OpenGL Quad only blending with background

Question

I have set up a quick 2D rendering system using OpenTK and OpenGL for a game. I have set up transparency using:

GL.Enable(EnableCaps.Blend);
GL.BlendFunc(BlendingFactorSrc.One, BlendingFactorDest.OneMinusSrcAlpha);

However, whenever I go to draw (the texture is a png file loaded with System.Drawing), it only blends with the color I clear at the start of the draw call:

I just ended up switching to MonoGame, as it's 2D rendering has already been sufficiently tested

That blend mode says use 100% of the source and 100%-source-alpha of the destination, i.e. 0% (unless your image has an alpha channel?). Equivalent to no blending — Kieren Johnstone, Dec 01 '12 at 09:11
I tried turning off depth testing, but that turns my text into a skewed image with no transparency at all — CallumDev, Dec 01 '12 at 09:17
Skewed, how so? Skew is equivalent to the sides being angled, like that? Depth testing should not do that — Kieren Johnstone, Dec 01 '12 at 09:26
Well not skewed, but the image goes funny when depth testing is _disabled_. Like this: http://img442.imageshack.us/img442/2056/textfunny.png — CallumDev, Dec 01 '12 at 09:29
@KierenJohnstone: (1, 1-src.alpha) blending mode is a perfectly valid mode for pre-multiplied alpha images. So that's probably not the problem. — datenwolf, Dec 01 '12 at 12:46
Whatever happens when you disable depth testing, it is weird and should not happen. Can you please post your full code? — datenwolf, Dec 01 '12 at 12:47
I ended up switching my rendering code over to MonoGame, which has a fully tested 2D Rendering System — CallumDev, Dec 01 '12 at 23:27
try looking at http://stackoverflow.com/questions/5984887/opengl-z-sorting-transparency question. — Stanislav Ageev, Dec 01 '12 at 23:44

score 2 · Accepted Answer · answered Dec 01 '12 at 08:27

Looks like you're not drawing in the proper order. To get correct blending, you need to sort your polygons from back to front, so that anything 'behind' gets drawn first.

Based on that picture though, you might be better served with just using alpha testing instead of blending. Alpha test just discards pixels with alpha less than a threshold. It's less expensive than blending and you don't need to sort objects to use it. You only need blending when you want to use semi-opaque objects (alpha between 0 and 1).

I drew the square, then the text. Also alpha testing made the text skewed — CallumDev, Dec 01 '12 at 08:40

OpenGL Quad only blending with background

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