Pixel Buffer

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Functions

A pixel buffer (Pixbuf) is a memory area that represents a grid of color dots or pixels. They allow direct access to information but are not optimized for drawing on the screen, so we must create an Image object to view them. They are very efficient for procedural generation or the application of filters, since reading or writing a value does not require more than accessing its position within the buffer.

• Use pixbuf_create to create a new pixel buffer.
• Use image_pixels to get the pixels of an image.
• Use pixbuf_width to get the width of the grid.
• Use pixbuf_height to get the height of the grid.

All operations on pixel buffers are performed on the CPU. They are efficient to the extent that we directly access memory, but they cannot be compared with alternatives that use the GPU for digital image processing.

1. Pixel formats

The format refers to how the value of each pixel is encoded within the buffer (Table 1) (Figure 1).

• Use pixbuf_format to get the pixel format.
• Use pixbuf_format_bpp to get the number of bits wanted for each pixel.

Value	Description
ekRGB24	True color +16 million simultaneous, 24 bits per pixel.
ekRGBA32	True color with alpha channel (transparencies), 32 bits per pixel.
ekGRAY8	256 shades of gray, 8 bits per pixel.
ekINDEX1	Indexed, 1 bit per pixel.
ekINDEX2	Indexed, 2 bits per pixel.
ekINDEX4	Indexed, 4 bits per pixel.
ekINDEX8	Indexed, 8 bits per pixel.

2. Procedural images

One way to "fill" buffers is through algorithms that calculate the value of each pixel. A clear example is found in the representation of fractal sets (Figure 2), an area of ​​mathematics dedicated to the study of certain dynamic systems. In Fractals you have the complete application.

• Use pixbuf_data to get a pointer to the contents of the buffer.
• Use pixbuf_set to write the value of a pixel.
• Use pixbuf_get to read the value of a pixel.

While pixbuf_set and pixbuf_get allow safe pixel manipulation, it may sometimes be necessary to get a little extra in terms of performance. In (Listing 1) we have some macros for direct access to the memory area returned by pixbuf_data. Use them with great care and knowing what you are doing, since they do not have error control methods, so segmentation failures are likely if they are not used correctly.

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#define pixbuf_get1(data, x, y, w)\
    (uint32_t)((data[((y)*(w)+(x))/8] >> (byte_t)(((y)*(w)+(x))%8)) & 1)

#define pixbuf_set1(data, x, y, w, v)\
{\
    byte_t *__ob = data + (((y)*(w))+(x))/8;\
    byte_t __op = (byte_t)((((y)*(w))+(x))%8);\
    *__ob &= ~(1 << __op);\
    *__ob |= ((v) << __op);\
}

3. Copy and conversion

During the digital processing of an image, we may have to chain several operations, so it will be useful to be able to make copies of the buffers or format conversions.

• Use pixbuf_copy to make a copy.
• Use pixbuf_convert to convert to another format (Table 2).

Source	Destiny	Observations
RGB24	RGB32	Alpha channel is added with the value 255
RGB32	RGB24	Alpha channel is removed with possible loss of information.
RGB(A)	Gray	RGB channels are weighted at a ratio of 77/255, 148/255, 30/255. Alpha channel is lost.
Gray	RGB(A)	RGB channels (gray, gray, gray) are duplicated. Alpha channel to 255.
RGB(A)	Indexed	The smallest distance between each pixel and the palette is calculated. Possible loss of information.
Indexed	RGB(A)	The palette will be used to obtain each RGBA value.
Indexed	Indexed	If the destination has a lower number of bits, out = in % bpp will be applied with possible loss of information.
Gray	Indexed	The Gray8 format will be considered indexed for all purposes.
Indexed	Gray	The Gray8 format will be considered indexed for all purposes.

pixbuf_create ()

Create a new pixel buffer.

Pixbuf*
pixbuf_create(const uint32_t width,
              const uint32_t height,
              const pixformat_t format);

Return

The buffer pixel.

Remarks

Initial content will be undefined.

pixbuf_copy ()

Create a copy of the pixel buffer.

Pixbuf*
pixbuf_copy(const Pixbuf *pixbuf);

Return

The copy.

pixbuf_trim ()

Crop a buffer pixel.

Pixbuf*
pixbuf_trim(const Pixbuf *pixbuf,
            const uint32_t x,
            const uint32_t y,
            const uint32_t width,
            const uint32_t height);

Return

A new buffer pixel with clipping.

Remarks

The function does not check that the limits are valid. You will get a segmentation error in such cases.

pixbuf_convert ()

Change the format of a buffer pixel.

Pixbuf*
pixbuf_convert(const Pixbuf *pixbuf,
               const Palette *palette,
               const pixformat_t oformat);

Return

The converted buffer.

Remarks

See Copy and conversion.

pixbuf_destroy ()

Destroy the buffer.

void
pixbuf_destroy(Pixbuf **pixbuf);

pixbuf_format ()

Get the pixel format.

pixformat_t
pixbuf_format(const Pixbuf *pixbuf);

Return

The format.

Remarks

See Pixel formats.

pixbuf_width ()

Get the width of the buffer.

uint32_t
pixbuf_width(const Pixbuf *pixbuf);

Return

Width.

pixbuf_height ()

Get the height of the buffer.

uint32_t
pixbuf_height(const Pixbuf *pixbuf);

Return

Height.

pixbuf_size ()

Get the buffer size (in pixels).

uint32_t
pixbuf_size(const Pixbuf *pixbuf);

Return

Width x height.

pixbuf_dsize ()

Gets the buffer size (in bytes).

uint32_t
pixbuf_dsize(const Pixbuf *pixbuf);

Return

Number of total bytes in the buffer.

pixbuf_cdata ()

Gets a read-only pointer to the contents of the buffer.

const byte_t*
pixbuf_cdata(const Pixbuf *pixbuf);

Return

Pointer to the first element.

Remarks

Correctly manipulating the buffer requires knowing the Pixel formats and sometimes using the operators at the bit level. Use pixbuf_get to correctly read a pixel.

pixbuf_data ()

Gets a read/write pointer to the contents of the buffer.

byte_t*
pixbuf_data(Pixbuf *pixbuf);

Return

Pointer to the first element.

Remarks

Correctly manipulating the buffer requires knowing the Pixel formats and sometimes using the operators at the bit level. Use pixbuf_get to correctly read a pixel.

pixbuf_format_bpp ()

Gets bits per pixel based on format.

uint32_t
pixbuf_format_bpp(const pixformat_t format);

Return

Bits per pixel.

Remarks

See Pixel formats.

pixbuf_get ()

Get the value of a pixel.

uint32_t
pixbuf_get(const Pixbuf *pixbuf,
           const uint32_t x,
           const uint32_t y);

Return

The color value.

Remarks

See Pixel formats to correctly interpret the value.

pixbuf_set ()

Sets the value of a pixel.

void
pixbuf_set(Pixbuf *pixbuf,
           const uint32_t x,
           const uint32_t y,
           const uint32_t value);

Remarks

See Pixel formats to correctly interpret the value.