If a Poser scene has:

• A main camera, which is being used for the current image.
• An object P which has a textured surface P2 which is visible on the main camera.
• A camera Dolly2, parented to some object such as a gun, e.g. to see what the gun's sight sees. Can I, within one Poser run (probably needing a Python script), render what Dolly2 sees, then feed the resulting rendered image into one of textured surface P2's texture nodes, then make the main render using the main camera?

I don't know if you had this question answered anywhere else, but it seemed like a nice little programming exercise, so I gave it a go. I chose a primitive, one sided square called 'square_1' as object 'P' and its material name is 'Preview' (P2) with an additional specification of the image_map node name 'Diffuse Map', so you could work with multiple images in that material and only change the right one. I've used 'Dolly Camera' as the gun sight cam, and 'Main Camera' as the render cam, with configurable resolutions for both of them (set as X,Y tuples).

Here's the script: (save as GunSightRender.py)

# GunSightRender.py
# (c) 2019 an0malaus (Geoff Hicks/GeoffIX/gwhicks)
#
# Requested on Renderosity by AnthonyAppleyard 2018
#
# v1.0  20190622    Initial version.
########################################################################################################################

version = '1.0'

import poser
import os

imagePath = 'Somewhere/on/your/computer/gun view.jpg'
imageType = 'jpg'
imageQuality = 80
#gunCamName = 'Dolly2'
gunCamName = 'Dolly Camera'
gunCamRes = (256,256)
renderCamName = 'Main Camera'
renderRes = (512,512)
picturePropName = 'square_1' # 'P'
pictureMatName = 'Preview' # 'P2'
imageNodeName = 'Diffuse Map'
imageNodeType = poser.kNodeTypeCodeIMAGEMAP
imageSrc = 'Image_Source'

scene = poser.Scene()
gunCam = scene.Actor(gunCamName)
assert gunCam and gunCam.IsCamera(), 'Camera "{}" not found!'.format(gunCamName)

renderCam = scene.Actor(renderCamName)
assert renderCam and renderCam.IsCamera(), 'Camera "{}" not found!'.format(renderCamName)

pictureProp = scene.Actor(picturePropName)
assert pictureProp and pictureProp.IsProp(), 'Prop "{}" not found!'.format(picturePropName)

mat = None
imageNode = None
try:
    mat = pictureProp.Material(pictureMatName)
except:
    pass
assert mat, 'Prop "{}" has no material "{}"!'.format(picturePropName,pictureMatName)

found = False
for layer in mat.Layers() or []:
    tree = layer.ShaderTree()
    for node in tree.Nodes() or []:
        if node.Type() == imageNodeType and node.Name() == imageNodeName:
            found = True
            break
    else:
        continue # try next layer
    break # Only get here if found
assert found, 'Prop "{}", material "{}" has no "{}" node "{}"!'.format(picturePropName,pictureMatName,imageNodeType,imageNodeName)

scene.SetCurrentCamera(gunCam)
scene.SetOutputRes(*gunCamRes) # Resolution of render output, tuple unpacked into parameters
scene.Render()
scene.SaveImage(imageType,imagePath,imageQuality)
assert os.path.exists(imagePath), 'Saved render "{}" not found!'.format(imagePath)

input = node.InputByInternalName(imageSrc)
input.SetString(imagePath)
tree.UpdatePreview()
scene.DrawAll()
scene.SetCurrentCamera(renderCam)
scene.SetOutputRes(*renderRes) # Resolution of render output, tuple unpacked into parameters
scene.Render()
print 'Done.'

### END ###

You'll need to put a real path in the imagePath string. I used a known folder on my Mac, which won't work for anyone else. Here's the material setup for the object P (square_1). Note the name set to 'Diffuse Map' which identifies which image_map node will be updated. Here's the view from the gun sight cam (Dolly Camera) before rendering

and here's the final render from the Main Camera

showing the texture automagically loaded onto the square prop.

Cheers, an0malaus

Thanks.