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C# XNA EffectPass.Apply() not doing anything

What I would like to do is to be able to draw a specific set of sprites within a spriteBatch with additive blending. The problem is that the draw order that they're drawn in needs to be preserved and I can't draw everything else in the SpriteBatch with additive blending, so I can't just do this:
spriteBatch.Begin(SpriteSortMode.FrontToBack, BlendState.AlphaBlend);
//Draw some stuff here
spriteBatch.End();
spriteBatch.Begin(SpriteSortMode.FrontToBack, BlendState.Additive);
//Draw stuff with additive blending here
spriteBatch.End();
So my solution would be to write a shader to do what I need and just do this:
spriteBatch.Begin(SpriteSortMode.FrontToBack, BlendState.AlphaBlend);
//Draw some stuff here
foreach(EffectPass pass in AdditiveShader.CurrentTechnique.Passes)
{
pass.Apply()
//Draw stuff with additive shader applied here
}
spriteBatch.End()
But pass.Apply() is literally doing nothing. Even if I try just using a BasicEffect and have it rotate a few degrees, it's doing nothing. The only way I can get it to do anything is to call it like this:
spriteBatch.Begin(SpriteSortMode.FrontToBack, BlendState.AlphaBlend,
null, null, null, AdditiveShader);
Then it actually does something to sprites, but that doesn't really help me because I want to only apply it to specific sprites and still retain the draw order.
What am I doing wrong when using pass.Apply()? Is there a way to draw a set of sprites with additive blending and another set with alpha blending and still keep the draw order? Any help would be greatly appreciated. Thanks.
EDIT: For clarification, I'm working in 2D.

OK, so what I found (Thanks to Shawn Hargreaves - http://blogs.msdn.com/b/shawnhar/archive/2010/04/05/spritebatch-and-custom-renderstates-in-xna-game-studio-4-0.aspx) is that I can use GraphicsDevice.BlendState = BlendState.Additive to change the BlendState on the fly.
So what I did was this:
spriteBatch.Begin(SpriteSortMode.Immediate, BlendState.Alpha);
//Draw with alpha blending here
GraphicsDevice.BlendState = BlendState.Additive;
//Draw specific sprites with additive blending here
GraphicsDevice.BlendState = BlendState.Alpha; //Revert the blendstate to Alpha
//Draw more stuff here with alpha blending
spriteBatch.End();
The problem is that the SpriteSortMode for the spriteBatch has to be set to Immediate or GraphicsDevice.BlendState = BlendState.Additive will do nothing.
So I suppose I'll have to use a custom DepthStencilState to replicate the functionality of SpriteSortMode.FrontToBack.

Getting rid of black background when using a render target

What I am trying to do is use a spritesheet to create a blended terrain, save that as a texture, and then pass it into a shader for additional effects. However, I am having some issues with the render target:
RenderTarget2D someNewTexture = new RenderTarget2D(GraphicsDevice, 256, 256);
GraphicsDevice.SetRenderTarget(someNewTexture);
GraphicsDevice.Clear(Color.Black);
spriteBatch.Begin();
{
// draw some stuff
}
spriteBatch.End();
GraphicsDevice.SetRenderTarget(null);
Obviously, this creates a black background with my texture on top of it. However, when I actually draw that texture onto my main scene, I don't want the background, only the texture (if I don't use clear, I get the default purple color instead). How do I fix this?

Use a transparent clear color:
GraphicsDevice.Clear(Color.TransparentBlack);
If you handle blending correctly, it will result in transparent pixels not to be drawn.

c# XNA 4.0 Camera zoom and sprite resizing

Inside my project, I have a sprite being draw of a box. I have the camera zoom out when clicking a key. When I zoom out, I want my box to scale it's dimensions so it stays consistent even though the camera has zoomed out and "shrunk" it.
I have tried multiplying the object's dimensions by 10% which seems to be the viewpoint's adjustment when zooming out, but that doesn't seem to work. Now this may sound dumb, but would scaling the sprite in the draw function also change the sprite's dimensions?
Let's say the box is 64x64 pixels. I zoom out 10% and scale the sprite. Does the sprite still have the boundaries as 64x64 or is the up-scaling also changing it's dimensions?

Scaling using SpriteBatch.Draw()s scale argument will just draw the sprite smaller/bigger, i.e. a 64x64 one will appear as 7x7 pixels (the outer pixels being alpha blended if enabled). However there are no size properties on the sprite, if you have your own rectangle, position variables for the sprite SpriteBatch.Draw() of course will not change those.
An alternative is draw the sprite in 3D space then everything is scaled when you move your camera, so the sprite will appear smaller though it will still be a 64x64 sprite.
How to draw a sprite in 3D space? Here is a good tutorial http://www.riemers.net/eng/Tutorials/XNA/Csharp/Series2/Point_sprites.php. (You will need to take time to learn about using 3D viewports, camera's etc, see here: http://msdn.microsoft.com/en-us/library/bb197901.aspx)/

To change sprite dimensions you need to change Rectangle parameter for SpriteBatch.Draw. To calculate zoom on rectange:
Rectangle scaledRect = new Rectangle(originalRectangle.X, originalRectangle.Y, (int)(originalRectangle.Width*zoom), (int)(originalRectangle.Height*zoom)); // where zoom default is 1.0f
When drawing use:
spriteBatch.Draw(Texture, scaledRect, Color.White);
Now I'm sorry to assume it, but without knowing why you doing what you doing - I think you doing something wrong.
You should use camera transformation to zoom out/in. It is done like that:
var transform = Matrix.CreateTranslation(new Vector3(-Position.X, -Position.Y, 0))* // camera position
Matrix.CreateRotationZ(_rotation)* // camera rotation, default 0
Matrix.CreateScale(new Vector3(Zoom, Zoom, 1))* // Zoom default 1
Matrix.CreateTranslation(
new Vector3(
Device.Viewport.Width*0.5f,
Device.Viewport.Height*0.5f, 0)); // Device from DeviceManager, center camera to given position
SpriteBatch.Begin( // SpriteBatch variable
SpriteSortMode.BackToFront, // Sprite sort mode - not related
BlendState.NonPremultiplied, // BelndState - not related
null,
null,
null,
null,
transformation); // set camera tranformation
It will change how sprites are displayed inside sprite batch, however - now you also must account for different mouse coordinates (if you using mouse input). To do that you must transform mouse position to transformed world matrix:
// mouse position, your tranformation matrix
public Vector2 ViewToWorld(Vector2 pos, Matrix transform)
{
return Vector2.Transform(pos, Matrix.Invert(transform));
}
I used the code without direct access to test it, so if something will not work - feel free to ask.
This is not answer to your question directly, if you could provide reason why you want re-size sprite when zooming instead of zooming camera - maybe I could better answer your question, also you should fallow markmnl link to understand world transformations and why you seem to need it in this situation.

Drawing textures with variable transparency

I am a moderately experienced C# developer, but I'm new to XNA and graphics in general. I'm making a 2D game and I'm trying to draw a texture that partially transparent. The desired transparency value is stored in a float variable. The only solution I've found is to directly edit the alpha values in the texture each frame, but that seems inefficient. I've tried using alpha blending, but I haven't been able to figure out how to use my own transparency value. Is there a better way to do it?
Edit: Added more information.

if you are using spritebatch is easy:
float alpha = desired_alpha;
spritebatch.Draw(texture, pos, source, Color.White * alpha);

You could try using this Color constructor to pass in your alpha value:
http://msdn.microsoft.com/en-us/library/dd231957(v=xnagamestudio.31).aspx

Sprite.Draw() draws my textures too small

I declared a device + sprite in a Windows.Form like this
PresentParameters presentParameters = new PresentParameters();
presentParameters.Windowed = true;
presentParameters.SwapEffect = SwapEffect.Copy;
var device = new Device(Manager.Adapters.Default.Adapter, DeviceType.Hardware, this, CreateFlags.HardwareVertexProcessing, presentParameters);
var sprite = new Sprite(device);
I loaded a texture via TextureLoader.FromFile(device, "image.png");
In my Draw method i startet the device scene, then the sprite scene, then i wrote
sprite.Draw2D(texture, PointF.Empty, 0, PointF.Empty, Color.White);
the drawing itself works, but it draws only a big portion of the image scaled up to the screen (like 90%)
i tried it with a source rectangle with the given texture size too, but the same bug occurred
any suggestions?

I am experienced in C++ DirectX, but not C# DirectX, so take this with a grain of salt.
In my experiences with the Sprite interface, you need to scale, rotate, and translate just like you need to with 3D objects. You may be forgetting to scale. Here is the code of my Update function.
void Button::Update()
{
Sprite->Begin(D3DXSPRITE_ALPHABLEND);
D3DXMATRIX trans;
D3DXMATRIX scale;
D3DXMATRIX world;
D3DXMatrixIdentity(&world);
D3DXMatrixTranslation(&trans, pos.x, pos.y, 0.0f);
D3DXMatrixScaling(&scale, scaleFactor, scaleFactor, 1.0f);
world = scale * trans;
Sprite->SetTransform(&world);
Sprite->Draw(buttonTexture, NULL, NULL, &D3DXVECTOR3(-width2, -height2, 0.0), whitecol);
Sprite->End();
}
Admittedly, this isn't a very object-oriented way of doing things, but it suits my needs.

Caveat: I am not an DirectX expert, but I had the same problem.
When you load the sprite it expands the sprite to fit a size where each dimension is a power of 2. For example, If you sprite was 200 x 65, the sprite will have a width of 256 (and the image will be expanded to a width of 256, increasing it slightly) by 128 (almost doubling the height).
When you draw the image, it will be almost twice the height you expected.
My solution was to modify my image file to have a height and width of a factor of 2 and then only draw the portion that was the original size.