Resampling and its effect on color
We can clearly see in many cases of resampling that some pixels are made from the average of several others. This is particularly true in upsample using cubic weighting. We have also said earlier that averaging two pixels in any color space can cause some odd results so this section is some gotchas to look out for and what to do about them. This is an area that I am least familiar with since I only use RGB and dont bother trying to fix these but its worth knowing anyway.
We noted that a blend from red to blue in RGB color space would give deep mauve - ok but look at this. This is an up sample of some colors which have some issues. Some of the corners here are pure green but the other is red & blue (magenta). When we average these two colors together the red & blue from one image drop halfway and the green from the other drops halfway. End result - all halfway to give mid gray ! Now its worth noting that for resample types that dont allow blends like the nearest neighbor then you dont get the issue. Also, it turns out that if you apply a gamma adjustment to the image, resample with a cubic and the apply and reverse gamma adjustment then the problem gets fixed. I havent fully convinced myself why this is true but I am assured it is.
Upsampling curvy black and white stuff
Reserved space - but this one came up recently too.
EDIT -- actually this comes up a lot and again today so here we go again... how to resample up just B&W stuff.
First, upsample anyway you like in factors of double (200%) in stages until one more is less than double (say 1.7).
For each stage double the size of the image which makes it pixellated - a nearest neighbor / pixel resize is fine.
Next, blur it - preferably using gaussian blur. The amount of blur radius can be experimented with but about a factor of 3 pixels or so.
Then use contrast to clamp it back to being B&W again. Actually I use about 95% or so not 100% but its up to you. Dont brighten it or darken it when you do it, just up the contrast.
Keep doing this in stages until the last stage is <200% in which case you might want slightly less blur than usual but not by much.
Here is the results. Everything is cool except for where there is an acute internal angle where it tends to start filling in depending on the amount of blur used. So less blur helps, but more blur is better to get rid of pixellation. You have to experiment with it.