Sprite used as mask

BlitzMax Forums/BlitzMax Beginners Area/Sprite used as mask

christian223 (Posted 2008) [#1]

Is there some way to use a sprite as a mask for another sprite?. So for example if a sprite inside this mask sprite goes out of the mask sprite shape then its not drawn, or viceversa, something like viewport but instead of beign a box it can be a custom shape. Posbile or not posible?.

Taron (Posted 2008) [#2]

Oh, I'd love to find out, too! :}

dmaz (Posted 2008) [#3]

there are no native bmax commands but you can do it low level through stencil buffers. or, depending on what you are doing you could fake it by depth, but I can't think of a way to fake it without masking a whole rectangle. -cls -setviewport to the imagerect -draw the items to be masked -draw the mask shadeblend can be used to create/draw the mask of a positive graphic

tonyg (Posted 2008) [#4]

I believe you can do it at GL/DX level using either single-pass or multi-pass multitexturing.

smilertoo (Posted 2008) [#5]

You mean for cookie cutter effects?

tonyg (Posted 2008) [#6]

I'm pretty sure with DX multitexture you can take the texture from one and the 'shape' of another. Having said that, I re-read the original post and don't think I understand what is being asked. My guess is there might be an easy answer if the question was understood. e.g. Non-rectangle viewport can be a rectangular viewport + overlay image Would that be enough?

TomToad (Posted 2008) [#7]

One way to do it would be to draw the image you want masked onto a newly cleared screen, draw the mask over it, then use GrabImage() to grab the masked portion. Then you draw that image over whatever background you want. Like this: Move the masked smiley face with the mouse, move the portion which is masked with the cursor keys. SuperStrict Graphics 800,600 AutoMidHandle True 'create the smiley face image Cls SetColor 255,255,0 DrawOval 0,0,100,100 SetColor 0,0,0 DrawOval 23,23,20,20 DrawOval 56,23,20,20 DrawRect 33,66,33,20 Local Smiley:TImage = CreateImage(100,100) GrabImage(Smiley,0,0) 'create the sprite mask Local mask:TImage = CreateImage(200,200) Cls Local Pixmap:TPixmap = LockImage(mask) For Local x:Int = -100 To 99 For Local y:Int = -100 To 99 If (x*x+y*y) > 900 WritePixel(Pixmap,x+100,y+100,$FF000000) 'black pixel with alpha = 1.0 Else WritePixel(Pixmap,x+100,y+100,$00000000) 'black pixel with alpha = 0.0 End If Next Next SetMaskColor 0,0,0 'mask color is black by default Local MaskedSmiley:TImage = CreateImage(100,100,1,DYNAMICIMAGE|MASKEDIMAGE) 'MASKEIMAGE flag will set alpha to 0 for any black pixels Local MaskOffsetX:Int = 0 'the mask offset from the smileyface Local MaskOffsetY:Int = 0 SetBlend AlphaBlend 'using alpha blend mode While Not KeyHit(KEY_ESCAPE) And Not AppTerminate() Cls DrawImage Smiley,50,50 'draw the smileyface DrawImage mask,MaskOffsetX+50,MaskOffsetY+50 'draw the mask over the smileyface GrabImage (MaskedSmiley,0,0) 'grab the now masked smileyface SeedRnd(1) 'Make sure random background is drawn same every frame Cls For Local i:Int = 1 To 20 'drawing some random shapes SetColor Rand(0,255),Rand(0,255),Rand(0,255) 'random color DrawOval Rand(0,800),Rand(0,600),Rand(10,100),Rand(10,100) 'random ovals Next SetColor 255,255,255 'set the color back to white Local MX:Int = MouseX() Local MY:Int = MouseY() 'get the mouse coordinates DrawImage maskedSmiley,MX,MY 'draw the image on the screen Flip If KeyDown(KEY_LEFT) Then MaskOffsetX :- 1 'move mask left If KeyDown(KEY_RIGHT) Then MaskOffsetX :+ 1 'move mask right If KeyDown(KEY_UP) Then MaskOffsetY :- 1 'Move mask up If KeyDown(KEY_DOWN) Then MaskOffsetY :+ 1 'Move mask down If MaskOffsetX > 50 Then MaskOffsetX = 50 'Limit the movement of the mask If MaskOffsetX < -50 Then MaskOffsetX = -50 If MaskOffsetY > 50 Then MaskOffsetY = 50 If MaskOffsetY < -50 Then MaskOffsetY = -50 Wend

christian223 (Posted 2008) [#8]

Thanks a lot for that piece of code TomToad, and thanks everyone else for the help also, your code gave me a different idea. First do a pixmapwindow() on the image i want masked, this will result in an effect like viewport, it will be masked just with a rectangle. Then i do like you did on your code but just inside a small portion of the screen, because otherwise i would need to alter my sprite management code wich is not posible. So, before i draw my masked image i take picture of the area i want to draw to, lets say a 50x50 pixels rectangle, then i fill that same area with black, then i draw my pixmap on that area, and then i draw my static mask on there, then i call grabimage, and then i redraw the picture i took before, and at last i draw my sprite with custom mask shape :D I fear that it will mess with the alpha thought, i didnt test this yet but i will tomorrow, thanks again.