Lesson 5 – Images
PV227 – GPU Rendering
Jiˇrí Chmelík, Jan ˇCejka
Fakulta informatiky Masarykovy univerzity
27. 10. 2015
PV227 – GPU Rendering (FI MUNI) Lesson 5 – Images 27. 10. 2015 1 / 22
Outline
1 Introduction
2 Post-processing effects
Gray Scale
Negative
Thresholding
Blurring
Convolution
PV227 – GPU Rendering (FI MUNI) Lesson 5 – Images 27. 10. 2015 2 / 22
DevIL
library for working with images,
simplifies loading textures to OpenGL,
download built SDK at http://openil.sourceforge.net/
(http://downloads.sourceforge.net/openil/
DevIL-SDK-x86-1.7.8.zip).
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DevIL (cont.)
update VC++ Directories (taken care of ;-)),
pass data from DevIL to OpenGL.
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Image Processing
image effects applied to a texture,
may be used as post-process on the framebuffer,
gray scale,
negative,
thresholding,
blurring,
general convolution.
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Texture Setup
same way as in fixed OpenGL,
texture unit ID passed to the sampler in the shader,
rendered using two triangles (quad),
camera setup so that only the quad is seen.
Figure: Rendered texture
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Texture Read
gvec texture(gsampler sampler, vec texCoord);
gvec is the texel type,
gsampler is the sampler type,
texCoord is in [0, 1] range (coordinates according to texture
dimension).
ivec textureSize(gsampler sampler, int lod);
ivec is the integer size of the texture (coordinates according to
texture dimension) ,
gsampler is the sampler type,
lod is level-of-detail (usually 0).
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Outline
1 Introduction
2 Post-processing effects
Gray Scale
Negative
Thresholding
Blurring
Convolution
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Gray Scale
Figure: Taken from wikimedia.org
linear combination of the RGB channels into luma (intensity),
texel is multiplied component-wise (dot product) with
the weights.
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Gray scale (cont.)
several options for choosing the weights,
NTSC weights: 0.299, 0.587, 0.114.
Figure: Grayscale
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Outline
1 Introduction
2 Post-processing effects
Gray Scale
Negative
Thresholding
Blurring
Convolution
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Negative
Inversion of each color channel,
alpha channel should not be inverted.
Figure: Negative
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Texture Fetch
in vec4 gl_FragCoord;
available only in fragment shader
contains the window-relative coordinates of the current
fragment (x, y, z, 1/w).
gvec texelFetch(gsampler sampler, ivec texCoord, int lod);
perform a lookup of a single texel within a texture
gvec is the texel type,
gsampler is the sampler type,
texCoord is in [0, textureSize] range (coordinates according to texture
dimension),
lod is level-of-detail (usually 0).
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Outline
1 Introduction
2 Post-processing effects
Gray Scale
Negative
Thresholding
Blurring
Convolution
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Thresholding
usually applied to gray scale images,
assigns white to pixels above threshold, black otherwise.
Figure: Thresholding
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Outline
1 Introduction
2 Post-processing effects
Gray Scale
Negative
Thresholding
Blurring
Convolution
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Blurring
averaging of the image,
the amount of blur depends on the kernel size,
blur type is controlled by the blurring weights,
the weights must sum to 1.
Figure: Gaussian 5x5 blur
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Outline
1 Introduction
2 Post-processing effects
Gray Scale
Negative
Thresholding
Blurring
Convolution
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Convolution
Figure: Taken from illinois.edu
used to compute any linear filter,
(f ∗ g)(t) ≡
∞
−∞ f(τ)g(t − τ)dτ,
(f ∗ g)(t) ≡ ∞
−∞ f(τ)g(t − τ).
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Sharpening
inverse of blurring (subtraction of neighbourhood),
the amount of sharpening depends on the kernel size,
sharpen type is controlled by the convolution weights,
the weights must sum to 1.
Figure: Sharpening 3x3
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Edge Detection
detects changes in intensity,
preferably blur the image before edge detection,
detection type is controlled by the convolution kernel,
the weights must sum to 0.
Figure: Laplacian edge detection
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More
PV131: Digital Image
Processing,
PA166: Advanced Methods of
Digital Image Processing,
PA170: Digital Geometry,
PA171: Digital filtering,
PA172: Image Acquisition
Principles,
PA173: Mathematical
Morphology,
. . .
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