Question

Working in Visual Studio 2008. I’m trying to draw on a PNG image and save that image again.

I do the following:

private Image img = Image.FromFile("file.png");
private Graphics newGraphics;

And in the constructor:

newGraphics = Graphics.FromImage(img);

Building the solution gives no errors. When I try to run it, I get this:

A Graphics object cannot be created
from an image that has an indexed
pixel format.

I don’t have much experience with using images in C#. What does this mean and how can I remedy this?

EDIT: through debugging, Visual Studio tells me that the image has a format8bppindexed Pixel Format.

So if I can’t use the Graphics class, what do I use?

EDIT2: After reading this, I think it’s safe to assume that I better stick to JPG files when working with GDI+, no?

EDIT3: my using-statements:

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Drawing.Imaging;
using System.Windows.Forms;

Answer 1

Without a better PNG library that supports indexed PNGs you’re out of luck trying to draw to that image since evidently the GDI+ graphics object doesn’t support indexed images.

If you don’t need to use indexed PNGs you could trap that error and convert your input to regular RGB PNGs using a 3rd party utility.

edit:

I did find this link http://fci-h.blogspot.com/2008/02/c-indexed-pixel-problem.html that gives a method to draw on your image, however it won’t affect the original, just a copy you can Save() if you require.

In case the link goes down:

Bitmap bm = (Bitmap) System.Drawing.Image.FromFile("Fci-h.jpg",true);
Bitmap tmp=new Bitmap (bm.Width ,bm.Height );
Graphics grPhoto = Graphics.FromImage(tmp);
grPhoto.DrawImage(bm, new Rectangle(0, 0, tmp.Width , tmp.Height ), 0, 0, tmp.Width , tmp.Height , GraphicsUnit.Pixel);