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#3
Members - Reputation: **96**

Posted 20 April 2012 - 05:03 AM

You should look up

decalsorprojected textures. There're many ways to do it and it depends on the engine you use.

I just want to implement it by only using DirectX. Any more information for me?

Is it that I have to find the the correct texture coordinates where the user-defined picture should resides in the corresponding texture, and then modulate the two texture to final texture for the mesh?

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#4
Crossbones+ - Reputation: **11958**

Posted 20 April 2012 - 05:23 AM

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#5
Crossbones+ - Reputation: **2894**

Posted 20 April 2012 - 05:49 AM

You may draw your decals in a separate pass using alpha blending. Drawing the decals in the same time as the base texture sounds overly complicated.

Technically drawing a decal isn't that difficult. It involves a little bit of matrix and vector math. I won't go into the math details since that isn't my field of expertise.

Roughly, however, you'll need a matrix which defines x and y rotation and scale of decal and a position of course. With matrix vector multiplication using this decal matrix and all vertex positions of your model you'll be able to calculate the texture coordinates for each vertex in your scene.

Redrawing the model using these texture coordinates and the decal texture allows you to project any texture on the model. Remember to set the texture address mode to clamp and blending mode to alpha blending (for example).

Obvious optimization is to not draw the faces which are totally outside of the decal area. Also, the projected texture coordinates may be calculated in a vertex shader.

Best regards!

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#6
Members - Reputation: **96**

Posted 18 May 2012 - 06:53 PM

Thank you, kauna.As you say, I have to recalculate the texture coordinates for the model, such as the wall. Is it right?By the way, do you have some simple sample or demo for me to learn? You know i can't work it out just with your reply. Thank you.Hi,

You may draw your decals in a separate pass using alpha blending. Drawing the decals in the same time as the base texture sounds overly complicated.

Technically drawing a decal isn't that difficult. It involves a little bit of matrix and vector math. I won't go into the math details since that isn't my field of expertise.

Roughly, however, you'll need a matrix which defines x and y rotation and scale of decal and a position of course. With matrix vector multiplication using this decal matrix and all vertex positions of your model you'll be able to calculate the texture coordinates for each vertex in your scene.

Redrawing the model using these texture coordinates and the decal texture allows you to project any texture on the model. Remember to set the texture address mode to clamp and blending mode to alpha blending (for example).

Obvious optimization is to not draw the faces which are totally outside of the decal area. Also, the projected texture coordinates may be calculated in a vertex shader.

Best regards!

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#7
Crossbones+ - Reputation: **2894**

Posted 19 May 2012 - 05:22 AM

Like I said, calculating the decal texture coordinates is rather simple. Here is some pseudo-shader-code:

float4x4 MVP; //Model*View*Projection matrix float4x4 Model; //just the model/world matrix float4x4 Decal; //Decal projection matrix, which transforms vertex from world space to decal space ... float4 worldpos = mul(input.position, Model); //calculate world position output.pos = mul(input.position, MVP); //Shown just for completeness output.uv = mul(worldpos, DecalMatrix); // calculate the decal uv coordinates based on the world position, may need some offsetting

So, what you still need is a decal matrix. There are lots of ways to calculate it depending on the needs. For example, you could use the wall surface normal to create desired projection matrix. Just remember to use the inverted version of that matrix in the shader.

You could start simply by putting your object to world origo and use an identity matrix for the decal matrix. This should give you a top down projection of your image.

Best regards!