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xujiezhige

Paste a new picture to mesh

6 posts in this topic

Hi, guys!

I want to make the effect, just like pasting a user-defined picture to the wall or any other place in counter strike 1.5.

How could do this?
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You should look up [i]decals [/i]or [i]projected textures[/i]. There're many ways to do it and it depends on the engine you use.
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[quote name='Ashaman73' timestamp='1334829304' post='4932750']
You should look up [i]decals [/i]or [i]projected textures[/i]. There're many ways to do it and it depends on the engine you use.
[/quote]

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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It depends on the engine or atleast rendering technique you use. You can create a "decal" model on the CPU, you can use projective textures in a forward renderer or you can use projective textures/decals in deferred renderer. DirectX is just an API, we need more information about what rendering technology you use.
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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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[quote name='kauna' timestamp='1334922575' post='4933150']
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!
[/quote]
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.
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Well, can you tell a bit of your environment? Which API are you using?

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

[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

[/code]

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!
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