I have attached some comparison pictures between a GAL imported picture (i.e. imported into the PICEDIT branch I was working on) and the corresponding AGI picture from the AGI version of the game. I have done this for PICTURE 6 and PICTURE 23. The GAL picture in each case is not exactly as it would have appeared in the original KQ booter versions but rather what it looks like when converted to the equivalent AGI picture codes.
The visual screens look almost the same. In the case of PICTURE 6 at the top, there are only a few pixels different, where there is a white pixel in the GAL picture that shouldn't be white. Now this obviously isn't what it looked like in the original booter version of the game, so it suggests that the GAL line or fill algorithm is slightly different. When comparing the picture commands for the GAL vs AGI versions, the AGI version usually has additional fill commands to fill in these white pixels that shouldn't be there. This suggests that the pictures were converted to the AGI format and then touched up afterwards. You'll notice that they missed one pixel in the upper middle part of the picture though, as it is still white in the AGI version.
The visual screen for PICTURE 23 really does look identical. The GAL picture on the right is how it appears in my PICEDIT app after being imported, so the end result, at least as far as the visual screen goes, appears to be identical in this case.
Where things get very different though is in the priority screens. For starters, the most distant priority colour, i.e. the one at the back, is blue in the case of GAL but red in the case of AGI. The red, cyan and green priority colours in GAL are just normal priority band colours and don't appear to have special meaning.
You'll also notice that black is used for water in the GAL picture but cyan is used in AGI.
Another thing that is fairly obvious is that the priority band division must be different, since the same objects, e.g. the trees, log, rock, are using different priority colours despite being placed in exactly the same part of the screen. This isn't surprising, given that red, cyan and green are used as normal priority colours, so there are more colours to cover the screen.
The other big difference is that the GAL picture doesn't have any "block" control lines drawn. I have seen in some of the GAL pictures that they use the white colour as a block line, in the places where an AGI picture might use the black control line. This seems to be used quite rarely though, and it makes me think that the block control lines must be placed into the picture using a different mechanism, and not generally part of the picture itself. But, as I say, some pictures do use white for a block line, meaning that white must have that meaning in GAL, i.e. the same meaning as blank in AGI.
I think that covers all the obvious differences. It is due to these priority screen differences that a tool like PICEDIT or WinAGI can't really support importing a GAL picture. It could import the visual screen and get very close, but the imported priority screen would not be compatible with AGI v1/2/3. I wondered whether the priority colours could be changed on import, but I think that would get very tricky to get right, since the tool would be making guesses about whether the base of the object is at ground level or not. You'll notice from the PICTURE 23 example that in one case, there is an object that is light green in the GAL picture but split between two different colours in the AGI picture (light cyan and light red). This shows that when they were touching up the converted image for the AGI version of KQ1, they must have decided that they needed to change the priority colour partway up that object. Not sure why yet, but it does suggest a manual conversion of the priority screen rather than automated. Maybe they automated it to some degree, but as I say, it would be difficult to get right.
And the absence of the control lines would certainly be an issue as well.
