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getPixel/setPixels or stencil eraser brush

hi im trying to build a eraser brush that you can clear away a texture on a gameObject with. i have succeeded in doing so using getPixel and setPixel however i am now encountering a dilemma. at the moment i create the brush around a central pixel then with a little math (and a nested loop) add each pixel in an area around the original pixels co-ordinates to an array with each being set to alpha=0, which is then processed into the new texture for the gameObject. the result of this sets a square of pixels around the mouse position to invisible. my question, is their a better and more efficient way to do this?

i have also attempted to implement a stencil script that paints a texture over another at the point you select, however i cant seem to see a way to make a section invisible if i use an image that is effectively empty.

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asked Oct 11, 2011 at 11:22 PM

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2 answers: sort voted first

In order to do the stencil thing, you could create your own shader that takes in the original texture and the stencil texture, and outputs rgb from the original texture and alpha of the stencil texture.

something like this

 Shader "Custom/CustomDiffuse"
         _MainTex("Main Texture (rgb)", 2D) = "white" {}
         _Color ("Main Color", Color) = (1,1,1,1)
         _Stencil("Stencil Texture (a)", 2D) = "white" {}
             #pragma surface surf Lambert alpha
             #pragma target 2.0
             struct Input {
                 float2 uv_MainTex;
             half4 _Color;
             sampler2D _MainTex;
             sampler2D _Stencil;
             void surf (Input IN, inout SurfaceOutput o) 
                 o.Albedo = tex2D(_MainTex, IN.uv_MainTex) * _Color;
                 o.Alpha = tex2D(_Stencil, IN.uv_MainTex).a;

Regarding the getpixel/setpixel, if you have unity pro, you could paint your changes to a RenderTexture. That way, the stencil generation is done on the GPU, which is usually a lot faster. The easiest is probably to generate your stencil texture first and then apply that stencil to your texture, either as a shader (as mentioned above) or premultiplied into the texture using another RenderTexture.

To create your stencil, you could create a small brush texture that is opaque in the center and transparent at the edges and use Graphics.DrawTexture to draw int onto the rendertexture at the desired location. The following code is one way of using graphics.blit and graphics.drawtexture to create a stencil (note that I haven't actually tested this, so there might be some bugs).

 void AddToStencil(Texture2D stencil, Texture2D brush, Vector2 brushPosition, int brushSizePixels)
     //Create temporary render texture
     int width = stencil.width;
     int height = stencil.height;
     RenderTexture rt = RenderTexture.GetTemporary(width, height, 0, RenderTextureFormat.ARGB32);

     //Copy existing stencil to render texture (blit sets the active RenderTexture)
     Graphics.Blit(stencil, rt);
     //Apply brush
     RenderTexture.active = rt;
     float bs2 = brushSizePixels / 2f;
     Graphics.DrawTexture(new Rect(brushPosition.x - bs2, brushPosition.y - bs2, brushSizePixels, brushSizePixels), brush);

     //Read texture back to stencil
     stencil.ReadPixels(new Rect(0, 0, width, height), 0, 0, true);

     RenderTexture.active = null;

Depending on how many of these stencils you want, and how much GPU memory you have, you could also consider having the stencil in a permanent RenderTexture, and save the first blit and the ReadPixels at the cost of having a permanently allocated RenderTexture.

Note that the stencil texture is now inverse, so in the shader above, you would change the alpha to be

   o.Alpha = 1 - tex2D(_Stencil, IN.uv_MainTex).a;

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answered Oct 12, 2011 at 06:58 AM

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hmmm interesting, just had a look at graphics.blit, how would you go about 'painting' the texture onto the rendertexture say in a small area rather than painting an entire image onto another, from what i can see pass might have something to do with it but im a little unsure how that works.

Oct 12, 2011 at 09:53 AM eradiuslore

updated answer to include some more details regarding rendertextures

Oct 12, 2011 at 10:30 AM KvanteTore

ok a few further questions (sorry!) should i use a rendertexture on gui or just on a plane/gameObject? im also a little unclear on how your actualy using Graphics.DrawTexture to draw the stencil onto the rendertexture. how does ReadPixels free up the render texture and return the stencil back to normal?

Oct 13, 2011 at 05:51 PM eradiuslore

I added an example of how you could update your stencil buffer using the GPU. We're using a similar technique for texture stamping in our avatar system, and if you have a lot of pixels to update it is a lot faster than setting pixels using SetPixel or SetPixels

Oct 13, 2011 at 09:07 PM KvanteTore

sweet. I would apreciate it if you mark my answer as accepted :)

Oct 26, 2011 at 10:02 AM KvanteTore
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How can this be done if you don't have Unity pro? I need something like this for iPhone and Get/Set pixels runs too slow on that device

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answered Apr 05, 2012 at 12:39 PM

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yaa....getpixel and setpixels r too slow in devices...lead to crash also..in low end devices...hw to resolve this prob with optimised solution...???!!

Sep 17, 2013 at 12:31 PM KrishnaMV
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asked: Oct 11, 2011 at 11:22 PM

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Last Updated: Sep 17, 2013 at 12:31 PM