Hi im just wondering if it's possible to add my own in-game gui without editing the ingamegui.class,

 

My plan is similar to the pumpkin how it adds a pumpkin gui shadow when added the helmet slot.

 

or has anyone got a different idea of doing this but with the same output?

Short answer:

Yes

 

Long answer:

You need at least three classes to do it.

 

The item or block itself

Packet Handler implements IPacketHandler

Gui Renderer  implements ITickHandler

 

These two lines in your base Init function:

overlayGui = new MyGUIrenderer();
TickRegistry.registerTickHandler(overlayGui, Side.CLIENT);

 

And this will get you drawing something like the pumpkin head.  Actually, 95% of this is the pumpkin head.

 

            	u = 32;
                float minU = (float)u/512; //my texture was 512x512
                float minV = 0;
                float maxU = (float)(u+32)/512;
                float maxV = 0.0625F;
            	ScaledResolution scaledresolution = new ScaledResolution(this.mc.gameSettings, this.mc.displayWidth, this.mc.displayHeight);
                int k = scaledresolution.getScaledWidth();
                int l = scaledresolution.getScaledHeight();
                GL11.glDisable(GL11.GL_DEPTH_TEST);
                GL11.glDepthMask(false);
                GL11.glBlendFunc(GL11.GL_SRC_ALPHA, GL11.GL_ONE_MINUS_SRC_ALPHA);
                GL11.glColor4f(1.0F, 1.0F, 1.0F, 1.0F);
                GL11.glDisable(GL11.GL_ALPHA_TEST);
                this.mc.renderEngine.bindTexture("/mods/MyMod/textures/gui/gui.png");
                Tessellator tessellator = Tessellator.instance;
                tessellator.startDrawingQuads();
                tessellator.addVertexWithUV(0.0D,      (double)l, -90.0D, minU, maxV);
                tessellator.addVertexWithUV((double)k, (double)l, -90.0D, maxU, maxV);
                tessellator.addVertexWithUV((double)k, 0.0D,      -90.0D, maxU, minV);
                tessellator.addVertexWithUV(0.0D,      0.0D,      -90.0D, minU, minV);
                tessellator.draw();
                GL11.glDepthMask(true);
                GL11.glEnable(GL11.GL_DEPTH_TEST);
                GL11.glEnable(GL11.GL_ALPHA_TEST);
                GL11.glColor4f(1.0F, 1.0F, 1.0F, 1.0F);