I spent a great deal of time trying to figure out how to create 7800basic compatible graphics with my first game, so I thought I'd share the steps I took to make it work. Using Gimp is the easiest way to get started (you can download it here).
First, open a 7800basic palette file, which you can download here.
Next, click on Windows->Dockable Dialogs->Palettes, we need the Palettes dialog box to be open.
Next, right click on the Palettes dialog box and click Import Palette.
Once the window is open, click on Image, Palette name (rename to Atari7800), and click on Import.
Click on Image->Mode->Indexed.
Click on Generate Optimum Palette and change the number to 13, then click on Convert.
Change setting back to RGB, just so we can go back and change it to Indexed again to make one more modification…
Click on Image->Mode->Indexed again to make one more change.
Click on Use custom palette, then the button next to it.
Click on the 7800palette that was just added, then Convert.
Click on open image, and open the tileset_rocks image from one of RevEng's samples in the distribution.
Click on View->Zoom->800% so you can actually see the image on the screen.
Resize the window so you can see the entire image.
Click on Windows->Dockable Dialogs->Brushes (we need the brushes dialog box open).
Change brush size to 1.0. Go to Windows Menu->Toolbox or press CTRL+B to open the Toolbox Tool Options if you don't see it on screen.
Click on color picker tool and choose one of the four colors to modify the image. The colors in this image don't matter yet, as they are changeable with the PxCx command in the code. One of the colors is transparent and will take the background color.
Click on the brush tool to change the image.
Use the brush tool to modify a 16x16 area, there are three side by side in the samples from RevEng, each is 16 pixels across. Don't worry about the colors, as they are set with the PxCx commands in software. You can use the same colors in the sample images.
When you're done, click on File->Export As to save.
Export the image as a PNG file and you're ready to go.
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