Hey guys,

I'm trying to display sprites from a sprite sheet, but mix-and-match alpha masks from the same texture.

So for example, I've got several types of grass and I want to plant them down using a variety of shoreline shapes that exists at other pixel offsets within the same texture's alpha channel.

For clarity's sake I'm showing the tiles and the alpha channel side by side...



Is this possible?

I thought had this problem nailed down, and wrote a pixel shader which basically sampled the color and then alpha at an offset, but I just learned the params passed to shaders are global to the pass.  :/  It looks great when I draw 1 test tile, but screws everything else up.

What strategy should I be pursuing to accomplish this?

I might be able to use a shader if it were possible the shader could identify which object it was drawing and use the right offsets, although I'd be reluctant to push a crapload of data to the shader for every object in the batch.

Can I continue to use SpriteBatch or do I need to use quads?

Any suggestions would be much appreciated.

You would normally do this by passing in an extra set of texture coordinates and using that for the choice of alpha mask. But I dont think you can change the vertex definition to sprite batch so you are stuck unless you write your own pixel/vertex shader and just use your own effects.

The only other thing you can pass in per sprite is the color, which does happen to have 4 floats, I'm not sure exaclty what the default vertex shader does with those but I'm guessing it passes them as vertex colors into the pixel shader. Maybe you can find a way to use those as an offset into you alpha maps.

Hi ZMan, thanks for the response.

Let me see if I understand your last comment correctly :

1. write and implement a vertex shader,
2. create a Vector4 float ( or whatever Color is ) and store my per-sprite alpha offsets in it,
3. pass it through SpriteBatch.Draw in the Color param,
4. catch it in the vertex shader,
5. pass it to the pixel shader,
6. sample my alpha value at the passed coords.

Sound like this will work?  I need to go learn how to write a vertex shader real quick, but it sounds good.  :D

You don't need a vertex shader for this (replacing the SpriteBatch vertex shader is something of an advanced topic and not really recommended).

There's 2 options:

1. Dont use sprite batch - its actually just a vertex/pixel shader anyway so you can write your own. If you write your own you can create a vertex buffer with 2 sets of texture coordinates and then its trivial to draw them using a custom vertex/pixl shader.

2. Use sprite batch and replace the pixel shader like you are doing now. However you need to find a way to pass in the extra texture coordinates (one for the texture, one for the alpha map). So use the color parameter since it has 4 floats. You will have to do the calculations yourself to work them out but it might work. The same color will be passed in for each vertex which means in the pixel shader you will get a constant value. You should be able to use this as an offset to the texture coordinates and resamplethe texture map. Note that you can't rewrite the vertex shader that comes with sprite batch only the pixel shader.

Woohoo, I got it working finally!

I managed to continue using SpriteBatch, offset my alpha masks, and I can still do tinting.

Basically the Color param on SpriteBatch.Draw passes 4 bytes as ARGB information.

I split the 3 bytes of ARG into two 12-bit values ( that was the tough part ) to store my xoff and yoff, and I reserved the B byte for passing a color index to my pixel shader.

Here is my shader...


Here is my sprite management class ( it has a wrapped Render method that takes our mask offset and tint color ).  I load all my sprite information from a .csv file on initialization, and create an object for every sprite.  They share the same loaded texture and I can draw them based on index number, which is what I store in my game map.


In my main game class where I draw all my tiles, I start the SpriteBatch, loop through my map data and draw my tiles...