GLCOPYCONVOLUTIONFILTER2D(3G)

✨ glCopyConvolutionFilter2D 💅

okay bestie, gather round, because this one is a function function — it's in the 3G manual (that's the OpenGL corner of the manpage universe), and it's giving "I copy pixels straight off your screen into a filter kernel" energy. no cap. 🫶

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🌸 NAME 🌸

glCopyConvolutionFilter2D — copy pixels into a two-dimensional convolution filter 💖

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🎀 C SPECIFICATION 🎀

void glCopyConvolutionFilter2D( GLenum target,
                                GLenum internalformat,
                                GLint x,
                                GLint y,
                                GLsizei width,
                                GLsizei height )

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🦋 PARAMETERS 🦋

• target — must be GL_CONVOLUTION_2D. that's it, that's the only option, no notes, periodt.
• internalformat — the internal format of the convolution filter kernel (yes, the original literally just says "internal" and drops the word, we love a low-effort moment 🙈). the allowable values are: GL_ALPHA, GL_ALPHA4, GL_ALPHA8, GL_ALPHA12, GL_ALPHA16, GL_LUMINANCE, GL_LUMINANCE4, GL_LUMINANCE8, GL_LUMINANCE12, GL_LUMINANCE16, GL_LUMINANCE_ALPHA, GL_LUMINANCE4_ALPHA4, GL_LUMINANCE6_ALPHA2, GL_LUMINANCE8_ALPHA8, GL_LUMINANCE12_ALPHA4, GL_LUMINANCE12_ALPHA12, GL_LUMINANCE16_ALPHA16, GL_INTENSITY, GL_INTENSITY4, GL_INTENSITY8, GL_INTENSITY12, GL_INTENSITY16, GL_R3_G3_B2, GL_RGB, GL_RGB4, GL_RGB5, GL_RGB8, GL_RGB10, GL_RGB12, GL_RGB16, GL_RGBA, GL_RGBA2, GL_RGBA4, GL_RGB5_A1, GL_RGBA8, GL_RGB10_A2, GL_RGBA12, or GL_RGBA16. pick one, any one, they're all iconic in their own way ✨
• x, y — the window space coordinates of the lower-left coordinate of the pixel array to copy. lower-left bestie, not upper-left, don't do that to me 😭
• width — the width of the pixel array to copy.
• height — the height of the pixel array to copy.

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💜 DESCRIPTION 💜

so. glCopyConvolutionFilter2D defines a two-dimensional convolution filter kernel — but the pixels come from the current GL_READ_BUFFER instead of from main memory. that's the whole plot twist: glConvolutionFilter2D pulls from main memory, this one pulls from where you're already reading. sibling energy fr fr. 🧚

The screen-aligned pixel rectangle with lower-left corner at (x, y), width width and height height is used to define the convolution filter. If any pixels within this region are outside the window that is associated with the GL context, the values obtained for those pixels are undefined. undefined. as in, nobody knows, not even the driver, we simply do not look 👀

The pixels in the rectangle are processed exactly as if glReadPixels had been called with format set to RGBA, but the process stops just before final conversion. Then the R, G, B, and A components of each pixel are scaled by the four 2D GL_CONVOLUTION_FILTER_SCALE parameters and biased by the four 2D GL_CONVOLUTION_FILTER_BIAS parameters. (The scale and bias parameters are set by glConvolutionParameter using the GL_CONVOLUTION_2D target and the names GL_CONVOLUTION_FILTER_SCALE and GL_CONVOLUTION_FILTER_BIAS. The parameters themselves are vectors of four values that are applied to red, green, blue, and alpha, in that order.) and listen — the R, G, B, and A values are not clamped to [0,1] at any time during this process. no clamping. they can go wherever they want. main character freedom 🌈

Each pixel is then converted to the internal format specified by internalformat. This conversion simply maps the component values of the pixel (R, G, B, and A) to the values included in the internal format (red, green, blue, alpha, luminance, and intensity). The mapping is as follows:

• GL_ALPHA — alpha ← A
• GL_LUMINANCE — luminance ← R
• GL_LUMINANCE_ALPHA — alpha ← A, luminance ← R
• GL_INTENSITY — intensity ← R
• GL_RGB — red ← R, green ← G, blue ← B
• GL_RGBA — red ← R, green ← G, blue ← B, alpha ← A

The red, green, blue, alpha, luminance, and/or intensity components of the resulting pixels are stored in floating-point rather than integer

(no babe I didn't cut that off, the sentence just ends there in the source. it's giving cliffhanger 💀 anyway—)

Pixel ordering is such that lower x screen coordinates correspond to lower i filter image coordinates, and lower y screen coordinates correspond to lower j filter image coordinates. so like, don't flip your brain about it, it lines up 🎀

Note that after a convolution is performed, the resulting color components are also scaled by their corresponding GL_POST_CONVOLUTION_c_SCALE parameters and biased by their corresponding GL_POST_CONVOLUTION_c_BIAS parameters (where c takes on the values RED, GREEN, BLUE, and ALPHA). These parameters are set by glPixelTransfer. yes, there's a second scale-and-bias moment after the convolution. it's a whole routine. 💅

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🔥 NOTES 🔥

glCopyConvolutionFilter2D is present only if GL_ARB_imaging is returned when glGetString is called with an argument of GL_EXTENSIONS. so vibe check your extensions first, bestie — if GL_ARB_imaging isn't there, this function isn't either 🙈

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💀 ERRORS 💀

• GL_INVALID_ENUM is generated if target is not GL_CONVOLUTION_2D.
• GL_INVALID_ENUM is generated if internalformat is not one of the allowable values.
• GL_INVALID_VALUE is generated if width is less than zero or greater than the maximum supported value. This value may be queried with glGetConvolutionParameter using target GL_CONVOLUTION_2D and name GL_MAX_CONVOLUTION_WIDTH.
• GL_INVALID_VALUE is generated if height is less than zero or greater than the maximum supported value. This value may be queried with glGetConvolutionParameter using target GL_CONVOLUTION_2D and name GL_MAX_CONVOLUTION_HEIGHT.
• GL_INVALID_OPERATION is generated if glCopyConvolutionFilter2D is executed between the execution of glBegin and the corresponding execution of glEnd.

🚨 not me telling you to check your values before the driver yells at you. width and height under zero? no. over the max? also no. and don't you dare call this inside a glBegin/glEnd sandwich, that's illegal and it will snitch on you 😤

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👑 ASSOCIATED GETS 👑

glGetConvolutionParameter, glGetConvolutionFilter

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🌟 SEE ALSO 🌟

glConvolutionFilter2D(3G), glConvolutionParameter(3G), glPixelTransfer(3G)

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and that's the whole page, bestie 🫶 this was GLCOPYCONVOLUTIONFILTER2D(3G) — filed under the 3G manual, footer and all, and yes the footer is just the page name staring back at you, which is very on brand for a manpage honestly ⭐ go forth and convolve. ate. 💖