the Rec.709 to the probable sRGB of the screen.
However, some applications, especially compositing applications, allow simulation (soft-proofing or proofing*) of the conversions that the images undergo once exported; in this case, several conversions take place between the workspace and the display:
• conversion from the workspace to the (simulated) export space (Rec.709 in the example)
• conversion from export to screen display (sRGB in the example)
But in no case is there a conversion to a Rec.709 display to be made. The activation/deactivation of this simulation should not, in theory, change the display of the image; the changes are only due to the loss of precision of the successive conversions that are simulated.
When playing a video, the colors are not the same as in the software that was used to create it.¶
Here are the parameters to check:
- Display problem in the software: the display color space in the software must match that of the screen (sRGB most often).
- Problem of output : the color space of output of the video is not standard ; check the space used in output of the application.
- Playback problem : the video playback is badly configured on the graphics card (see below When a video is played on the computer (…) the colors appear “dull”.)
If a (very slight) variation persists, it may be due to the conversion from RGB to YUV, or the compression of the video format in particular. In this case, there is nothing to do…
“Banding” : colored “bands” appear in the fine gradations¶
Most often, this problem comes from the fact that the application workspace is too small and the depth* is too small compared to the output format.
Here is what to check and what can correct the problem:
- The depth in the application should be higher than the output (16 bpc minimum to output in 8 bpc, etc.), especially if the workspace is linear.
- If it’s not possible to increase the depth of the workspace, when possible a space with the same gamma* as the output one can help reduce the problem.
- Choose a workspace at gamut* that is larger than the output one.
- If the gradient is part of an imported file, check the above points in the application that created the file.
- Banding problems may occur due to compression; check if they are still visible in lossless formats (openEXR, PNG, Quicktime Animation, etc.)
- Adding a very slight, imperceptible noise or grain to the image often “fixes” this problem.
A fringe, a line that appears blurred, appears in colored or high contrast areas, especially on vertical or horizontal lines¶
This problem is usually a consequence of chrominance subsampling, when switching from RGB to YUV. If possible, increase the subsampling (4:4:4 or 4:2:2 instead of 4:2:0 for example). If this is not possible, and the image is a still image, it is sometimes possible to move the image a pixel or two in the video to correct the problem.
When you play a video on your computer, in most players and on videos on websites, the colors look “dull”. Whites are light gray, blacks are dark gray.¶
In the parameters of the graphics card, a “video” option often allows to change this setting; you have to choose Full if it is a computer screen that is connected, or Limited if it is a TV.
Sources and references