Fastest way to create a palettized image?

[TheRealQuantam]

I'm trying to use ImageMagick to save palettized images generated by my program. Currently I'm using the following process:

1. Create the image with MagickImage img = new(MagickColors.Green, (uint)Width, (uint)Height);
2. Set img.ColorType = ColorType.Palette; and img.ColormapSize = 256;. Note that for the purpose of my issue (slow), it does not matter if step 2 is merged into the construction step 1.
3. Copy the pixel data from my buffer to the image with SetByteArea, passing 4 identical bytes of the palette index for each pixel (so I don't have to care which byte is used to store the palette index).
4. Set up the palette with SetColormapColor

This works, but step 2 (specifically setting ColorType is EXTREMELY slow; for the image I'm working with (8192x7680) it takes 18 seconds. Step 1 is also slower than I'd expect, taking about 2 seconds. How can I drastically speed up this process?

[TheRealQuantam]

Manually constructing a TGA file in a buffer (almost instantaneous) then passing it to the MagickImage constructor reduces the construction time to about 2 sec; not great, but tolerable. However this really isn't an answer to the question as the entire point of using Magick.NET was so that I wouldn't need to write any file format code...

[dlemstra]

Right now there is no way to create an image with a colormap without taking the steps that you wrote. But I could probably add a method that creates an image that has a colormap with a single color. This should be possible with the internal api of ImageMagick.

am not sure what would be a good place to add this to the current api. I don't think I can/want to add it as a constructor overload. Maybe as a setting inside the MagickReadSettings. But that would only make sense for the color constructor. An other option would be adding a new method to the MagickImageFactory that can be used for this. This looks like the best idea for now. Do you have any ideas on how I should add this to the api?

[TheRealQuantam]

I'm not the best person to ask as this is my first time using ImageMagick, but if it were me I'd be inclined to make it a new constructor (actually I wasn't even aware of MagickImageFactory until you mentioned it). There's already a constructor for a blank image of specified size with a single color, and it seemed odd to me that it didn't allow you to specify a color mode.

Though even if you leave the API exactly the same if the step to change to palettized were fast it wouldn't be an issue to begin with. I'm not at all clear why it takes that long at all, when the entire image is a single color.

While we're at it, if there isn't already a way to do it that I don't know (and there very well might be), it would be good to make a function for IPixelCollection that took a simple byte array/list (1 byte per pixel) and would put it in the right place for a palettized image, rather than having to manually expand it into multiple bytes per pixel.