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Transparency

Transparency. - Blending colors. - Frame buffer limitations. - Compositing. Basic Alpha Blending. Transparent front color with opacity a. Solid back color. Basic Alpha Blending. Transparent front color with opacity a. Solid back color. 2. 1. 3. Blending Alphas.

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Transparency

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  1. Transparency - Blending colors - Frame buffer limitations - Compositing

  2. Basic Alpha Blending Transparent front color with opacity a Solid back color

  3. Basic Alpha Blending Transparent front color with opacity a Solid back color 2 1 3

  4. Blending Alphas Transparent back color with opacity ab Transparent front color with opacity af Process in either front to back or back to front order But can’t insert surface between them later - why?

  5. Premultiply alpha Transparent back color with opacity ab Transparent front color with opacity af Notice - rgb always appears multiplied by it’s a Can store rgb’s as premultiplied by a

  6. Frame Buffer - What about z value in depth buffer? • Manage z-values: glDepthMask(GL_FALSE)

  7. Transparency in OpenGL glEnable(GL_BLEND); glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA) Draw polygon that has alpha values set glDisable(GL_BLEND);

  8. “Screendoor” transparency - No blend - use alternating pixels from different surfaces What about z value in depth buffer?

  9. Stipple in OpenGL Uses 32x32 pattern • GLubyte halftone[] = { • 0xAA, 0xAA, 0xAA, 0xAA, 0x55, 0x55, 0x55 , 0x55 • …repeat 16 lines • } glEnable(GL_POLYGON_STIPPLE); glPolygonStipple(halftone); draw polygon glDisable(GL_POLYGON_STIPPLE);

  10. Compositing • Without pixel z values • With pixel z • - use depth at pixel corners to interpolate partial coverage

  11. 10 6 9 9 10 6 9 9 Compositing Z buffer: keep z values with color buffer Compare z values at corresponding pixels Keep all or nothing

  12. 9 8 9 10 9 6 10 6 9 9 9 6 10 9 8 9 10 6 9 9 9 10 9 6 9 8 Compositing • Z buffer: keep z values with color buffer • Compare z values at corresponding pixels • 2. Compute partial coverage: • Interpolate corner z values • Compare corner values for pixel and blend

  13. Compositing Alpha Channel Value between 0 and 1 Combined partial coverage and transparency Computed during rendering in front of a null background 2 1/2 D blend based on alpha of image in front alpha RGB 32 bit pixel values

  14. Compositing - example

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