I recently got a custom SkyRenderer working by copying the vanilla overworld code, and have been figuring out how it works. I created a third celestial object using the same rendering code used for the sun and moon, and for the most part, it functions perfectly:

        GlStateManager.enableTexture2D();
        GlStateManager.tryBlendFuncSeparate(GlStateManager.SourceFactor.SRC_ALPHA, GlStateManager.DestFactor.ONE, GlStateManager.SourceFactor.ONE, GlStateManager.DestFactor.ZERO);
        GlStateManager.pushMatrix();
        float f16 = 1.0F - world.getRainStrength(partialTicks);
        GlStateManager.color(1.0F, 1.0F, 1.0F, f16);
        GlStateManager.rotate(-90.0F, 0.0F, 1.0F, 0.0F);
        
        GlStateManager.rotate(world.getCelestialAngle(partialTicks) * 360.0F, 1.0F, 0.0F, 0.0F);
        
        float f17 = 30.0F; //size of the sun? default is 30.0F
        this.renderEngine.bindTexture(SUN_TEXTURES);
        bufferbuilder.begin(7, DefaultVertexFormats.POSITION_TEX);
        bufferbuilder.pos((double)(-f17), 100.0D, (double)(-f17)).tex(0.0D, 0.0D).endVertex();
        bufferbuilder.pos((double)f17, 100.0D, (double)(-f17)).tex(1.0D, 0.0D).endVertex();
        bufferbuilder.pos((double)f17, 100.0D, (double)f17).tex(1.0D, 1.0D).endVertex();
        bufferbuilder.pos((double)(-f17), 100.0D, (double)f17).tex(0.0D, 1.0D).endVertex();
        tessellator.draw();
        
        f17 = 20.0F;
        this.renderEngine.bindTexture(MOON_PHASES_TEXTURES);
        int k1 = world.getMoonPhase();
        int i2 = k1 % 4;
        int k2 = k1 / 4 % 2;
        float f22 = (float)(i2 + 0) / 4.0F;
        float f23 = (float)(k2 + 0) / 2.0F;
        float f24 = (float)(i2 + 1) / 4.0F;
        float f14 = (float)(k2 + 1) / 2.0F;
        bufferbuilder.begin(7, DefaultVertexFormats.POSITION_TEX);
        bufferbuilder.pos((double)(-f17), -100.0D, (double)f17).tex((double)f24, (double)f14).endVertex();
        bufferbuilder.pos((double)f17, -100.0D, (double)f17).tex((double)f22, (double)f14).endVertex();
        bufferbuilder.pos((double)f17, -100.0D, (double)(-f17)).tex((double)f22, (double)f23).endVertex();
        bufferbuilder.pos((double)(-f17), -100.0D, (double)(-f17)).tex((double)f24, (double)f23).endVertex();
        tessellator.draw();
        
        GlStateManager.rotate(-world.getCelestialAngle(partialTicks) * 360.0F, 1.0F, 0.0F, 0.0F); //reverse rotation for celestial angle
        
        GlStateManager.rotate(-80, 1.0F, 0.0F, 0.0F);//rotate drawing pad to horizon
        GlStateManager.rotate(75, 0.0F, 1.0F, 0.0F);//rotate drawing pad to sideways
        
        //custom third celestial object
        float jupSize = 65.0F; //size of the sun? default is 30.0F
        this.renderEngine.bindTexture(JUPITER_TEXTURES);
        bufferbuilder.begin(7, DefaultVertexFormats.POSITION_TEX);
        bufferbuilder.pos((double)(-jupSize), 100.0D, (double)(-jupSize)).tex(0.0D, 0.0D).endVertex();
        bufferbuilder.pos((double)jupSize, 100.0D, (double)(-jupSize)).tex(1.0D, 0.0D).endVertex();
        bufferbuilder.pos((double)jupSize, 100.0D, (double)jupSize).tex(1.0D, 1.0D).endVertex();
        bufferbuilder.pos((double)(-jupSize), 100.0D, (double)jupSize).tex(0.0D, 1.0D).endVertex();
        tessellator.draw();
        
        GlStateManager.rotate(-75, 0.0F, 1.0F, 0.0F);//rotate back upright
        GlStateManager.rotate(80, 1.0F, 0.0F, 0.0F);//rotate drawing pad back to default position for vanilla celestial objects
        
        GlStateManager.disableTexture2D();

This creates a third visible 2d texture that always appears slanted on the horizon, which is perfect. The only issue is, I want the sun to render behind it. I tried drawing the third object both before and after the sun is drawn, but either way, the sun always appears on top of it, rather then behind it. How could I force the third image to overlap the sun? I'm new to working with minecrafts' rendering code, and I haven't been able to find any helpful resources for it.

 

EDIT: The moon overlaps it too...

EDIT2: I've figured out that the second argument in bufferbuilder.pos(#,#,#) controls depth, but it still doesn't seem to help the draw issue; textures behind other textures still render over for some reason.

Edited September 5, 20196 yr by pacguy

I was able to find a fix on my own! Turns out, it appears the way it does because of these lines earlier in the vanilla code:

        GlStateManager.disableAlpha();
        GlStateManager.enableBlend();

the blend feature caused all the images to appear transparent relative to each other, making it seem like they always overlapped. running disableBlend(), enableAlpha, then modifying the images to use transparency did the trick! Everything now renders based on the order it was drawn like expected :3

 

Posting this for reference, in case someone has a similar issue.