I've written a pixel shader to mask 2 textures (both belong to a spritesheet). This works great but adding support to flip the resulting image horizontal is proving to be very difficult.

// Tile.
uniform extern texture ScreenTexture;
sampler screen : register(s0) = sampler_state
{
Texture = <ScreenTexture>;
MinFilter = POINT; // Stops pixels bleeding together.
MagFilter = POINT;
};

// Alpha map (mask) texture to blend with.
uniform extern texture maskTexture;
sampler mask : register(s1) = sampler_state
{
Texture = <maskTexture>;
MinFilter = POINT;
MagFilter = POINT;
};

float2 spritePos; // Screen coordinates of the tile.
float2 maskPos; // Screen coordinates of the mask.
bool flip;

float4 PixelShaderFunction(float2 inCoord: TEXCOORD0) : COLOR
{

    // Convert the mask position to 0-1 texels.
    float2 maskTexel = (maskPos - spritePos) / 512;
    float2 pos = inCoord + maskTexel;

    float x = (1 - inCoord.x) + maskTexel.x;
    if (flip) pos = float2(x, pos.y);

    // Blend!
    float4 color = tex2D(screen, inCoord);
    if (maskPos.x < 0) return color;
    float4 maskColor = tex2D(mask, pos);
    if (maskColor.a > 0) color.rgba = 0;
    return color;
}

technique
{
    pass P0
    {
        // The xbox can only run pixel shader 2.0 and for this purpose that is plenty for us too.
        PixelShader = compile ps_2_0 PixelShaderFunction();
    }
}

From what I've read, 1 - x is all that is required to flip horizontally so I'm guessing combining this with the spritesheet is what's causing the issue?

I've tried for several hours and I can't get it to anything apart from a black square, upside down and backwards or squashed!

Any suggestions?

Cheers,

John

Thanks for the help but still no luck.

The project I'm working on is 2D and this isn't a major part of it but I'd prefer to keep it all within a pixel shader.

// textureLeftTC, textureRightTC are left/right TCs for the particular sprite in the atlas
if( flip )
{
   newX = textureLeftTC + (textureRightTC - x); // move the TC the same distance from the left side as it was from the right side
}

That code does work, however, it will draw the incorrect tile.

This is what I used:

float2 rightMask = maskPos;
rightMask.x = maskPos.x + 22;
float2 rightSprite = spritePos;
rightSprite.x = spritePos.x + 22;

float2 left = (maskPos - spritePos) / 512;
float2 right = (rightMask - rightSprite) / 512;

float x = left.x + (right.x - inCoord.x);
float2 pos = float2(x, inCoord.y + left.y);

This is really starting to frustrate me!

EDIT:

Ok, I can get it to work using only the base sprite. Once I attempt to mask (different texture with different coordinates), that's where the problem is.

I've come up with a solution which works. Just to clarify, I'm only trying to flip the mask horizontal, not the sprite.

// Sprite texture.
uniform extern texture ScreenTexture;
sampler screen : register(s0) = sampler_state
{
    Texture = <ScreenTexture>;
    MinFilter = POINT; // Stops pixels bleeding together.
    MagFilter = POINT;
};

// Alpha map (mask) texture to blend with.
uniform extern texture maskTexture;
sampler mask : register(s1) = sampler_state
{
    Texture = <maskTexture>;
    MinFilter = POINT;
    MagFilter = POINT;
};

float2 spritePos; // Screen coordinates of the tile.
float2 maskPos; // Screen coordinates of the mask.
bool flip;

float4 PixelShaderFunction(float2 inCoord: TEXCOORD0) : COLOR
{

    // Convert the mask position to 0-1 texels.
    float2 maskTexel = (maskPos - spritePos) / 512;
    float2 pos = inCoord + maskTexel;

    // Flip the sprite horizontally - this will be corrected during SpriteBatch.Draw.
    float x = inCoord.x;
    if (flip) x = (spritePos.x / 512) + (((spritePos.x + 22) / 512) - inCoord.x);

    // Blend!
    float4 color = tex2D(screen, float2(x, inCoord.y));
    if (maskPos.x < 0) return color;
    float4 maskColor = tex2D(mask, pos);
    if (maskColor.a > 0) color.rgba = 0;
    return color;

}

technique
{
    pass P0
    {
        PixelShader = compile ps_2_0 PixelShaderFunction();
    }
}

Since flipping the mask is the issue, I simply flip the sprite instead (which does work) and then flip when draw to the screen.

spriteBatch.Draw(texture, pos, source, Color.White, 0, Vector2.Zero, 1, SpriteEffects.FlipHorizontally, 0)

This causes the sprite to be flipped back to its original form and the mask to be flipped horizontally! This may not be the most elegant solution but certainly works!