Working with Textures, Models, and 2D - Mastering Unity Scripting - page 315

• Lines 062-072 : If a collision was detected, the UV coordinates of the hit
location should be retrieved through the textureCoord variable of the
RaycastHit structure. More information on this variable can be found
online in the Unity documentation at http://docs.unity3d.com/
ScriptReference/RaycastHit-textureCoord.html . This member, is
only valid if the intersected mesh has MeshCollider , as opposed to other
collider types, such as BoxCollider or CapsuleCollider . However, this
means that any object used as a texture-painting destination should feature
a MeshCollider component, since it contains UV data. Lines 63-72 then
convert the UV coordinates into absolute pixel positions, centering the brush
source texture at the position of the cursor. The result of this code is to clearly
identify a position on the source brush texture that should be the pivot or
origin point, and to establish a pixel x , y coordinate location at which the
source texture should be painted into the destination texture.
• Line 075 : Finally, the PaintSourceToDestTexture function is called to
perform the paint operation itself.
• Line 081 : The PaintSourceToDestTexture function accepts four arguments:
Source , Dest , Left , and Top . On the basis of these, the Source texture is
painted onto the Dest at the positions Left and Top . This function is declared
as static, which means that no instance of this class needs to be declared.
• Lines 084-087 : The first step in the texture-painting process is to retrieve
all pixels in both the Source and Dest textures. This is achieved using the
GetPixels function. More information on GetPixels can be found online in
the Unity documentation at http://docs.unity3d.com/ScriptReference/
Texture2D.GetPixels.html . Now, although each image is visually a two-
dimensional array of pixels, the returned array from GetPixels is in fact
linear (one-dimensional). This is the reason for both the GetPixelFromArray
and SetPixelFromArray functions, which convert pixel x and y positions
into linear array indices.
• Lines 89-101 : Here, each pixel is retrieved from the Source texture and
painted onto the destination. This checks to ensure that the brush texture is
painted with the destination bounds and allows clipping, if required. This
is necessary because a brush mark could, in principle, be made close to the
texture edge; in this case, only a part of the brush would actually be painted
onto the destination, as some pixels would be "cut off". Pixels are retrieved
from the Source texture with GetPixelFromArray , and destination pixels
are set with SetPixelInArray .